FWIW, for 160 I used my old 97-foot Rohn 25 tower, with 2 tribanders and
a 40M 2-el on it, shunt fed at about 50 feet. I had a pair of 300 uf
variable caps at the bottom, one in series with the feed and the other
in parallel. It proved to be easy to tune to low SWR once I discovered
that the local 500-watt radio station on 1500+ KHz went to low power at
6 pm, so my MFJ analyzer didn't freak out from the received signal.
73, Pete N4ZR
On 12/21/2023 4:55 PM, Jim Brown wrote:
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
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