Hi, Bill,
Even though the beams on your monster tower have all elements insulated
from the boom, the driven elements will still add to the top loading
because their feedlines running down the tower to the ground will couple
the driven elements to the tower when on 160m. Therefore, the electrical
height of the structure will be quite a bit greater then 230 feet.
Unless your soil conductivity is "very good" or better, there will be no
advantage as far as gain is concerned over a 1/4-wave vertical as per my
findings using EZNEC (in fact, gain could be slightly less). Besides,
there are other disadvantages shunt-feeding a tower close to 1/2-wave
electrically such as very narrow 2:1 SWR bandwidth and extremely high RF
voltages across the shunt-feed capacitor(s).
I would recommend that you use Herb's (KV4FZ) suggested method or
establish the feedpoint close to 1/4-wave electrically down from the top
of the tower. Attach at least four 1/4-wave elevated radials to the
tower at that point and gamma or omega match it there.
73, de Earl, K6SE
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