Archive for the 'Amateur Radio' Category

Six Meter Interrupted Loop Antenna

Six meter AM operation using restored vintage equipment is regaining popularity because the band is rarely crowded and brings back memories.   A number of informal  local six meter AM nets operate on a regular basis.  In most cases, the net participants are located at different bearings within a 10-15 miles circle from a central reference point.   Using an omnidirectional antenna for net meetings eliminates the latency of beam antenna rotation and the coverage nulls posed by other types of antennas.

One type of low cost easy to build six meter horizontally polarized omnidirectional wire antenna is the interrupted loop (IL).  The antenna can be mast or tower mounted and will perform acceptably at a height of 10′ above average ground.  By using light weight construction materials, the antenna can also be suspended between trees.

IL antennas have been around for quite a while and are known by different names including — Halo, Squalo, HO-loop,  IL-ZX, etc.  A properly constructed six meter IL antenna will produce a nearly concentric horizontally polarized radiation pattern with moderate gain at heights as low as 10′ above average ground.

Figure-1 is a diagram of a six meter  IL wire antenna optimized for operation at 50.4 MHz  situated 10′ above average ground.  The antenna is fed at the center of the side opposite the gap with 50 Ω coax transmission line.   The width of the gap is 6″.

NOTE:  Double click figures to see a full size display.  Close the display window to return to the article.

il-diagram

Figure-1. Diagram of 6 meter IL antenna.

Figure-2 shows the predicted 3-D radiation pattern of the IL antenna at 50.4 MHz.

6m-interrupted-loop-10-ft-3d-radiation-pattern

Figure-2. Predicted 3-D radiation pattern.

Figure-3 shows the model predicted azimuth radiation pattern of the IL antenna at 50.4 MHz.

6m-interrupted-loop-10-ft-azimuth-radiation pattern

Figure-3. Predicted azimuth radiation pattern.

Figure-4 shows the model predicted elevation radiation pattern at 50.4 MHz.

6m-interrupted-loop-10-ft-elevation-radiation pattern

Figure-4. Predicted elevation radiation pattern.

Figure-5 is a bar graph of the model predicted SWR for the frequency range 50.1 – 50.6 MHz.

6m-interrupted-loop-10-ft-swr-curve

Figure-5.  Bar graph of predicted SWR.

TEST ANTENNA

I constructed a test version of the IL antenna optimized for operation at a height of 10′.  The frame was made with 2 X 2 treated furring strips and attached to a center support made with a piece of 2 X 6 treated lumber.  The center support was attached to a 10′ section of  1″ diameter galvanized pipe inserted into a 150 lb.  square concrete base pedestal.    The radiators were made with #14 AWG THHN stranded copper wire looped through porcelain insulators screwed to the top of the frame.  Light weight rigid landscape edging was used to maintain the 6″ gap and provide connection points.  Overlapping sections of edging were  used at the feed point to provide connection points and a means of tightening the wire loop.

Figure-6 is an annotated photo of the IL test antenna.

IL-antenna-2

Figure-6. Annotated photo of IL test antenna.

An AIM-4170C antenna analyzer was used to measure the test antenna’s impedance characteristics.  As predicted by the EZNEC+ V6 & AutoEZ models, capacitance needed to be added at the feed point.  This was accomplished by placing equal lengths of 20 gauge solid copper insulated twisted doorbell wire on either side of the feed point and incrementally trimming the wires until the best SWR was obtained for the frequency range 50.1 – 50.6 MHz.  Based on the average of several AIM-4170C capacitance measurements, 1′ of the doorbell wire yields ~19 pF of capacitance at 50.4 MHz.

Figure-7 shows a comparison of the predicted SWR curve and the measured SWR curve after the test antenna feed point impedance was adjusted to provide the lowest possible SWR.   A 75′  length of RG-8X (50 Ω) coax was used as the transmission line.

6m interrupted loop doorbell wire 11-24-2015 SWR comparison

Figure-7.  Model vs.  measured SWR comparison (Z0 50 ohms)

TUNING NOTES

Differences between model predicted SWR and measured SWR can be influenced by a number of factors including ground effect, surrounding objects, minor construction variances, and inherent measurement errors.  Antenna models are mathematical representations that cannot fully take into account “real world” factors such as those mentioned above.

If you want to achieve the lowest possible SWR for the antenna at a particular frequency,  it will be necessary to insert an impedance matching network at the feed point.

For the test antenna described in this article,  a simple matching network can be easily designed using the measured Rs and Xs values, target frequency, and transmission line characteristics.  Most modern antenna analyzers will record the results of a frequency sweep in a machine-readable file.   The Rs, Xs, and frequencies  for the scan range will be stored in the file.  The values can be entered into the ARRL TLW (Transmission Line for Windows) program.   TLW will design a matching network based on the values supplied and provide additional useful information.  The measured SWR at 50.4 MHz for the test antenna was ~1.9:1 (Rs=44.67 Ω; Xs=29.83 Ω) after adding twisted pair door bell wire capacitive loads on either side of the feed point.

By entering the measured values into TLW along with the length and type of transmission line (75′ RG-8X),  the L network shown in Figure 7a was generated.  You can provide the required inductance by winding a coil using #10 or #12 AWG enameled magnet wire.  A surplus adjustable capacitor with the proper range and power rating will furnish the needed capacitance and provide adjustment capability.  Notice the high loss associated with using 75′ of RG-8X at 50.4 MHz.

IL-10'-L-matching network-RG-8X

Figure-7a. Feed point L network matching circuit (RG-8X)

Take note of the significant decrease in loss  (see Figure 7b) by substituting Belden 9913-7F for RG-8X.

IL-10'-L-matching network-9913-7F

Figure-7b. Feed point L matching network (9913-7F)

CONSTRUCTION NOTES

Dimensions and wire geometry are very important factors in the construction of a 6 meter IL antenna.  For transmission line distances over 25′, consider using low loss coax such as 9913-7F (1.1 dB loss/100′ @ 50 MHz) or even better Ecoflex® 15 Plus (1.87 dB loss/100 m @ 50 MHz) to minimize transmission losses.  Be sure to use a current balun rated for operation up to 54 MHz,  snap-on ferrite sleeves, or split ferrite beads over the transmission line at the feed point to mitigate potential common mode current problems.  Environmental surroundings will definitely affect the characteristics of the antenna so plan to spend some time tuning the antenna for best performance.

OPERATIONAL TEST

With ~30 watts of transmitter power (AM mode)  from a restored Lafayette HA-460 transceiver, the IL test antenna was able to provide clearly discernible omnidirectional contacts up to a distance of ~13.7 miles from my QTH.   Depending upon terrain and receiving antenna configurations, the IL antenna at a height of 10′ above average ground provides acceptable omnidirectional communication at distances of 10 -15  miles making it suitable for many 6 meter nets.   At a height of 20′, the antenna produces a concentric double lobe 3-D radiation pattern.  At a height of 33′,  the antenna produces a concentric triple lobe 3-D radiation pattern.

Figure-8 shows the predicted 3-D radiation pattern of the IL antenna at a height of 20′.

IL-antenna-3D-radiation-pattern

Figure-8. Predicted 3-D radiation pattern at 20′.

Figure-9 shows the predicted 3-D radiation pattern of the IL antenna at 33′.

IL-antenna-3D-radiation-pattern-33'

Figure-9. Predicted 3-D radiation pattern at 33′.

The EZNEC+ V6 and AutoEZ models of the antenna at 10′, 20′, and 33′ are available at the link below.  Special thanks to AC6LA for the help in modeling and optimizing the 6 meter IL antenna.  If you’re an EZNEC antenna modeler,  be sure to evaluate AutoEZ.   It will definitely make a great addition to your antenna modeling tool kit.  The user defined variables and optimizer features of AutoEZ will greatly enhance your modeling capabilities.  Be sure to take a look at AC6LA’s free  antenna software TLDetails and ZPLOTS.

Download the antenna models (zipped format) from the link below.

IL antenna models

I mounted my 6 meter IL on a 33′ crank-up mast.  The antenna provides omnidirectional coverage for our vintage 6 meter AM net that meets every Sunday evening at 1900 on 50.4 MHz.  Contacts at distances up to 30 miles from my QTH are routinely made using a restored Utica 650 transceiver with a 6 meter linear amplifier putting out about 30 watts.

