2006 KØS, Kurt N. Sterba Strange Antenna Challenge

Hosted by Erik Weaver nØew and Dwayne Walker wb5plj,

Memorial Weekend, May 27, 28, 2006

    This was the third and final antenna for Saturday, and it is the antenna we used for the rest of the special event. It is a "semi-vertical" design. The ground plane is a trampoline that measures about 20-diameter, and stands about 4-feet off the earth. The primary radiating element is a 4x50-foot roll of chicken wire rolled into a tube. This tube is tied off close to the far edge of the trampoline, which serves as our feed point, and the far end of the tube is hauled up into a nearby tree. The spacing wasn't quite right, so about 10-feet of the upper end of the chicken wire tube is bent back at a slight downward angle. We estimate the main section of the tube is rising at about 30-degrees, and it is pointing toward the south.

    We added 40-feet of LMR-400 to the 20-foot section we had been using, for a total of 60-feet of LMR-400 servng as our transmission line.

    As we were taking the impedance readings with the MFJ-269 Antenna Analyzer we could tell this antenna had good potential. After a while you start to get a feel for these things, which is of course one part of the reason why we enjoy this special event!

    Here are the readings we took with my MFJ-269 Antenna Analyzer. The battery voltage measured 15.5 vdc so the measurements may still be relied upon. (The unit has a low battery voltage warning, however, it warnes you about low battery voltage well after the meter is no longer able to provide reliable readings, so you have to pay close attention to the voltage upon start up. If you leave it on for an extended period of time, recheck the battery voltage, and especially as you approach 12 vdc.)

    This was our best antenna this year. We contacted several Canadian stations, an Alaska station, and three stations in Europe!

    We didn't even have to use the manual transmatch on this antenna. The Icom-746's internal tuner tuned this on 20-meters in only a couple seconds. Nor did we experiment further with this antenna. If you take at look at the MFJ-269's measurements for 20-meters, you will see there is not much reactance present, however the pure resistance is lower than ideal.

    (Theoretically the ideal impedance for a dipole reasonably high above the earth is 72-73-Ohms, and it is half that amount for a vertical antenna (which is half of a dipole, as you may recall).)

    If we had wished to improve this antenna's feed point impedance, and bring the pure resistance closer to the ideal 36-37-Ohms, we would have needed to think of ways to improve the current flow back to the feed point along the earth, or close to the earth. We could have also tried to find a means of raising the vertical element (chicken wire) closer to beng perpendicular to the earth, or we could have shortened the chicken wire tube. Any of these changes may have improved the antenna's feed point impedance. At the same time, one would have to monitor the change to reactance, especially when you have very little RF energy caught up in reactance.

    (Reactance is resistance to current in an alternating circuit. You can think of it as energy that is performing no useful work. Instead it is caught up in a circle, or eddy, moving around quite a bit but not doing anything useful. One of the reasons this is detrimental in an antenna is the RF energy you are supplying to the antenna system is finite. Therefore, energy caught up in these loops, or eddys, is unavailable as radiation from the antenna (as radio waves).)

    Just for fun, and a change of pace, we occasionally spun through the bands listening for calling stations, and trying to break pile ups. We weren't able to break them all, but we did break our fair share. Below is our log for this antenna. There is one entry we did NOT work. I left that entry in the log because we were part of a three-station round-robin in which the other two stations worked one another with the guy in North Carolina using a tree as his antenna. Yes, a tree! I wasn't able to hear him on the tree, but he was able to be heard by a mobile station near Greenbay Wisconson. Neat stuff!