Saturday, September 24, 2011



Several years back, I started looking at various directive arrays for 160 meters. Computer analysis of various configurations didn't match unsolicited reports from users. On-the-air observations also showed a great variation in practical results. In order to sort this all out, I started building "scale models" of these various configurations. I chose 40 and 30 as practical bands to build and test these arrays since that size would fit in the yard space I had available at the time.

Quite a few of the common 4-square variations were built and tried including the sloping dipoles, dipoles folded back to the support base, 1/4 wave wires sloped from the top of the structure, 1/4 wave wires bent, elevated radial systems, radials laying on top of the grass, and buried radials. As well as the physical variations, I tried different methods of phasing.

The one thing that was common to all the models built and tested was that the performance was sensitive to the radial configuration. After much test and measurement it was found that elevated radials could cause unpredictable variations in the element phasing. With 4 or less radials, the pattern could be skewed so much that the array was unusable. I did find that more radials (8 or more) on each element reduced this sensitivity dramatically. Also physical symmetry of the radial wire was very necessary. Some of the problem was traced to the radials of one element coupling to radials of another element in an unpredictable manner. They would also couple to surrounding objects. In an antenna dense environment like I had this was an unworkable situation.

To make a long story short, I wound up with buried radials producing measured results which closely tracked computer models.

A permanent working 40 meter system was then constructed using the experimental results as a guideline. The phasing system chosen was the parasitic method. This is essentially identical to the phase sloping dipoles that have been described in the ARRL Handbook and Antenna Book for many years. This system will not give quite the forward gain or as good F/B as the hard phased system but it has several advantages over the usual phasing method. First of all it is simple. Four relays and you are done! The lines to each element are cut for about 135° electrical length and the "front" element is relay selected and fed directly. The thing is 50 ohms and as broad as 40 meters is wide. Another interesting feature is that you are not required to have 1/4 wave spacing between each of the 4 elements. 0.2 wavelength spacing is great and the actual spacing is not critical as long as all four are the same. Additionally, you can add more elements to give more switchable directions. Just add additional relays and elements. The forward gain goes up a little bit as you add elements but it is not significant. A four element version spaced at 0.21 wavelength was used for several years at the old AZ QTH before it was disassembled for the move to WA. On-th-air A/B testing showed it to be consistently 6-8 dB better that the two element yagi at 80 feet that I had at the time. Of course, part of that was due to the exceptionally high conductivity earth I had at that QTH.

This is a photo of the control box. The fifth relay switches in a matching network for 30 meters. The 40 meter version works very well on 30 if you just match it! You can see that there is not much to it. No complex networks to adjust. this is just the ticket for those who are challenged by phasing networks.

After moving to WA, the array was re-assembled using 1/4 wave spacing since I had more space available. After running the array for two seasons, I decided to try phasing it by using the traditional 0/90/180 forced feed system. The method used was the Lewellen system and was designed using the information in ON4UN's fourth edition of "Low-band DX'ing". The array was computer modeled and the phasing box was assembled and pre-tuned on the bench for the predicted component values. It was then connected to the array and the tuning was touched up using an oscilloscope to verify phase and amplitude.

Here is a photo of the phasing box in place. As you can see, it is very much more complicated than the parasitic box.

In the above photo you can see the scope probes in place for the alignment operation. The phasing lines out to the individual elements are an electrical 1/4 wave. 75 ohm coax works best for this system and common RG11 has a velocity factor that makes the lines too short to reach the center! I used a good quality TV cable (Belden 9116) RG6 type. This cable is very low loss and will easily handle well over 1500 watts when used in this application. The only real problem is connecting to the cable. It has an aluminum shield so you can't solder to it. I solved the problem by obtaining a high quality brass body F connector and soldering a shield pig tail to it. I WX-proofed it by coating the open end with common black ABS pipe cement. The network is built on a piece of salvaged aluminum and enclosed in a cheap plastic food container. These containers work OK as long as you spray paint them to prevent degradation caused by sunlight. There are small holes in each of the four corners to let condensation out. The holes where the cables enter the box are sealed with "duct seal" after all the alignment is done.

Here's the setup for alignment:

The scope is a Tek 7603. RF was supplied by my old trusty FT301 at a low power output. There is a fifth 4x4 pole in the center of the array because I wanted to be able to add a fifth element in the center at a later date if I wanted to without tearing up the radial system.

Here is the element feed point:

There are about 65 1/4 wave long radials under each element. I didn't bury them, but I hope eventually they will be covered by grass (and weeds!)

The element support structure:

Each element is supported by a pressure treated 4x4 cemented into the earth. The holes were about 27 inches deep. No insulators are necessary. The element itself is fabricated from steel tubing, tapered buggy whip style, and the joints welded. Several coats of Rustoleum keep the corrosion in check. This method of fabrication results in a very strong element at a fraction of the cost of using aluminum. Also no guy lines to get in the way of the mower!

And finally, the whole array:

Preliminary results compared to the original parasitic system indicate a better F/B as expected on RX. The overall signal to noise ratio didn't seem to improve much, as was expected, because of the wide beam width and the fact that the parasitic system was adequate to begin with. Forward gain is supposed to be about 1.5 dB better, but that's pretty hard to judge on the air. At least it's no worse than it was before! Now that I have the concept verified and am experienced in the phasing method, I'll have to start working on the 160 model!

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