Figure-10 is a photo of my 6 meter IL at a height of 33′.

6m-IL-33ft-photo

Figure-10. Photo of my 6 meter IL antenna at 33′.

W4RNL IL-ZX 6 Meter Omnidirectional Antenna

L. B. Cebik (W4RNL, SK) wrote an article (last updated 04/03/2006) describing an experimental 6 meter omnidirectional antenna.  He referred to the antenna as an “interrupted loop – impedance transforming” antenna abbreviated IL-ZX.  The antenna is unique because it produces a nearly omnidirectional horizontally polarized radiation pattern with good gain and take-off angle at relatively low heights.

Figure-1 is a diagram of the IL-ZX antenna.  Notice the gap located in the center of the right end of the diagram.  The gap represents the interrupted loop portion of the antenna’s name.  The feed point is located at the center of the lower loop opposite the end of the antenna with the gap.  The impedance transformation characteristic of the antenna is due to the the wire geometry and spacing that is chosen to produce a feed point impedance near 50 Ω.

NOTE:  Double click the figures in the article to see full size images displayed in a separate window.  Click the close tab for the display window to return to the article.

6m IL-ZX diagram

Figure-1. 6m IL-ZX antenna diagram

Due to the significant inductive feed point reactance, Cebik suggested using twisted pair series capacitive loads (one on either side of the feed point) to neutralize the reactance.  The capacitance needed (each load) for this version of the antenna (50.4 MHz)  is ~9.2 pF.  Since the loads are capacitive and in series they act like resistors in parallel.  The net capacitance at the feed point is ~4.58 pF.  The twisted pair loads can easily be made using #20 AWG twisted doorbell wire available from a number of sources.  The lengths of the twisted pair loads needed to provide the capacitance can be measured using an antenna analyzer.  Be sure that each load measures as close to 9.2 pF as possible.

Figure-2 is a diagram of the twisted pair capacitive loads at the feed point.

IL-ZX feed point impedance

Figure-2. Twisted pair feed point capacitive loads.

Figure-3 is a table of wire dimensions (inches) for an implementation of the antenna using #14 AWG THHN stranded copper wire at a height of ~20′ and optimized for operation @ 50.4 MHz.  The width of the gap is 1″ and the vertical distance between the two horizontal loops is 4″.

table of IL-ZX measurements

Figure-3.  Table of wire lengths (inches).

Figure-4 is a bar graph of the predicted SWR for the frequency range (50.1 – 50.6 MHz).

6m IL-ZX SWR bar graph

Figure-4.  IL-ZX predicted SWR bar graph.

Figure-5 shows the predicted 3-D radiation pattern (cursor elevation 14°) @ 50.4 MHz.

6m IL-ZX predicted 3-D radiation pattern

Figure-5. Predicted 3-D radiation pattern.

Figure-6 shows the predicted radiation pattern (elevation slice) @ 50.4 MHz.

6m IL-ZX elevation pattern

Figure-6. Predicted radiation pattern (elevation slice) @ 50.4 MHz.

Figure-7 shows the predicted azimuth radiation pattern @ 50.4 MHz.

6m IL-ZX azimuth pattern

Figure-7. Predicted azimuth radiation pattern @ 50.4 MHz.

The IL-ZX is certainly an interesting antenna and well worth building especially if you don’t want to deal with an antenna rotator.  The antenna also provides good gain at a height of 10′ but presents a somewhat higher takeoff angle than at a height of 20′.

Special thanks to AC6LA for the help in modeling the IL-ZX antenna discussed in this article.  Modeling the antenna was greatly facilitated by using AutoEZ.  The optimizer and resonate features of AutoEZ were especially helpful in determining the optimum wire lengths and the feed point capacitive load values.  Thanks also to W5BIG for the tips on how to measure the capacitance/ft. of #20 AWG twisted pair doorbell wire with an AIM-4170C antenna analyzer.  If you’re looking for an antenna analyzer be sure to review the highly acclaimed AIM-4300.

The EZNEC+ V6 and AutoEZ models of the antenna are available from the link below in zipped format.

W4RNL 6m IL-ZX models

Compact Low Profile 6 Meter Antenna

The six meter band is popular with the DX community but  it can also provide enjoyment  for simplex operation within localized geographical areas.  A number of 6 meter simplex nets routinely “revisit  the past” by operating in the AM mode using restored vintage radios such as the Clegg Zeus,  Utica 650, Lafayette HA-460, etc.

Want to participate in a local net or operate simplex with a friend but don’t have much space?  Perhaps you’re severely constrained by Home Owners’ Association (HOA) CCR’s (covenants, conditions, and restrictions) or municipal restrictions?  How about building yourself a low-profile 2-element 6 meter rectangle antenna?   The antenna frame can be constructed using treated wood and attached to your deck or unobtrusively located in your backyard.   It may even fit in your attic.  The wire elements are #14 AWG THHN insulated stranded copper wire  available from a variety of sources.

The six meter band is broad (50 – 54 MHz).  A portion of the band (frequency range)  should be selected for antenna design and optimization purposes.   Since a lot of 6 meter phone activity occurs in the  50.1 – 50.6 MHz portion of the band,  that is a good frequency range to select.  For a specified design frequency,  50.3 MHz  makes sense because it is near the center of the selected frequency range.

The next step is to compute the approximate lengths of the driven element, reflector element, and inter-element spacing for the specified design frequency (50.3 MHz).  Fortunately, there is free software available to help with the calculations.  The software (MOXGEN) was written by AC6LA.  Read about MOXGEN and download it from the link below.

MOXGEN

NOTE-1:  The Moxon rectangle antenna is a modification to a much earlier antenna designed by John L. Reinardz (K6BJ, W1QP, W3RB,  SK).   Reinardz published an article about the antenna in the October 1937 issue of QST magazine (pp. 27-29).

NOTE-2:  MOXGEN computes the element lengths and inter-element spacing for Moxon rectangle antennas at a specified frequency.   Normally a Moxon rectangle antenna produces a “beam” radiation pattern.  By making some simple modifications to the  wire geometry generated by MOXGEN,  it is possible to configure a 6 meter antenna that produces a “dipole like” horizontal radiation pattern with good gain at a relatively low “take-off” angle.   The modifications result in a compact low-profile antenna suitable for local 6 meter use.

NOTE-3:  Click the figures to view them enlarged in a separate window.  Click the X in the upper right corner of the window to return to the article.

Figure-1 shows the typical MOXGEN display.  A NEC or EZNEC model is produced by clicking the Generate Model button.

moxgen-compact-6m-antenna

Figure-1. MOXGEN display.

IMPORTANT:  Be sure to read the instructions  on the MOXGEN page carefully.  You will have to make modifications to the EZNEC or NEC model produced by MOXGEN to account for antenna height above ground, ground type, wire characteristics, etc.  If you are using insulated wire, be sure to enter its characteristics (dielectric C and thickness) in the model wire definitions.   For THHN insulated wire use 4 for the dielectric C and .022″ for the thickness.

Starting with the  horizontally polarized model generated by MOXGEN,  reorient the antenna placing it in a vertical plane with the long sides parallel to the ground and the shorter length of the sides at the top.  The antenna is fed at the center of the top element using 50 Ω coaxial transmission line.   The transmission line should be connected at the feed point using a 1:1 current balun or choke (rated for operation at 50 – 54 MHz)  to suppress common mode current and prevent the transmission line from radiating.   As an alternative,  a snap-on ferrite sleeve can be placed around the transmission line near the feed point.  Figure-2 depicts the target antenna after it has been reoriented.

compact-6m-antenna-diagram

Figure-2.  Diagram of target antenna.

Figure-3 is a bar graph of the predicted SWR for the target antenna.

compact-6m-antenna Custom

Figure-3.  Predicted SWR graph of target antenna

Figure-4 is a graph of the predicted radiation pattern for the target antenna @ 50.3 MHz.

6m-moxon-unoptimized-radiation-pattern

Figure-4.   Graph of predicted radiation pattern @ 50.3 MHz.

Optimized Version of the Antenna

By using the optimizer feature of AutoEZ, the predicted SWR curve and gain of the antenna were improved by adjusting the lengths of the wire elements and inter-element spacing.

