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″.
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Figure-2 shows the predicted 3-D radiation pattern of the IL antenna at 50.4 MHz.
Figure-3 shows the model predicted azimuth radiation pattern of the IL antenna at 50.4 MHz.
Figure-4 shows the model predicted elevation radiation pattern at 50.4 MHz.
Figure-5 is a bar graph of the model predicted SWR for the frequency range 50.1 – 50.6 MHz.
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.
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.
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. A simple L network constructed with components that provide the required range of impedance matching capability will suffice.
For the test antenna described in this article, a simple L 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 tuning with twisted pair door bell wire.
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.
Take note of the significant decrease in loss (see Figure 7b) by substituting Belden 9913-7F for RG-8X.
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.
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′.
Figure-9 shows the predicted 3-D radiation pattern of the IL antenna 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.
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′.