Since my back went out, I've had some time to think about may antenna
system. A brief foray into the CQ 160m contest convinced me I need to
put up some kind of 160m antenna, rather than trying to load my low 80m
dipole.
So, here's my situation - I have a 15m tower with a small tribander
(A3S). The tower is currently shunt-fed for 80m. The shunt consists of
two 12 gauge stranded wires about 11" apart, spaced about a foot away
from the tower. Shunt goes to the 42 foot level, which is the top of
the triangular portion of the tower. Matching network is about 120 pf
in series, and 25 pf across the coax. This gives a good match at 3800
kHz.
The 80m shunt works great, due to 25 1/4 wave radials. In contemplating
solutions for 160m, I'd like to take advantage of these radials without
affecting the performance of the 80m shunt. Seems like I have three
options:
A) Use the existing shunt -- build a matching network for 160m.
B) Drop another shunt arm on the other side -- with appropriate
matching network.
C) Put up an inverted L a couple of feet away from the tower, using the
same radial field.
--
A) seems like the simplest approach, but I have no idea where to start.
I've tried various combinations of larger series and shunt caps, but
without any luck. Reading part of W8JI's pages indicates that I
probably need a series inductance -- but how big an inductance?
B) is similar to A but has the advantage that I don't need to remotely
switch the 80m and 160m matching networks, I can just add a coax run.
But the matching network remains a problem.
C) Avoids the matching network problem entirely. However, I'm concerned
that the proximity of the 160m inverted L will affect the 80m shunt,
and possibly the tribander.
Any suggestions? Where do I start on the matching network problem? I'd
appreciate any assistance.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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