Figure 5 is a comparison of the predicted SWR before and after optimization.  The blue bars show the predicted SWR after optimization.

compact-6m-combined-swr

Figure-6 is a graph of the predicted radiation pattern @ 50.3 MHz after optimization showing a slight improvement in gain from 2.38 dBi to 2.63 dBi @ 24º elevation.

6m-moxon-optimized-radiation-pattern

Figure-6. Graph of predicted radiation pattern after optimization.

Figure-7 is a diagram of the optimized antenna.  Notice the significant increase in inter-element spacing.

compact-6m-antenna-diagram-optimized

Figure-7.   Diagram of optimized antenna.

The wingspan of the antenna is a little over 7′.   With the lower element 3′ above ground,  the total height of the antenna is around 6′.

Construction Tips

  1. 2 X 2 treated furring strips are suitable for the antenna frame.
  2. Small screw-type porcelain standoff insulators work well at each corner to maintain the shape of the antenna.  Figure-9  is a photo off a small screw-type porcelain insulator available from farm supply stores.
screw-type-porcelain-insulators

Figure-9. Small screw-type porcelain insulator.

  1. The inter-element gap is important.  You’ll want to use a couple of equal length pieces of lightweight non-conducting material  as spacers to maintain the proper gap and provide physical connection points for the wire elements.  Strip a small amount of insulation off the ends of the  wires, crimp or solder ring terminal lugs on the ends of the wires,  and fasten the wires to the spacer with #8 stainless steel pan head machine screws, washers, lock washers, and nuts.  Be sure to maintain equal spacing between the wires (gaps) on both sides.   Use COAX-SEAL or some other suitable non-conducting protective coating over the connections.  You’ll want to use shrink wrap tubing over the spacers  and associated element connections to prevent shorting during wet weather.
  2.  Provide a support arrangement at the feed point to prevent the weight of the balun/choke and transmission line from causing the top wire element to sag.

The EZNEC+ V6 and AutoEZ models for the optimized antenna can be downloaded from the link below (zipped format).

compact 6m antenna models

Fabricated Antenna

I built a test version of the antenna so it could be analyzed and tested on the air.  Figure-10 is a photo of the test antenna.  The antenna frame was constructed with scrap treated lumber.   The pedestal was made from 50 lbs. of quick setting concrete poured in a square wooden form.   The mast is a 1″ diameter section of galvanized pipe inserted into a section of  1 1/4″ diameter galvanized pipe.    The upper end of the inner pipe section is threaded into a 1″ galvanized floor flange.  The flange is attached to the lower wood cross member with (4)  galvanized carriage bolts.   The lower end of the outer pipe section is threaded into a 1 1/4″ diameter floor flange embedded in the concrete pedestal.   A short piece of plate metal was bolted to the underside of the flange with 3″ carriage bolts.  The metal plate keeps the inner pipe section from scoring the concrete when rotated and the long carriage bolts embedded in the concrete provide additional support for the outer mast section.   The concrete pedestal facilitates relocation of the antenna and eliminates issues associated with digging in areas  containing underground service.   The test antenna transmission line is a 50′ length of Belden 9913-F7 (RG-8U) low loss coax.   The 6 1/2″ inter-element spacers are made of black rigid landscape edging.  The wires are attached to the edging with ring terminals and #8 stainless steel hardware.  Rigid landscape edging is light weight, durable, and easy to work with.

fabricated-antenna

Figure-10. Fabricated test antenna.

Figure-11 is an SWR graph produced from an AIM-4170C antenna analyzer scan of the test antenna.

6m lo profile 10-12-2015 SWR graph

Figure-11. SWR graph of test antenna

The antenna performed acceptably at 50.4 MHz AM with ~7 watts output power from a restored restored Lafayette HA-460 transceiver.   Signal reports of (S4-S5)  were obtained within a radius of 14 miles from the transceiver location.

The antenna is suitable for local use  in limited space or legally constrained environments.   It can be easily attached to the corner of a deck with metal clamps or situated in a garage or attic.

6 meter Hentenna

Looking for a cheap and easy to build 6 meter wire antenna with good performance that fits into a small space?  How about a Hentenna? The Hentenna was developed in Japan in the early 1970’s.  Because of the strange characteristics the antenna exhibits, the Japanese word for strange, “hen”, was substituted for the “an” in antenna and it became known as the Hentenna.

The modeled dimensions for a horizontally polarized 6 meter Hentenna optimized for operation between 50.1 to 50.6 MHz are shown in Figure-1.  The modeled antenna elements are made of #15 AWG bare aluminum fence wire.  The lower end of the antenna is approximately 5′ above average ground.  The antenna is tuned by sliding wire 4 up or down as needed to obtain the lowest SWR.  The antenna feed point is the center of wire 4.  The model produced the best SWR curve when a 2:1 balun was inserted at the feed point.  The modeled transmission line consisted of 25′ of RG-8X coax.

Hentenna Characteristics

If the antenna is mounted long sides up, the resulting RF field is horizontally polarized.  If it is mounted long sides parallel to the ground, the resulting RF field is vertically polarized.  Antenna gain is relatively high and take-off angle is low.  Figure-1 is a diagram of the antenna.

6m-HENTENNA-diagram

Figure-1. 6 meter Hentenna diagram.

Figure-2 depicts the modeled antenna’s predicted radiation pattern at 50.2 MHz.  Click the image to display it full size.

6m-HENTENNA-radiation-pattern

Figure-2. Predicted radiation pattern at 50.2 MHz.

Figure-3  is a bar graph of the predicted SWR  for the frequency range 50.1 – 50.6 MHz.  Click the image to see display  it full size.

6m-HENTENNA-horizontal-swr-graph

Figure-3. Predicted SWR bar graph.

 Construction Tips

1.  Consider using #15 AWG bare aluminum fence wire and porcelain “standoff” insulators to build the antenna.  The wire and insulators can be purchased from your local farm supply outlet.  Using bare aluminum wire facilitates the tuning process.  Tuning is performed by sliding wire 4 up or down to achieve the best SWR curve.  You can make wire 4 (tuning wire)  using set screw type lug connectors at either end.  Make sure you keep wire 4 as level as possible and move each end the same amount during the tuning process. Don’t forget to insert the loop wire through the set screw lugs and position the wire 4 lugs above the lower porcelain insulators before the loop is tightened and connected at the bottom.  You can use a small aluminum turnbuckle to connect and tighten the wire loop at the bottom.   The tuning process will compensate for the insertion of the turnbuckle.  If you need to optimize tuning for different portions of the 6 meter band, record the vertical distances of each optimal tuning point in a table or mark them on the long support member in different colors.  To re-position wire 4, loosen the set screws, move the lugs to the desired colored position, and tighten the set screws.

2.  When you build the supporting frame for the wire, consider providing a center support member to relieve the strain on adjustable wire 4 (feed point) especially if you are using a balun otherwise the combined weight of the balun and transmission line will cause wire 4 to sag in the middle. You can use a zip tie around the balun and center support to secure the balun in place when tuning is complete.  If you need to re-position wire 4 to tune for different portions of the 6 meter band, use an adjustable garden clamp to secure the balun so it can be easily moved.

3.  With a little work, the antenna can be made manually rotatable using 1″ diameter galvanized pipe inserted into a 4′ section of 1 1/4″ diameter galvanized pipe embedded in a concrete pedestal.  It is easy to build a square wood form and make the pedestal using fast setting concrete.  Grease the lower 4′ of the 1″ diameter pipe with good quality lithium grease to ensure easy rotation and retard rust formation.  Drill and tap the 1 1/4″ section of pipe near the top to accept a 1/4″ X 20 TPI cap bolt.  Tighten the cap bolt to prevent “freewheeling.”  Attach a split pipe clamp threaded for a bolt to the 1 1/4″  fixed section of pipe to serve as a reference pointer to North.  Use a compass to find magnetic North.  If  you need the antenna to be positioned relative to true North, use the link below to find the declination for your latitude/longitude.  Add or subtract the declination to your magnetic North compass bearing to determine true North.  Rotate the clamped index pointer to the North you have selected (magnetic or true) and tighten it.  You can also add a split pipe clamp threaded for a bolt to the 1″ diameter rotating mast to serve as a bearing pointer relative to the North index pointer attached to the fixed lower section of pipe.

