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.

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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.

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Figure-2.  Diagram of target antenna.

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

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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.

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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.

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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.

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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.

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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.
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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.

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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.

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