I am building a feed system for a quad with wires for 5 bands. My plan
is to run 1/4 wavelength of 72 ohm coax from each element to a switch
box on the boom. I will use a single 52 ohm coax from the switch box
to the shack. This will provide a match for about 113 ohms at the
antenna.
I trimmed a piece of 72 ohm coax to 1/4 wavelength using an Autek RF1
instrument getting Z=0 at a design frequency of 21.150 Mhz with the
coax left open..
The switch box I have (from ICE) uses a few inches of copper strip to
make the switched connections between the coax connectors and relays.
I decided to find out how the switch box might change the tuning of a
quarter-wave coax transformer.
I found with the coax connected to the box, and the relay activated
to switch the coax to the common input connector, I get Z=20 ohms
with nothing else connected to the switch box. Sweeping the analyzer
to find where Z=0 yielded 18.700 Mhz.
The change is obviously due to the relay contacts and wiring inside
the switch box. My question is should I do anything about it? Should I
trim the coax to 1/4 wavelength with it attached to the box and
switched to the common coax input connector?
K5ESW
Paul
Paul.Ferguson@pobox.com
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