[AMRadio] Antenna Relays


JAMES HANLON knjhanlon at msn.com
Fri Jan 23 14:28:02 EST 2015


Steve, I like to operate QSK (break-in) CW and I use a relay to switch the 50 ohm antenna line coming from my transmatch and going to either my receiver or my transmitter.  I use a Western Electric mercury wetted relay, they type housed in an octal-based tube casing about the size of a metal 6V6.  You can find suitable relays on the Surplus Sales of Nebraska web site at https:// blu184.mail.live.com/?pages=compose .  The 276G relay has one SPDT contact and a 700 ohm coil that they rate for 12 to 24 vdc.  It sells for $10.  Actually the contacts are a little more complicated than just the normal SPDT switch.  Each stationary contact, on both the normally closed side and the normally open side, has two electrodes mechanically and electrically separated from one another.  The "swinger" contact has a bar which makes contact with both of the stationary contacts and shorts them together.  They were made this way, I think, to perform a logic function in the old, electromechanical telephone exchanges.  But you can just wire the two normally-open contacts together and the two normally-closed contacts together and treat them like a standard SPDT relay.   The DC rating for the 276 relay contacts is 5 amps maximum.  I have a time-sequenced keying circuit so that I don't turn on the transmitter until after the antenna relay has completed its switching cycle - I don't "hot switch" my antenna relay.  Used in this service, my 276 relay holds the output of my Heathkit SB-200 amplifier, about 600 watts, into a nominal 50 ohm load.  That works out to about 3.46 amps of RF current.  5 amps would work out to 1250 watts through the relay. The reasons why I use a mercury-wetted relay are as follows.  They are "fast" as relays go, they operate and release in less than 1 millisecond.  They have very low contact resistance, under 20 milliohms.  They have absolutely no contact "bounce" like normal, dry contact relays have.  Once the contacts close, they stay closed, once they open they stay open.  Now that I think about it, the 276 type relay actually has a short period of time during the operating cycle when the NO contacts, the swinger, and the NC contacts are all bridged together.  But this gives me no problem since I don't turn my transmitter on until after the antenna relay switching cycle is completed and I turn the transmitter off before I release the antenna relay.   One caution, the mercury wetted relays must be operated reasonably close to vertical, upright, with the octal base down.  If you turn them "upside down," the liquid mercury inside can flow down onto the contacts and the swinger and short them out.   I have no noticeable problem with an "impedance bump" with this antenna relay through 10 meters.  I operate it with both tube-type transmitters and with a Ten Tec Triton IV transceiver.   If you'd be interested in further information on my QSK keyer, I can supply a copy of an article that I wrote about it for Electric Radio Magazine, October 1996, page 20.  My keyer switches the antenna relay and generates separate, nested keying closures for the oscillator and amplifier stages of my transmitters.  So the keying sequence when I press and release my key goes:  1-Antenna relay transfer from receiver to transmitter and receiver cutback,  2- Oscillator key closure, 3-Amplifier key closure, 4-Amplifier key opening, 5-Oscillator key opening, 6-Antenna relay transfer from transmitter to receiver and receiver recovery.  This all happens fast enough so that I am able to listen to what's going on between dots and dashes to well over 30 wpm.  I'm not enough of a CW speed demon to ever have pushed it to the point where it doesn't recover between characters.    Hope this is interesting, Jim Hanlon, W8KGI 		 	   		  


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