Switching Open Wire Feedline Bewtween a Transmitter and Receiver?
|Aug 8th 2011, 13:34|
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An ARRL Member has asked:
I have a home-brew 160 foot dipole fed with (8 gauge wire) ladder line, spaced 5 inches apart. My question is, where or how can I get a switch to toggle between my transmitter and receiver?
I have a Viking Ranger 2 transmitter and a Hammarlund HQ-170-AC receiver."
From Moderator Bob Allison, WB1GCM:
"While the Johnson Ranger 2 transmitter will load up into a wide range of antenna impedances, the usual method of switching between separate receiver/transmitter pairs of that rea (50-60's) is to use a coaxial relay, such as a Dow-Key Relay. The output of the relay feeds an antenna tuner, which tunes your open wire fed dipole. While this method involves using more equipment, the Dow-Key Relay also has provisions for switching receiver muting, or other control circuits for reducing the receiver's RF gain.
My method of operating TX/RX pairs is to use an SWR / Wattmeter after a TR switch. Next in line towards the antenna is a coax switch. I use the coax switch to switch between a 50 ohm dummy load and an antenna tuner. I always load up into the dummy load first. This greatly reduces QRM on and near the crowded AM windows. Once the transmitter is matched to 50 ohms, I reduce the output power to about 10 watts and tune the antenna for the appropriate ham band. With SWR as close to 1:1 as possible, I then increase the output power, if needed. You may have to tweak up the dip in the final amplifier plate current once you're on the antenna just a bit. Once the antenna is tuned for a certain frequency, I write the antenna tuner settings on a chart. I actually make up a chart for each band the antenna loads to; it makes antenna tuning a snap.
The advantage to the above method is having the ability to switch the antenna from the TX to the RX, provide muting contacts, provide a procedure for causing minimal QRM, and provide the opportunity to measure output power and antenna performance. Dow-Key Relays are found plentifully on eBay.
Of course, you asked how to switch open wire line. This can be done with an open frame relay. You will need a double pole, double throw relay with large contacts. Fair Radio Sales in Lima Ohio sells many varieties of open frame relays of these.
I have also used a good old fashioned double pole, double pole knife switch to switch window line. Make sure the plate is off when you switch one of those!"
Bob Allison, WB1GCM
ARRL, the national association for Amateur Radio™
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