NOAA Magnetic Declination Estimated Value

4.  The 6 meter Hentenna has a relatively small footprint.  You can clamp the outer 1 1/4″ pipe to the corner of a deck or a length of pressure treated 4 X 4 buried in the ground.  Use a threaded pipe cap attached to the lower end of the outer pipe to keep the rotating mast from dropping through the bottom.

5.  By adding a motor driven antenna rotator, you can easily convert the Hentenna into a remotely controlled rotatable antenna.   A TV antenna rotator can easily provide the necessary torque to rotate the antenna.

The EZNEC and AutoEZ models referenced in this article can be downloaded from the link below in zipped format.  EZNEC provides the capability to view radiation pattern polarization components (horizontal and vertical) in different colors.  Check the EZNEC user manual for instructions on how to display radiation polarization.

6 meter Hentenna models

Optimizing the W3EDP Antenna

The W3EDP multi-band HF wire antenna is popular among QRP enthusiasts because it is light weight and cheap. It is also used as a portable antenna for Field Days and other similar events.  I decided to see if the W3EDP antenna could be optimized by adjusting the counterpoise lengths for each band.

Figure-1 is a diagram of the original W3EDP antenna. See the article entitled An Unorthodox Antenna  published in the March 1936 issue of QST.  A technical explanation of the antenna can be found on pp. 33-34 of Practical Wire Antennas  by John D. Heys, G3BDQ.

tape-measure-orig-diagram-mod

Figure-1. diagram of original W3EDP antenna

 

SAFETY NOTE:  The W3EDP antenna counterpoise may radiate when you transmit.  Take proper steps to ensure that no one goes near the counterpoise or the lower end of the 84′ foot radiator while you are operating.

psep

PHASE I

My experiment was divided into two phases.  Phase I entailed construction of a close approximation of the original W3EDP antenna.  After construction, the SWR measurements for the (80 – 6 meter bands) were taken with my AIM-4170C and recorded for comparison purposes.  A counterpoise length of 33’ was used for the (80 – 40) meter bands.  A counterpoise length of 17’ was used for the (20 – 6 meter bands).   I draped both lengths of counterpoise wires (#14 AWG copper stranded THHN insulated) over a pair of plastic fold-up sawhorses.  the counterpoises were spaced roughly 2′ apart and ~3′ above ground.   Figure-2 is a diagram of the Phase I antenna.

tape-measure-orig-diagram

Figure-2. Diagram of Phase I antenna.

PHASE II

Phase II consisted of modifying the Phase I antenna to provide continuously variable counterpoise lengths, adjusting the counterpoise lengths for the (80 – 6 meter bands),  and recording the SWR measurements taken with my AIM-4170C.  A  33’ X 1″ coated steel tape measure was used as an adjustable counterpoise.  I cut the tip off the tape measure and attached a spade lug to facilitate connection to the balun.  Be sure to extend the tape about a foot and lock it securely before cutting the tip off and attaching the lug.  Otherwise, the tape will retract inside the case.  Attaching a pair of locking pliers or a hemostat to the tape near the enclosure entrance slot and the tip is also a good idea in case the lock fails to hold the tape in place.  An LDG RBA-4 4:1 balun was used.  When extended, the tape measure was draped over fold-up plastic sawhorses at a height of ~3 above ground.   Figure-3 is a diagram of the Phase II antenna.   The optimum counterpoise lengths  (best SWR curve) for each band were recorded and entered into a table shown in Figure-4.

tape-measure-L2-diagram

Figure-3. Diagram of the Phase II antenna

tape-measure-L2-table

Figure-4. Table of optimum tape measure counterpoise lengths by band

Analysis

The measured SWR curves from Phases I and II  were overlaid on a graph for each band to facilitate comparison.  The red lines with embedded circles represent Phase I SWR curves.  The light orange lines represent the Phase II SWR curves.  Click a band band to see the SWR comparisons.

80 meters
40 meters
20 meters
17 meters
15 meters
12 meters
10 meters
6 meters

 Summary

The SWR curves for every band tested were improved by adjusting the counterpoise to an optimum length. I tested the antenna on 17 meters using my ICOM-706MKIIG at 100 watts (SSB) to see how well it would work with the tape measure counterpoise.  I set the tape measure to a length of 12′, found an open frequency and tuned up using the default 17 meter settings (52/284) for my Palstar AT-500 manual tuner.  I didn’t have to make any adjustments to the tuner.  I cruised around the band until I heard CQ from G0EPU (Maltby, England) on 18.120 MHz.  Had a nice QSO with him.  He reported receiving me at 5-5 to 5-9.  The 17 meter band was fluctuating quite a bit at the time.

W3EDP Multi-band Antenna

A description of the W3EDP antenna  entitled An Unorthodox Antenna was published in the March 1936 edition of QST. The article was authored by Yardley Beers, W3AWH.  The antenna described in the article was developed using  “the cut and try method” by H. G. Siegel, W3EDP probably around 1934.  W3EDP’s objective was to build a multi-band antenna.  Today, the W3EDP antenna is a favorite of many QRP (low power operation) aficionados because it is simple, compact, and easy to deploy.

I decided to take a look at the W3EDP antenna as a possible multi-band antenna suitable for fixed location use and as an easily deployable portable antenna for events such as Field Days.  After several hours of Internet searching, I collected and read a number of articles, explanations, and testimonials about the antenna.  The most useful information was gleaned from the March 1936 QST W3EDP article available from the ARRL archives and a technical description of the W3EDP antenna [pp. 33-34] published in Practical Wire Antennas (ISBN 0 90061287 8) by John D. Heys, G3BDQ.

Undoubtedly W3EDP was aware of the much publicized Zepp(elin) antenna that was patented in 1909 by Hans Beggerow (German patent #225204).  See figure-1a.  Early Zepp antennas were 1/2 WL long (or multiple) and fed with a 1/4 WL (or multiple) open wire feed line which uses only one of the wires. The feed line acted as a matching section for the transmitter.  The antenna was typically connected to the transmitter with a tuned link coupled network.  As it turns out,  the HF antenna on the dirigible Graf Hindenburg was 85′ long.  W3EDP settled on a length of 84′ for the long leg of his antenna.  Apparently much of  his experimentation with wire lengths was centered on optimum counterpoise lengths for the U. S. amateur radio bands circa 1936.

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Figure-1a. 1909 Zepp antenna patent diagram

What is the W3EDP antenna?  The best way to answer that question is to provide a simple diagram of the original antenna and go from there.  Figure-1b is a diagram of the original W3EDP antenna. W3EDP is reputed to have used over 1,000’ of wire to come up with the dimensions shown in Figure-1b.  Wire L1 is 84′ long and attaches to one end of a parallel tuned tank.  The distinguishing characteristic of the antenna is wire L2 that W3EDP referred to as a “counterpoise.”  L2 was attached to the other end of the parallel tuned tank.  The use of the term counterpoise to describe L2 has been questioned by some who have studied the antenna.   See the L. B. Cebik, W4RNL (SK) article entitled Counterpoise? On the Use and Abuse of a Word ; AntenneX ~ December 2006 Online Issue #116.  To avoid entanglements, I shall simply call it element L2.

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Figure-1b. original W3EDP antenna

NOTE:  W3EDP calculated that an L2 length of 17′ gave the best results for the 160, 80, 40, and 10 meter bands.  He selected an L2 value of 6.5′ for the 20 meter band.  According to the March 1936 QST article, W3EDP worked 75 countries in all continents within a two year period using his antenna with 50 watts input to the final P.A. (power amplifier) of his transmitter.  The article doesn’t specify the model of  his transmitter or whether he operated using CW, AM, or both.

Construction Phase

I decided upon a W3EDP configuration in common use today for my antenna.  The design uses one half of a 17’ length of 450 ohm balanced line for element L2.  The other half of the 17′ balanced line is connected to a 67′ length of wire to complete the 84′ L1 element.   L2 is left unconnected at the top but is connected at the bottom to the source.  Figure-2 is a diagram of the configuration.

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Figure-2. W3EDP project configuration.

