Saturday, February 11, 2012


No Traps or Tuner Required

In the March 1996 Edition of The Canadian Amateur an article by Bob Eldridge VE7BS described an interesting multi-band antenna concept that aroused my curiosity. His information came from an article published in R-F Design by Gary Breed K9AY. My first reaction to an article published by an amateur with "BS" as the suffix to his call was one of skepticism; after all it was getting close to April fools' day. However, after due consideration I decided to contact Bob in an effort to learn more about this antenna. Bob quickly dispelled my concerns and provided the backup information that his article was based on. My question at the time was could the concept be used with a vertical antenna. The article indeed stated that the approach would work with a vertical antenna although Bob cautioned that he had not heard of anyone who had actually tried building one. Since I already had a 40 meter vertical antenna, which incidentally worked quite well on 15 meters, I decided to give it a try.

The 40 meter vertical that I planned to modify for the experiment had been in use for about 10 years and was constructed using aluminum conduit as the radiating element had 16 radials of varying lengths and was guyed using nylon (not polypropylene) rope. The base of the antenna was about 15 feet above ground at the peak of my garage.

I had decided to see if I could use the concept to come up with an antenna that would work on 20 meters as well as 40 and 15 meters. The approach was really quite simple. Using a scrap CB antenna plus some other aluminum tubing that I just happened to have laying around I put together a 20 meter 1/4 wave element. The length was determined using the handbook formulae 234/f (MHz). This element was mounted alongside the 40 meter radiator with the bottom spaced about 6 inches away and the top about 12 inches away. The bottom was then connected to the counterpoise (the junction of the 16 radials).

The moment of truth had arrived. Using my ICOM 735 I proceeded to check the VSWR of the antenna on 40 and 15 meters. To my surprise the match on these bands was still good and had not changed. With more than a little bit of apprehension I then checked the 20-meter band. The VSWR was not good. It quickly became apparent that the spacing between elements was much more critical than I had anticipated. At the bottom end of the 20 meter element using a piece of threaded rod and a right angle bracket I constructed a mechanism that would allow me to precisely control the spacing between elements.  The top end spacing was not quite as critical so I used a short piece of wood and some cord to hold the element in place while measured the VWSR. The spacing top and bottom were adjusted until a 1:1 VSWR was obtained. The acid test of course was the antenna performance on the air. On air testing proved to be a pleasant surprise. Good signal reports and solid QSO's were the norm. As a precaution the VSWR was again checked on 40 and 15 meters where the match as previously measured was unchanged.

By now I was wondering just how far could I go using this approach. I decided to go a little farther and over the next several years added 10,12, 17, 20 and 30 meter elements. Using the knowledge gained while adding the 20-meter element it was a much easier job to mount and tune the additional elements. The end result was a 7 band vertical antenna that really worked without resorting to traps or an antenna tuner.

The VSWR was later checked with a MFJ 259 antenna analyzer. While not exactly the same as in my previous measurements using the ICOM 735 were still less than 2:1 on all bands. Since the 16 radials were of random length I then tuned them for minimum VSWR on 40 meters. Overall the tuning of the radials improved the VSWR on some bands slightly but I came to the conclusion that the changes were too small to be of significance. The one thought that did come to mind was that possibly one set of radials tuned to each of the different bands might be interesting.  At a later date I did add one full sized radial cut to 40 meters and ran it horizontally to a nearby tree. This improved the match slightly on 40 meters. If you are wondering where the radials are in the picture I covered all but the ends with shingles during the construction of the garage. The ends are accessible inside the garage. As you can see, this makes for a very neat and tidy looking antenna.

At one point I decided to try the same approach with a loaded 80-meter element. This didn't work. However when I connected the loaded 80-meter base to the base of the 40-meter element rather than the counterpoise as I had with the other elements I was pleasantly surprised. There was a good match on 80 meters but with a very limited bandwidth. The next step was obvious; I replaced the loaded element with a 1/4-wave length of wire, which ran vertically for about 10 feet then horizontally to nearby support about 60 feet away. Now the bandwidth had increased and signal reports indicated performance close to that of a full sized dipole. I have also used this approach with a 160 meter element.

In April 2000 I retuned the system after modifying the base ground circuitry (see Fig: 3 for details). It had been my intention to add a 6 meter element but before adding the element I checked the system with the antenna analyzer. To my surprise I found a good match. As luck would have it there was an Aurora display on April 6 th and I was able to make 3 contacts in short order using the antenna as is. Checks on 6 meters on Nov 30 2000 during an opening to South America showed that while the antenna works it is in no way equivilent to a beam under these conditions. In any event it does work and it seems that I now have an 8 band vertical antenna.

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