On Dec 11, 2010, at 6:57 PM, Kelley wrote:
> Multitudes of web pages show, well, multitudes of setups for shunt feeding a
> tower. Many appear to use a couple of variable capacitors in order to adjust
> for lowest SWR. Having a limited junk box, what kind of parts should I be
> looking for to build a generic matching network?
It really depends on the shunt-feeding arrangement. Some can get by with a
single shunt capacitor, others use an omega match, still others use L or T
It really all hinges on the complex impedance that shows up at the feedpoint.
I have a short tower that I have shunt-fed for 160 and 80m. It works pretty
well. Just about any installation is unique though -- how tall is the tower,
how big are the antennas at the top, how high and how far away is the shunt
wire(s), etc. K9AY suggested I run a shunt wire as high as I could, and then
measure the impedance at the base and build a matching network to suit. And
that's what I did, basically.
You can read the entire article on my blog here:
I didn't have any sophisticated equipment to measure the impedance, so I
designed an L-network through trial and error.
If you want to run more than 100 watts or so, you are likely to need pretty
beefy components in the matching network. Capacitors with 1-4 kV ratings
recommended. Coils (if you need them) would be very large.
Bill Coleman, AA4LR, PP-ASEL Mail: firstname.lastname@example.org
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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