During the construction step, I marked the lower end of the half of the balanced line used for L2 with a distinctive color so it could be identified when connected to the source.  After completing the antenna, I hoisted the balanced line end to a height of ~20’.  The other end of the antenna was ~25’ above ground. 

Analysis I

The lower end of L2 of was connected to the ground port of an LDG RBA-4 4:1 balun which is what I had available for use at the time.  See the note below regarding baluns.  The lower end of the  L1 section of the antenna was connected to the antenna port of the RBA-4.  Figure-3 shows the balun connection.

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Figure-3. Balun connection

 NOTE:  For an unbalanced antenna like the W3EDP, you should use a 4:1 current balun to provide the required impedance match as well as avoid ground/counterpoise losses associated with voltage baluns.  At the time of the experiment,  I only had a 4:1 voltage balun available.

The RBA-4 was directly connected with a double-ended PL-259 coupler to the input port of an AIM-4170C analyzer.  The analyzer was used to calculate and store the SWR curves for the 80 through 6 meter bands. The curves for each band are displayed below.  Click the images to see the full-size display.

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Figure-4. 80 meter band SWR curve

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Figure-5. 40 meter band SWR curve

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Figure-6. 20 meter band SWR curve

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Figure-7. 17 meter band SWR curve

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Figure-8. 15 meter band SWR curve

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Figure-9. 12 meter band SWR curve

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Figure-10. 10 meter band SWR curve

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Figure-11. 6 meter band SWR curve

Analysis I Summary

The antenna is multi-banded but not particularly efficient on any band.  A tuner with wide-range impedance matching capability will be required to use the antenna.  A 4:1 balun will definitely help with impedance matching.

Modification and Analysis II

 I decided to see what would happen if I connected a 32’ length of AWG #14 stranded THHN coated wire to the ground port of the RBA-4 and just let it lay on the ground.  I inserted an insulated “spade type” connector at the half-way point (16’) so I could easily disconnect the last 16’ feet of wire.  Figure-12 shows the Analysis II configuration.

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Figure-12. Analysis II configuration

I used the AIM-4170C to analyze each band (80 – 6 meters) again with the configuration shown in Figure-12.  For the 80 and 40 meter band analysis, the full 32’ length was used (L3 + L4).  For the 20 – 6 meter band analysis, only the 16’ length (L3) was used.

The Analysis II curves are shown below with the light orange curve representing SWR with L3 or L3 + L4 connected.  The Analysis I curves are displayed in red with embedded circles.  Click the images to see the full-size display.

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Figure-13. combined 80 meter band SWR curves

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Figure-14. Combined 40 meter band SWR curves

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Figure-15. combined 20 meter band SWR curves

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Figure-16. combined 17 meter SWR curves

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Figure-17. combined 15 meter SWR curves

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Figure-18. combined 12 meter SWR curves

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Figure-19. Combined 10 meter SWR curves

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Figure 20. Combined 6 meter SWR curves

Analysis II Summary

Adding the L3 and L3 + L4 wires significantly improved the measured SWR curves on nearly all of the bands analyzed.   You can decide what to call L3 and L3 + L4.

NOTE:  Be sure to properly ground your equipment and follow applicable safety precautions when using the L3 or L3 + L4 wire configuration.  In this configuration,  L3 and L3 + L4  radiate and can introduce RF at the station.  If the equipment is not properly grounded,  you may notice extraneous noise and possibly get “bitten” if you touch metal components when transmitting.

Operational Test

I hooked up my equipment and gave the modified antenna configuration an “on-the-air” test.  I was curious to see how much impact the L3, and L3 + L4 wire element radiation losses would have on antenna performance.  Figure-21 shows the station equipment used in the test.  In the diagram, the AT-500 is a Palstar manual antenna tuner with wide-range impedance matching capability, the RBA-4 is an LDG RBA-4 4:1 voltage balun, and the IC-706MKIIG is an ICOM transceiver.  All testing was done using 100 watts (SSB).  My QTH is located in Cary, North Carolina.

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Figure-21. “on-the-air test” station equipment configuration.

 Operational Test Results

80 meters:  the AT-500 tuned the antenna with no problem.  However, no activity was heard on the 80 meter band which was not surprising given that the test was conducted around lunch time.

40 meters:  the AT-500 tuned the antenna with no problem.  The 40 meter band was “jumping” and I quickly made contacts in South Carolina and Connecticut with 59 signal reports.  I have a 40 meter bowtie wire antenna that works very well.  I used the coax port switch on the AT-500 to check the W3EDP antenna against the bowtie antenna.  The bowtie proved to be better than the W3EDP but not by much.

17 meters:  the AT-500 tuned the antenna with no problem.  The 17 meter band was “jumping” and I quickly made contacts in California and Spain.  I have a 17 meter N4GG antenna that works exceptionally well.  I used the coax port switch on the AT-500 to check the W3EDP antenna against the N4GG antenna.  The N4GG antenna proved to be better than the W3EDP but again not by much.

I ran out of time and terminated the operational test without trying the 20, 15, 12, 10 and 6 meter bands.  I plan to test these other bands as time permits.

The modified W3EDP is definitely a multi-band antenna.  However,  an antenna tuner capable of matching a wide range of impedances is required.  A 4:1 balun will definitely help with impedance matching.  Undoubtedly, some autotuners will have difficulty handling the impedances presented by the antenna.

TIP:  You may want to consider using a 3-position switch to control which “counterpoise/radial” wire is selected.   Figure-22 shows a diagram of the switch.

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Figure-22. switching arrangement for the 16’ and 32’ wires

Analysis III

I decided to configure and analyze an antenna that closely resembled the original W3EDP antenna.  I removed the balanced line portion of the antenna used for Analysis I and II.  For L1,  I attached a single 17′ piece of AWG #14 copper stranded THHN coated wire to the existing 67′ piece of the same kind of wire using a spade connector.  For L2, I used two different lengths of AWG #14 copper stranded THHN coated wire.  A 17′ length for the 20 – 6 meter bands and a 33′ length for the 80 – 40 meter bands.   I used a 3-position knife switch to select the appropriate length of L2. Figure-23 is a diagram of the Analysis III configuration.

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Figure-23. Diagram of Analysis III configuration

For L2, I draped both lengths of wires over a pair of plastic fold-up type sawhorses spaced so the wires remained roughly parallel to and ~3′ above ground.  The wires were spaced ~2′ apart over the sawhorses.  This arrangement aligned the wires at almost a right angle to L1 at the balun which was located on top of a portable work table.  I analyzed the configuration for the 80 – 6 meter bands using my AIM-4170C.  The SWR curves for each band are shown below.  Click the images to see the full-size display.

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Figure-24. Analysis III – 80 meter band SWR curve (L2 = 33′)

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Fiigure-25. Analysis III – 40 meter band SWR curve (L2 = 33′)

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Figure-26. Analysis III – 20 meter band SWR curve (L2 = 17′)

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Figure-27. Analysis III – 17 meter band SWR curve (L2 = 17′)

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Figure-28. Analysis III – 15 meter band SWR curve (L2 = 17′)

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Figure-29. Analysis III – 12 meter band SWR curve (L2 = 17′)

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Figure-30. Analysis III – 10 meter band SWR curve (L2 = 17′)

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Figure-31. Analysis III – 6 meter band SWR curve (L2 = 17′)

Analysis III Summary

I have to say that based on my Analysis III measurements,  H. G Siegel, W3EDP did an excellent job of coming up with the dimensions for a multi-band HF wire antenna.

With some experimentation,  employing optimized L2 lengths for each  band of interest will significantly improve SWR curves over just switching between the fixed 33′ and 17′ lengths.  As a test,  I connected an old steel tape measure (33′ X 1″) to the ground port of the 4:1 balun and adjusted its length to obtain the best SWR curve for the 17 meter band.  I was able to reduce the measured SWR from ~7:1 (L2 = 17′ of #14 AWG copper stranded THHN insulated wire) to slightly over 1:1 by adjusting the tape measure to a length of about 12′.  See the combined measured SWR curves below. The light orange curve was produced with the tape measure attached to the ground port of the 4:1 balun.  Click the image to see the full-size display.

