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Thread: AC operated antenna switch question....

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  1. #1

    Default AC operated antenna switch question....

    In a sk estate, this ham had a bunch of military parts. I've come across a couple of two terminal anteann switch that operate by ac. It had 110 ac on a small silver part and 3 female pl 259. Are these realys safe? Is it possible for them to malfunction and shoot ac into the antenna line? Do they introduce noise into the receiver?

    Thanks Brian kb4mng

  2. #2
    Join Date
    Feb 2004
    Location
    Bakersfield, CA
    Posts
    4,697

    Default

    Some of the older transmitters used 120VAC for the operation of a T/R relay. There were usually auxillary contacts that would be used to mute the receiver or place it in standby during transmission.
    To introduce noise into the coax the 120VAC lines would have to be the one carrying the noise. The chances are the noise would be picked up by the receivers antenna and not by the coil on the relay.
    The common household 120VAC is too low in voltage to arc any great distance. A seperation of 1/4" would be much larger then the 120VAC can arc.
    These relay can be safe if properly installed. Having no exposed contacts. The older way it used to be done was sometimes quite dangerous. The relay would be just sitting behind the equipment and wired into the transmitters circuits. This was also the day of the open cathode keying which put a sizable voltage on the CW key contacts. Many an amateur had a very rude awakening from that arrangement.
    Most installations today utilize low voltage switching, like 12VDC. Some of the linear amplifiers out there still have the older style of keying the amp. These voltages could be enough to give you a healthy jolt.
    There are modifications for those units to convert them to the safer low voltage switch. The rigs today are made mostly to key that low voltage. Some have relay contact in them that can handle just about anything on the keying circuit.
    73
    Gary

  3. #3
    Join Date
    Apr 2010
    Location
    Wilson, NC
    Posts
    583

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    Yes Brian - they are safe. Hams have been using them for decades. Before transceivers we had to switch the antenna between the transmitter and the receiver and these realys were made to do that. What you have acquired is called a Dow Key relay. Google it for more info. The Boatanchor guys use them. It probably has the shape of a "Tee" - the single SO-239 at the bottom of the Tee is the common (antenna) and the two back-to-back connectors at the top of the tee are the normally open (transmitter) and normally closed (receiver) contacts. Some have an external set of "leaf" contacts mounted outside of the housing for muting a receiver during TX. Good find!
    73
    Bill

  4. #4
    Join Date
    Jan 2002
    Location
    NYC
    Posts
    10,476

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    Yes, coax relays have been used forever and seem to last forever. If I see one at a hamfest for about $25 I buy it - and after about 10 haven't gotten a bad one. Dow-Key is a brand name for the more popular coax relays, but not all coax relays are Dow-Key. Dow-Key coax relays have a rectangular box that's painted black. The others are usually (always?) bare metal. But they're all good.

  5. #5
    Join Date
    Jan 2002
    Location
    NYC
    Posts
    10,476

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    'WB:

    The older way it used to be done was sometimes quite dangerous. The relay would be just sitting behind the equipment and wired into the transmitters circuits.
    And then there was the "novice TR switch" - an open DPDT (for balanced line) or SPDT for coax knife switch! Lots of fun!

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