Years ago, after putting up a heavy-duty crank-up tower with several antennas on a tall mast at the top, I was interested in loading up the whole metal tree for 160 and/or 80. However, I didn't like the prospect of transmitted power getting back into the shack via the existing feedlines, causing all kinds of problems including losses. From a surplus vendor I obtained four square slabs of ferrite (no spec's) and taped them together to form a box-shaped common mode choke around the existing feedlines (and rotor control cable etc). I didn't have radials at the tower base, but a couple of long and wide copper strips buried and connected to ground rods to which the tower was grounded. I opened up the tower ground connection and I had a feed point. Finding resonance was not as easy as using my antenna analyzer. The signal from the analyzer was overwhelmed by picked-up broadcast signals, rendering the analyzer unuseable. I got by using a custom measurement setup. It turned out that the entire metal tree resonated in the broadcast band and was inductive at 160. I tuned it with a serial variable capacitor, and found the antenna worked very well on 160. However, I didn't continue using it, as I didn't feel safe not to have the tower grounded. I pondered schemes to add some kind of gamma-like matching device, but never got around to it. Also I never tried it on 80, but I suspect it would not been ideal for low angle radiation.
Later I added more antennas to the mast, and with the added cables, the whole bundle would no longer fit in the makeshift ferrite choke. Out of curiosity I once again tried ungrounding the tower to check on its characteristics as a vertical antenna. I could no longer find the resonance I had seen and used before. Apparently, the ferrite choke had been a crucial part of the scheme. At this point I still don't have an antenna for 160 or 80. (I did try an inverted vee off the tower for 80, but it caused terrible de-tuning of the 40m part of my beam on the tower, so I gave up on that. Maybe I should try a sloper.) I am thinking of putting up a dedicated vertical, but on my small lot it would couple to the tower. Perhaps it would be better to give the tower another look as my low-band vertical? My source of ferrite slabs dried up years ago. I wonder if anyone else on the list has used a similar approach and found a good way to choke off RF on a bundle of feedlines? Individual chokes don't seem very attractive to me as I have many cables, but if one has to go that route then it would make sense to look very carefully at the choice of chokes. I would also be interested in knowing about others' experiences with feed systems that leave the tower grounded. Thanks in advance for any useful ideas! 73, Erik K7TV ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