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Figure-32. tape measure 17 meter band SWR curve

See Optimizing the W3EDP Antenna.

For fixed location operation, a rotary multi-position switch box to select the optimum length of L2 for each band would be fairly inexpensive to build.  A light-weight portable non-conducting spreader bar arrangement could be made to suspend the L2 wire array ~3′ off the ground.

The W3EDP antenna is a good fixed location HF wire antenna as well as a fine portable antenna.  You will need a tuner to match your rig to the impedances presented by the antenna.  A 4:1 balun will also help.

40 Meter Bowtie Antenna

A bowtie antenna is a type of antenna that reputedly provides higher gain at lower radiation angles than a center-fed dipole antenna at heights considerably less than 1/2 wavelength above ground.  Bowtie antennas have been around since the “spark gap” days of radio.  I was curious about HF bowties so I decided to learn more about this type of antenna.

Phase I – Antenna Modeling

I selected a simple horizontal form of the bowtie antenna for modeling purposes. Figure-1 shows the target bowtie configuration.

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Figure-1. 40 meter bowtie antenna configuration

How long should the radiators (L) be and how far apart (W) should they be at the ends?   I started with a commonly used formula for computing the radiator lengths of a 1/2 wavelength dipole (468 / freq. MHz).

L = 468 / 7.15 MHz = 65.45’;  65.45 / 2 = ~32.72’ = 33’ (rounded up)

For W, I decided to start with 20% of the length of L.  Why 20%?  I had to start somewhere and I knew that I could easily adjust the value of W in the model.

W =  33’ * .20 = ~6.6′ = 7’ (rounded up)

The antenna was specified to be in a horizontal plane 25’ feet above real ground.  The radiators were defined as #14 AWG stranded THHN insulated wire.  The antenna was configured to be center-fed using Wireman 554 ladder line with a 4:1 balun at the source for impedance transformation purposes.

I plugged in the starting values of L & W and adjusted the model to get the best predicted SWR curve.  After some experimentation,  I found that a value of 6’ for W and radiator lengths (L) of ~31.25’ produced the best predicted SWR curve.

The EZNEC models used in this project can be downloaded from the links below.

center-fed dipole model      bowtie model

Note:  To see the predicted effects of insulated wire radiators take a look at the EZNEC models below.

center-fed dipole – insulated radiators   bowtie – insulated radiators

Predicted radiation Patterns

Figure-2 shows the EZNEC predicted elevation radiation pattern for a 40 meter center-fed dipole at a height of 25’.

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Figure-2. 40 meter center-fed dipole predicted elevation radiation pattern

Figure-3 shows the EZNEC predicted elevation radiation pattern for the target 40 meter bowtie antenna at a height of 25’.

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Figure-3. 40 meter bowtie predicted elevation radiation pattern

 Predicted SWR Curves

 Figure-4 shows the EZNEC 40 meter center-fed dipole predicted SWR curve.

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Figure-4. 40 meter center-fed dipole predicted SWR curve

Figure-5 shows the EZNEC predicted SWR curve for the target 40 meter bowtie antenna.

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Figure-5. 40 meter bowtie antenna predicted SWR curve

Before you get concerned about the predicted bowtie antenna SWR curve, remember that balanced line will be used for the transmission line.  High quality balanced line provides low transmission loss even in the presence of relatively high SWR values.  What we really care about is antenna efficiency which is a measure of the effective power delivered to the antenna feed point.  We will calculate the efficiency of the completed antenna later in the article when the antenna analyzer measurements are available.

Let’s take a quick look at the proposed 40 meter bowtie antenna predicted efficiency.  We can use a free online transmission line loss calculator and the EZNEC SWR predictions to estimate efficiency.  The online loss calculator is generally accurate to within ±5%.  The Wireman 554 ladder line (50′) was selected for the transmission line.  Figure 5a shows the predicted antenna efficiency at three frequencies across the 40 meter band.

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Figure-5a. 40 meter bowtie antenna predicted efficiency

Comparison of Center-fed Dipole and Bowtie EZNEC Models

As far as predicted radiation patterns go, the center-fed dipole is a “cloud warmer” with negative gain at radiation angles less than 45 degrees.  On the other hand, the bowtie exhibits modest gain at radiation angles of 25 degrees and higher. The 40 meter bowtie predicted SWR curve and antenna efficiency prediction are acceptable.

According to the model predictions, a 40 meter bowtie antenna should provide good local/regional coverage. Encouraged by the predictions, I decided to build a bowtie antenna.

Phase II – Construction

The characteristics of the wire used and environmental conditions present will affect the physical lengths of the radiators needed to achieve the best SWR curve.  If you build the antenna, be sure to allow extra radiator length to compensate for wire characteristics and factors such as ground quality and “capacitance” caused by proximity to trees or buildings.  The extra wire can be left dangling at the radiator ends and trimmed as needed to achieve the best SWR curve.

TIP:  If you don’t have 65’ feet of horizontal space for the antenna, let the radiators dangle at the far ends so you can fit the antenna in the space you have.  Keep the  lengths of the dangling portions of the radiators the same length and limited to 20% or less of the radiator length (L) and the antenna will still perform well.

End spreaders: The spreaders used to maintain the end spacing were made from 2 X 2 X 8 treated wood furring strips.  To facilitate attaching the radiators and support ropes, I inserted 1/4 X 20 eyebolts at the ends of the spreaders.  Figure-6 shows a diagram of the spreader bar.

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Figure-6. diagram of spreader bar

TIP:  If you use 2X2 furring strip spreaders, select the strips carefully to find two that aren’t full of knots, split, bowed,  or dripping with preservative.  After you insert the eyebolts, spray paint the spreaders and bright metal hardware with dull finish camouflage paint to reduce visibility.

Figure-7 shows one of the completed end spreaders.

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Figure-7. completed spreader bar

Feed Point Connector and Balanced Line Support

The feed point connector was made by inserting an eyebolt in the center of a plastic dog bone insulator.  The balanced line support was made from rigid composite landscape edging.  The support “sandwiches” the balanced line between two pieces of edging. Nylon bolts (1/4 X 20) and wing nuts were used to fasten the two halves of the support.  The support is attached to the feed point connector with a large plastic zip tie.  Ring terminals were crimped on the ends of the radiators and balanced line. The radiators were connected to the balanced line with 3/4” long #8 pan head stainless steel machine screws, flat washers, lock washers, and wing nuts. Figure-8 shows the completed feed point connector and balanced line support.

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Figure-8. feed point connector and balanced line support

Phase III – Analysis

The completed bowtie antenna was hoisted and analyzed with an AIM-4170C.  Figure-9 shows the measured SWR curve after the antenna was tuned.  Notice the measured SWR curve is lower than the model predicted SWR curve and is relatively flat across the 40 meter band.

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Figure-9. 40 meter bowtie AIM-4170C measured SWR curve

40 Meter Bowtie Calculated Antenna Efficiency

The AIM-4170C SWR readings and the online transmission line loss calculator were used to calculate the efficiency of the completed bowtie antenna.   Figure-10 shows calculated antenna efficiency at three frequencies across the 40 meter band.

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Figure-10. 40 meter Bowtie antenna efficiency (online calculator)

Antenna Matching

A balanced line matching network is the preferred approach to matching the bowtie antenna to a transmitter.  Figure 10a is a diagram of a balanced line antenna matching network.

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Figure-10a. Balanced line matching network

Unfortunately, most of us don’t have a balanced line matching unit readily available.  Integrated autotuners found in most amateur radio transceivers are designed to accept unbalanced coax transmission lines.

The good news is that we can connect the balanced line to an unbalanced antenna tuner by inserting  a balun between the balanced line and the tuner input. Since the experimental antenna has a 4:1 balun connected to the balanced transmission line, a matching circuit such as a high pass T-network can be used.

We can get an idea of how a high pass T-network matching circuit will affect antenna performance by using a free T-network tuner simulator (courtesy of W9CF).  Figure 10b is a diagram of a typical adjustable T-network matching circuit.

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Figure-10b. Typical adjustable T-network circuit

To use the simulator we need to know the load R ±j values at each frequency of interest.  Figure-10c shows the source R ±j values measured by the AIM-4170C.

