Topband: Voltage along coax feeder

donovanf at donovanf at
Thu Mar 28 11:18:09 EDT 2013

Its very wise to use an isolation transformer -- at least at the receiver end -- so a shorted capacitor can never result in a blown radio front end! As W8JI has pointed out, be very careful to take precautions to prevent water and moisture infiltration into your coaxial cables and connectors which can result in an elevated noise floor. 

As KM1H reminded us, very inexpensive 24 volt microwave SPDT RF relays sealed in a DIP package from Electronic Goldmine are superb high isolation relays for receiving antennas. It takes only 8 milliamps to drive the coil. I've never had a relay failure. 


----- Original Message -----

From: "Tim Shoppa" <tshoppa at> 
To: "David Cole" <g3rcq at>, topband at 
Sent: Wednesday, March 27, 2013 2:32:25 PM 
Subject: Re: Topband: Voltage along coax feeder 

September 1997 QST, K9AY relay and switch box. 

Usually done with two DPDT relays (giving 4 possible permutations) rather than three SPST relays. 

Google will find you many slight variations of the remote relay and station controller boxes showing different ways to couple in the control voltage over the coax. Some use chokes others use transformers. Some use autotransformers and some use isolation transformers. I use isolation transformers at both ends of CATV 75 ohm coax. 

Tim N3QE 

-----Original Message----- 
From: Topband [mailto:topband-bounces at] On Behalf Of David Cole 
Sent: Tuesday, March 26, 2013 4:49 PM 
To: topband at 
Subject: Topband: Voltage along coax feeder 

Could anyone please point me in the direction of a circuit to feed +12 -12 and AC along the feeder to my beverages to switch 3 relays - thanks all from Dave in the UK All good topband ops know fine whiskey is a daylight beverage. 
Topband Reflector 
All good topband ops know fine whiskey is a daylight beverage. 
Topband Reflector 

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