I am planning improvements to my 160/80/40 TX vertical to improve efficiency
and bandwidth. Today it is mounted on a 4.5 ft post in a salt marsh and has a
44 ft long radiator. I remotely switch bands by shorting out parts of the base
coil. I want to increase height to 60 ft and top load it. I can not get
beyond 60 ft. and properly guy everything.
My main question is the following: As antenna height is increased to improve
efficiency and bandwidth, I get into the situation where feedpoint impedence on
40M becomes very high. As I pass over 1/2 wavelength, but stay under 5/8
wavelength, the impedence decreases somewhat, so that on 40M EZNEC predicts
1245 - j370. Most of what I read suggests feeding such an antenna with a tapped
parallel tuned circuit, but when I calculate a standard L/C L network, the
components seem practical - 80 pf shunt across the antenna and 5.8 micro-h in
series with the feedline. According to EZNEC the bandwidth should be OK.
1. What determines the choice of a parallel curcuit over an L-network. Is
there any reason NOT to go the L network route? Maybe the losses the deciding
2. What can you suggest about the form factor and wire (copper tubing maybe)
size for the L-network coils for all three bands - especially 160M where I need
25 micro-h in the series element. I want to handle 1500W max.
Thanks for any input - Bob AF4OX
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