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Figure-10c. AIM-4170C source impedance measurements

Fortunately,  we can use the free TLDetails transmission line loss calculator (courtesy of AC6LA) and the AIM-4170C measured source R ±j values (Figure-10c) to get the load R ±j values needed to use the tuner simulator.  Figure 10d shows the TLDetails load impedance values for 7.15 MHz.  Look for the cursor arrow (at load) on the left side of the results panel.

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Figure-10d. AIM-4170C source impedance measurements

Enter the frequency and load impedance values from TLDetails into the tuner simulator.  Click Autotune.  The simulator will display the SWR match and estimated tuner loss.  The values Autotune selected for C1, L, and C2 will also  be displayed.  We can vary the tuner component values using Setup to reflect the characteristics of a real T-network tuner.  Figure 10e shows the tuner simulator display for 7.15 MHz.  The simulator indicates that a SWR of 1:1 can be achieved.  Calculated tuner loss is 4.8%.

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Figure-10e. T-network tuner simulator display for 7.15 MHz

We can refine calculated antenna efficiency by subtracting estimated tuner loss from net power available.  There will be additional loss incurred with the 4:1 balun depending upon the impedance.

The completed bowtie is reasonably efficient for a 40 meter wire antenna at a height of 25’ (~.28 wavelength).  Most autotuners should have no problem matching  the antenna to the transmitter.

Phase IV – Operational Test

The operational test configuration for the bowtie antenna consisted of a IC-706 MKIIG transceiver and a Palstar AT-500 manual antenna tuner.  The Wireman 554 balanced line TL was connected to an LDG RBA-4 4:1 balun. The RBA-4 was connected directly to the AT-500 using a double-ended PL-259 connector.  The AT-500 was connected to the IC-706MKIIG with an 18″ RG-8X coax jumper cable.  Figure-11 shows the operational test configuration.

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Figure-11. Operational test configuration

Bowtie Operational Test Results

An operational test was conducted on September 17, 2013.  QTH Cary, North Carolina running 100 watts SSB.

I’m a member of the Navy Amateur Radio Club (K4NAR.org) that operates a 40 meter net on 7.245 MHz (0800-1000) daily.  I was able to check in with the duty net control located in Georgia with a 59 signal report.  The net routinely has check-ins from all over the eastern half of the United States.  I was able to clearly hear check-ins from New York to Florida and as far west as Ohio. Check-ins from North Carolina were also clearly heard.  I checked in with the daily Communications Ontario Net (Chatham, Ontario, Canada) on 7.153 MHz with a 59 signal report.  I was able to hear numerous net check-ins from all over the Eastern half of the country on ECARS (7.255), MIDCARS (7.258), and SOUTH CARS (7.251).

CONCLUSION

The bowtie antenna performs well as a local/regional antenna.  In my case, it covered the Eastern half of the United States and lower Canada.  The bowtie antenna would make a good Field Day antenna that could easily be suspended between trees or a couple of portable masts.  As a bonus, the antenna will perform acceptably on the 20, 15, 12, and 6 meter bands if an antenna tuner is used.

Coil Winding Tips

Wire coils (inductive loads, traps, band-pass filters, etc.) are used in a number of amateur radio projects especially antenna projects.  The free single-layer air-core inductor design tool by K7MEM will provide the construction parameters for specified target inductance values.  There is an option available for taps.  The parameters generated by the design tool will be in the “ball park.”  However,  the characteristics of the wire (diameter, velocity factor, etc.) used to wind the coil will affect its inductive properties.  Be sure to measure the inductance of the completed coil.   An antenna analyzer such as the AIM-4300 can be used to produce a graph of the inductance curve and display the equivalent circuit characteristics of the coil.

Let’s start with coil forms.  PVC pipe is an acceptable coil form stock that is relatively inexpensive and readily available.  Two inch inside diameter Schedule 40 PVC pipe is my favorite for making coils.  The pipe can easily be cut to the desired length with a handsaw.  Eyebolts can be inserted through PVC end caps that are glued to the pipe to provide a strong inline suspension capability.  Figure-1 shows an unassembled coil form.

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Figure-1 

To lower the visibility of coils, spray paint the coil form and end-caps with a camouflage paint such as dull black before you wind the coil.  If the coil is to be deployed outside, use stainless steel hardware.  Figure-2 shows a popular brand of camouflage spray paint.

image Figure-2

The process of winding the required number of turns of wire around the coil form is the most challenging step in making coils because wire tends to be springy and difficult to hold in place. If you are making a loosely wound tapped coil, the task becomes more difficult because uniform spacing between turns is desirable.  Also sufficient space between coil turns has to be provided so a lead can be attached to the appropriate tap to obtain the desired inductance.

A cheap and effective way to significantly reduce the effort required to wind coils is to attach a length of PVC “Z” flashing to the coil form with pan head screws.  For closely wound coils, wind the coil and use the flashing to sandwich the coil to the form to hold it in place.  You may even want to use two pieces of flashing situated on opposite sides of the form.   For loosely wound and tapped coils,  drill holes through the side of the flashing at uniform intervals to provide the desired spacing between coil turns.  The hole diameter should be just large enough to accommodate the wire used to wind the coil.    The wire can then be easily threaded through the holes.   Figure-3 shows a section of PVC Z flashing attached to a coil form.  PVC Z flashing can be purchased in 10’ lengths from home supply stores.

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Figure-3

To attach a radiator to the coil,  crimp solderless ring terminals to the ends of the coil lead and the radiator.  Use #8 pan head screws and washers to fasten the ring terminals to the form and make an electrical connection between the radiator and coil end.  Figure-4 shows a coil end connection.

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Figure-4

It is a good idea to document the properties of completed coils including their inductance curves.  Attach a unique identifier to each coil so the identifier can be used to retrieve the archived properties when needed.  In the documentation, include coil type (load, trap, band-pass, etc.), inductance curve, dimensions and material used for the form, characteristics of the wire used, number of turns, number of taps and tap locations.

Ultra-compact Multi-band Wire Antenna

Live in an area that prohibits external antennas?  Maybe your lot is just too small to put up one of the popular multi-band wire antennas.  If you have an attic, garage, workshop, or deck that is a little over 12’ in length you can operate on 40 meters through 6 meters with a relatively cheap and easy to build wire antenna thanks to WB2JNA.  An article describing his antenna design was published in the ARRL Antenna Compendium, Vol. 6  (ISBN: 978-0-87259-743-3).  The antenna design is also included in Simple and Fun Antennas for Hams (ISBN 978-0-87259-862-1), Chapter 12,  pp. 7-8.

This article discusses my implementation of the WB2JNA antenna.  Figure-1 is a diagram of the antenna.

Figure-1. Cheap & easy to build  multi-band antenna diagram

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ANTENNA MODELING

If you have the capability, it’s a good idea to model an antenna before you build and evaluate it.  EZNEC+ models for the 40-meter and 20-meter bands are available at the link below.  Thanks to AC6LA for the tips to improve the models using AutoEZ.

If you are an EZNEC user, you should seriously consider adding AutoEZ to your toolkit.  AutoEZ is an attractively priced add-on that interfaces with EZNEC+ and enables you to easily refine your models.  AutoEZ includes a powerful optimizer feature that can be used to find optimum radiator lengths, coil inductance values, and many other important aspects of an antenna.  I used the AutoEZ optimizer feature on this project to find the loading coil inductance values and counterpoise lengths that produced the best predicted SWR curves for the 40-meter and 20-meter bands.

NOTE: AutoEZ indicates that the SWR curves for each band can be significantly improved by adjusting both the inductance of the loading coil and the length of the counterpoise.  You may want to consider using optimum length counterpoise for each band of interest.   A rotary switch could be used to switch between counterpoises   Another possibility is to use a retractable steel tape measure for the counterpoise  Using an antenna analyzer, you can compile a table of the optimum coil tap positions and counterpoise lengths for each band of interest.

