Topband: Inverted L Question

[Jim Brown]

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