Gary, K9AY wrote:
"One easy way is to use two wires spaced the width of the tower face. On
a typical 60-80 foot tower with beams, go all the way to the top."
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Coincidentally, yesterday I modeled SM4CAN's shunt-fed tower which uses
this feed method, but with the shunt wires attached at the 50-ohm "sweet"
spot. Kent was concerned why the 50-ohm tap point was so high on the
tower. Obviously his shunt feed system works well, because he has one of
the better topband signals from Europe.
His tower is 24m high with a Telrex 6-el 20m beam for top loading. The
structure's resonant frequency is about 1557 kHz. His tap point for the
two #8 shunt wires is at 23m and the wires are spaced 0.3m from the
tower. It takes about 500 pF to tune out the reactance at the base of
the shunt feed.
Modeling his tower, I came up with almost exactly the same figures as he
sees in the real world, confirming the correct 50-ohm tap point is at
23m, with 0.3m spacing. EZNEC computed the correct capacitor value at
557 pF.
Out of curiosity, I modeled this same structure, but used only one #8
shunt feed wire. The 50-ohm tap point dropped down to 21m with 0.25m
spacing. The required "gamma" capacitor value changed to 265 pF.
The 2:1 SWR bandwidth (assuming zero-loss coax) for the two-wire feed was
from 1769 kHz to 1891 kHz. With the single wire shunt feed, the 2:1 SWR
bandwidth was 1781 kHz to 1876 kHz, so the two-wire shunt feed has a
better bandwidth by 27 kHz..
K9AY's suggestion to attach the shunt feed to the top of the tower and
use an L- or T-network to tune the feedpoint to 50 + j0 ohms will work
fine, but there's nothing much simpler and trouble-free than a single
series ("gamma") capacitor if a little bit of extra effort is made to
find the 50-ohm attachment point.
73, de Earl, K6SE
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