[Boatanchors] T/R switch question _ 221122

[Richard Knoppow]

    Old fashioned non-electronic TR switching, by which I mean the use 
of a mechanical relay and not a TR tube, generally have an extra set of 
contacts for muting the receiver. An example is the TR switch built into 
the Johnson Matchbox's. These have a heavy duty antenna relay operated 
by AC from the transmitter. Many transmitters had a switched AC output 
for running an external T-R relay. The relay in the Matchbox has a set 
of independent contacts for the receiver mute. Other antenna relays are 
similar. Most receivers (but not all) require a closed contact to 
receive. This may be on the B+ or a bias line for muting. In a few cases 
the receiver requires a closed contact for muting, for instance, the 
break-in relay in the Collins 51-J series. In that case a low voltage 
for the internal receiver relay can be had by putting a wall wort of the 
right voltage on the same AC line that activates the antenna relay. When 
the AC comes on switching the antenna from receiver to transmitter this 
supply activates the receiver break-in relay muting it.
    However, most receivers have a muting arrangement that requires a 
closed contact to receive. It seems to me that requiring one to vary the 
gain controls on the receiver to transmit is extremely inconvenient.
    In my original station, which was all home brew or modified surplus, 
I switched the mute connection on the receiver and also had a small 
relay right across the antenna terminals such that it shorted the 
terminals in "transmit". When on CW I could hear my own signals by 
putting the front panel switch in "receive", keeping the receiver from 
completely muting but the shorting relay kept the residual signal small 
enough so it would not overload the receiver. Worked very well.
     A note: there is a device called a T-R switch that uses a vacuum 
tube. It is derived from a device used in radar. I am not sure I 
remember how they work well enough to explain it here but there is 
material on the web. I think the Barker and Williamson switch may be of 
this kind.

On 11/22/2023 6:28 PM, Rob Atkinson wrote:
> I'm not familiar with the B&W switch.  T/R switching varies with
> transmit power level.  If you are using a 100 w. transmitter and
> receiver the Dow Key relay shoujld be fine.  Some vintage transmitters
> have an AC connection on the back of the rig for a relay coil that
> will power the relay when you transmit.  You don't have to have a
> second set of contacts for muting.  Receivers mute in different ways.
> I guess it's nice to have that but I have never had an automatic mute
> switch here.  I just turn down the AF or RF gain on the receiver to an
> acceptable level just before I transmit.
> 
> 73
> 
> Rob
> K5UJ
>
-- 
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998