WB2JNA EZNEC Models (zipped)

CONSTRUCTION

The radiator is 12’ long.  The counterpoise is either 16′ or  32’ long.  Read about counterpoises. The term counterpoise in this context means a length of insulated wire with one end attached to the shield of the coax transmission line at the antenna feed point.  The counterpoise for this antenna is situated above physical earth ground and is made of black #14 AWG copper stranded THHN insulated wire.  The counterpoise has a SPST switch in the middle (16’ point) to facilitate band switching.  The loading coil is wound on a short piece of Schedule 40 2” ID PVC pipe.  Twenty feet of black #14 AWG copper stranded insulated hookup wire was used to wind the coil.  The coax connector block for the transmission line was made from a short piece of 1 1/4” ID schedule 40 PVC pipe, (2) couplings, (2) plugs, and a SO-239 connector.  The coil tap lead (red) has an alligator clip on one end.  The other end of the coil tap lead is connected to the coax center at the connector block.  The counterpoise is connected to the coax shield at the block.  The connector block is attached to the coil form with a plastic zip tie.  Figure-2 shows the coil and coax connector block.  The coil form has eyebolts in either end for suspension purposes.  The connector block has an eyebolt in the top so the block can be attached to the end of the coil form.

NOTE:  You may want to add a few more turns to the loading coil to provide a wider range of inductance in case it is needed to for your implementation.  You can also buy loading coils and tapping clips from Barker & Williamson.

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Figure-2. Loading coil and coax connector block

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LOADING COIL MEASUREMENT

The coil was analyzed on the 40 meter and 20 meter bands with an AIM-4170C.  For the 40 meter band,  the end-to-end coil inductance measurement was ~23.7 uH.  Figure-3 shows the 40 meter analysis.  Figure-4 shows the 20 meter analysis

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Figure-3. 40 meter band coil analysis

The 40 meter coil inductance curve is relatively flat across the 40 meter band.

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Figure-4. 20 meter band coil analysis

The 20 meter coil inductance curve varies significantly as the frequency increases.

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40 METER BAND SWR CURVE MEASUREMENT

The antenna was hoisted to a height of approximately 10’ and the SWR curves for different bands were plotted using the AIM-4170C.  The SWR curves for all bands analyzed were significantly improved by coiling 10 turns of the RG-8X coax transmission line at the source end to form a choke.   The coil was secured with plastic zip ties.  The tap of the loading coil was moved around to find the best SWR curve for the 40 meter band.  The counterpoise length was set to 32’ (SPST switch closed).  Figure-5 shows the 40 meter band SWR curve.

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Figure-5. 40 meter band SWR curve

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20 METER BAND SWR MEASUREMENT

For the 20 meter band SWR analysis,  the counterpoise length was reduced to 16’ (SPST switch open) and the loading coil tap was moved around to find the best SWR curve.   Figure-6 shows the 20 meter band SWR curve.

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Figure-6. 20 meter band SWR curve

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17 METER BAND SWR MEASUREMENT

The 16′ foot counterpoise was used and the loading coil tap moved around to find the best SWR curve.  Figure-7 shows the 17 meter SWR curve.

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Figure-7. 17 meter band SWR curve

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6 METER BAND SWR MEASUREMENT

The 16′ foot counterpoise was used and the loading coil tap moved around to find the best SWR curve.  Figure-8 shows the 6 meter SWR curve.

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Figure-8. 6 meter band SWR curve

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15, 12, 10 METER BANDS

By adjusting coil tap position,  length of the radiator,  and length of the counterpoise, it is possible to achieve acceptable SWR curves for the 15, 12, and 10 meter bands.

OPERATION

NOTE: This is a compromise antenna and will not provide the performance of a properly configured full-size antenna.  It’s purpose is to get  you on the air in a very limited physical space.  The use of an automatic antenna tuner is recommended due to the significant variations in impedance presented by the antenna at different frequencies. You can expect some differences in SWR curves from those included in this article due to construction variances and environmental factors at your QTH.

Some reasonably priced automatic tuners:  LDG AT100PROII MFJ-925; SG-237

The antenna performed reasonably well on 40 meters.  I was able to check into several regional 40 meter nets with signal reports ranging from 55 to 59.  Although I didn’t transmit on 20 meters and 17 meters, I could hear plenty of stations.   It’s certainly an interesting antenna and well worth building especially if you’re in a limited space situation and need an ultra-compact multi-band antenna.

SAFETY NOTE:  The counterpoise may radiate on some bands.  Be sure to take steps to ensure no one comes in contact with the 12′ radiator or the counterpoise when transmitting.  Keep the transmitter power to 100 watts or less if you’re using the antenna indoors.

“Homebrew” Balanced Line Support

You can build your own multi-purpose balanced line antenna feed point support in just a few minutes using readily available “off-the-shelf” parts.  Balanced line transmission lines should be securely attached at antenna feed points to prevent breakage due to “wire fatigue” caused by the flexing effects of wind and the weight of ice loading.

The most common types of balanced line used as antenna transmission lines are show below.

imageThe balanced line antenna feed point support discussed in this article consists of two parts.  The top part is a modified plastic dog bone insulator.  The bottom part is a “sandwich” type clamp made of durable lightweight weather resistant material.  The clamp is attached to the modified dog bone insulator with a stout plastic zip tie.

Top Part

You can make the top part of the support by drilling a hole (slightly larger than 3/16” diameter) through the center of a plastic dog bone insulator.  The next step is to insert a 3/16” diameter stainless steel eyebolt through the hole and secure it with a stop nut.  It’s a good idea to use flat stainless steel washers on either side to bridge the ridges in the insulator.

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top part

Bottom Part

The bottom part “sandwich” clamp can be made from weather resistant lightweight composite material such as 3 1/2” wide rigid landscape edging available in 20’ rolls.  Other materials such as Plexiglas can be used to make the clamp.   It turns out that a single clamp can be made by cutting a 9” long piece of edging and ripping it exactly in half (1 3/4” wide).  If you go with the edging,  be sure to use a bi-metal or ceramic saw blade because of the abrasive properties of the material.  Also be sure to wear eye protection and a mask when cutting the edging.

 

rigid landscape edging

The next step in constructing the bottom part is to drill holes for the nylon bolts that will secure the two halves of the clamp and cut two threading slots.  Since we want the clamp to work with any of the three types of balanced line discussed above,  hole/slot placement is important.  Drill an appropriately sized hole at the top of the clamp for the zip tie that will secure the clamp to the modified dog bone insulator.  Lay a section of each type of window/ladder line on the clamp and mark the spots (centered) where the top edge of the line will catch a nylon bolt.  Drill two holes at the spots slightly larger than the diameter of a 10/24 nylon bolt.   Drill two additional holes (same diameter) near the bottom of the clamp.  These two holes and the hole (second from top) will accommodate the three nylon bolts needed for configuration #1 shown below.  Lay a section of TV/FM twin lead on the top half of the clamp and mark the spots where the slots will be cut.  Use a drill and appropriately sized bit to cut the two slots.  Allow enough spacing between the slots so the TV/FM twin lead is not bent up at a sharp angle as it passes through the slots.

Configuration #1 (300 ohm TV/FM twin lead)

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300 ohm TV/FM twin lead

Thread the TV/FM twin lead through the slots and tighten the clamp using three nylon bolts and wing nuts.

Configuration #2 (300 ohm & 450 ohm window/ladder line)

 

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300 ohm window/ladder line

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450 ohm window/ladder line

The window/ladder line is placed in the clamp so the two nylon bolts catch the top edges of the gaps in the line.

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Tighten the wing nuts to secure the window/ladder line snugly between the two halves of the clamp.

Hanging the support

To hang the support, secure the clamp to the top part (modified dog bone insulator) with an appropriately sized plastic zip tie.  The ends of the wire radiators are inserted through the ends of the dog bone insulator and appropriately secured.  To connect the antenna, strip the ends of the transmission line and the ends of the wire radiators, and join the corresponding ends of the transmission line to the radiators.   Be sure to provide enough slack in the connections so the strain on the transmission line is at the zip tie junction and not on the wire ends.

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zip tie used to secure clamp to modified dog bone insulator

TIPS:

(1) Keep a spare balanced line support handy as a template for future use.  You will be able to lay the template over the material to be used and mark the spots for the holes and slots.

(2) To lower visibility of the finished antenna, use black THNN coated wire for the radiators, black dog bone insulators, black zip ties, and spray paint the bright metal, nylon bolts/wing nuts and clamps with camouflage dull black paint.

It won’t take you very long to make several of these supports so you will be ready for future antenna projects.