Rich,
Thanks for your reply; very helpful! Something similar to your shunt feeding method is what I have had in mind. Since my tower is a crankup (55 ft topped by about 22 ft of mast+antennas), I do have some misgivings about managing the wire running parallel to the tower, in the scenario where the tower sections are telescoping and releasing the tension on the wire. Maybe I should first try attaching the feed to the top of the bottom section rather than the top of the tower. The most interesting part of your installation is your grounding of most of the coax cables at the tower base, and also at the entry box. When I installed my tower in 1993 I had read a recommendation to let the cables form a gentle curve almost touching ground, which is what I did. The idea of grounding them at the tower base did not occur to me, and doesn't seem compatible with the gentle curve concept. At the house I did think about feedthrough panel that could be grounded, but I was in a hurry and didn't do the work. Some of my vhf/uhf cables are of the thick variety (much thicker than RG-213, and extra connectors cost significant money. Since then, it has been on my to-do list to go back, cut the cables, install connectors, and install a grounded plate or box. At the time when I wanted to try the tower as a 160 vertical, I didn't feel confident that doing that work would be as effective as choking off common mode currents with ferrites, and that did seem to work well as long as the cables fit in the device. After reading your post I get the impression that instead of obtaining choke(s) for the cable bundle as it is today, I should do the work of grounding the cables at the house entry and also at the tower base. For non-coax control cables I could apply my choke. You state that not all of your cables were grounded. For such a situation my first thought is that the result would depend on cable length and frequency, and if there is no problem in your situation then luck might have played a role. You did not mention trying your setup for 80. In my case, I have more need for 80m than 160, and I am still unsure if the whole metal tree might be on the big side for 80. That should be easy to determine with a model. Thanks for the details of your ground system. If I were to erect a separate vertical for 80, I would have to install a lot of radials, so doing the same around the tower is not a big deal. Like in your case, most of the radials would be on one side of the tower (directed away from the house). When I did feed the tower on 160 as an experiment years ago, I had (and still have) just two ground rods at the base, plus copper ribbon, which was meant to encircle the house, a project that was never finished, but has been on my to-do list ever since. I see a lot of similarity with your situation, but I need to put in some work to catch up! Now, supposing I get it all done, and it all works as expected, the next step is seeing the resulting 80m capability in the context of a 2-radio system. Obviously, only one radio can be on 80 at a given time, and it will use a bandpass filter but not a multiplexer. The 80m antenna (=tower) will be closer to the beam than my high-band vertical.The 80m rf will flow in very close proximity to the beam, the use of which by another radio is protected by both the multiplexer and the applicable bandpass filter. Will that be enough protection, given that the proximity situation is worse than with the high-band vertical? I am not totally sure, but it seems a reasonably good bet to me. 73, Erik K7TV ---------------------------------------------------------------------------- ---------------------------------------------------------------------------- ------------------------------------------- Erik, I've been feeding K3 RF to my house bracketed, 75 ft Rohn 25 tower for years with great success on 160 Meters. The tower is topped with HF and VHF yagis which act as a capacitive top hat. The 50 Ohm coax feedline is connected to a tap on the coil of a parallel tuned circuit. The top of the LC circuit is connected to a #16 THHN wire which runs parallel to the tower, about 18" outside one of the legs and connects to a bolt around 10 ft down from the top of the tower. At the base of the tower I simply tune the capacitor and find the best spot on the coil to tap to obtain the lowest SWR. There are 16 quarter wave radials around the tower, most of them on the tower side of the house from south to north and a few wrapping around and under the back deck toward the northeast. There are around a dozen 8 ft ground rods connected to the tower with #6 wire and the tower also is connected to a perimeter ground system surrounding the house. The shields of most, but not all, of the coax cables leaving the tower are grounded at the tower base. Before all coax and control cables enter the shack they all are grounded at the steel entry box around 30 feet from the tower base. So how does a shunt fed 75 ft tower work with a barefoot K3 feeding it? With 100 Watts I've confirmed 229 countries on the Topband and have worked 89 countries on all continents with QRP 5 Watts. You can check out the 160M QRP WAC cards at: http://www.qrz.com/db/K1HTV If you haven't tried shunt feeding your grounded tower, give it a try. You may be surprised at how well it can work on 160 Meters. 73, Rich - K1HTV ______________________________________________________________ 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
