On 12/21/2023 12:54 PM, Paul Dulaff via Topband wrote:
Ran a basic EZNEC model with no tower present for your 80 ft X 45 ft inverted L at 1.825 Mhz. The base impedance for this is 28.5 - j 130 ohms. The get rid of the reactance I extended the top wire an additional 20 ft so 80 X 65 ft and base impedance is 37.2 + j0. The tower is definitely influencing the inverted L.
Neighbor K6RB described a 160 vertical running next to his tower, and I don't remember him do anything to detune the tower. Yes, the tower becomes part of the antenna, but that isn't a bad thing as long as we take it into account to match it to the line. AND -- unless the line is quite long or uses lossy coax (like RG58), excess loss on 160M is quite low.
I've seen (and used) a simple equation for determining the effective diameter of a triangular tower. An NEC model should include that, as well as aluminum at the top.
For my Tee, I adopted the ancient and accepted practice of making the horizontal element long enough that the feedpoint Z became 50 +jX at the desired center of operation, and added series C at the feedpoint to equal -jX. I can do this because my Tee is strung between tall redwoods.
I ended up with C in the range of 800-900 pF. What's required here is capacitors with very low loss, but not particularly high voltage, because they're at a high current point, not a high voltage point. So their voltage rating is tied to peak value of TX power. I had a stash of low loss caps in the 2-3kV range, and used those in parallel, in a weatherproof box.
73, Jim K9YC _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
