Jim is correct. The Phase shift is 180 degrees and is added to two equal
lengths of coax making the antennas In-Phase with the switch in one position
and Out-Of-Phase in the other position.
I took the idea from Joe Carr's book "Practical Antenna Handbook" Chapter 11
pgs 181 & 182. (figure 11-4)
I am not going to try get the last db out of the antenna but want a way to turn
off the South American Taxi Drivers that plague me. I have two small had
operated antenna switches that I am going to install in the center box, but
that will entail going out to the antenna and manually switching both to put
the phasing length in and take it out. The relay will allow me to switch it
from the desk.
Clint - W5CPT
----- Original Message -----
From: Jim Lux
To: Clint Talmadge ; towertalk@contesting.com
Sent: Sunday, April 08, 2007 11:38 AM
Subject: Re: [TowerTalk] DPDT Coaxial Relay question or alternative
At 06:42 AM 4/8/2007, Clint Talmadge wrote:
>I am building a set of Phased verticals for 30M. I was going to
>switch in a quarter wave length of coax using a DPDT coaxial relay,
>to steer the signal 90 degrees (Broad side to End fire).
First off.. a quarter wave of coax won't change the phase 90 degrees
(unless your verticals are a LONG ways apart.. wavelengths) because
of the reactive feedpoint impedances and the mutual coupling between
the antennas. The ARRL Antenna book and ON4UNs book both cover this
in some detail, and I have an excel spreadsheet that might be useful
for you. There are some "cookbook" lengths for things like resonant
1/4wave verticals that might work, but in, general, the coax lengths
are not 1/4 wave different. You might want to google for "Christman
phasing" or "K3LC"
> I went looking for the appropriate relay and found some (Surplus
> Sales) but the cost was more than I have in the two verticals and
> the cabling. Does anyone have an inexpensive source for one or an
> idea on how I might do it with a "regular" DPDT relay? I have some
> Potter Brumfield that will take the power but how much impedance
> "bump" will that insert and should I worry about it?
Almost any relay will work, assuming it can take the voltage and
current. Sure, there's some wierd impedances there, but you can
"tune those out".. adjusting the lengths of the coax, or changing the
antenna length slightly, or, just ignoring it (because it will be small).
If what you're looking for is just the ability to go from broadside
to endfire, and you're not going to obssess about the last 1/10th dB,
you can be pretty casual.
Jim, W6RMK
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