Electronic T/R switch

Garey Barrell k4oah at MINDSPRING.COM
Wed Mar 7 10:52:12 EST 2001

Bill -

I used an electronic T/R switch for several years in the 60's.  The one I
used was designed to be mounted inside the transmitter, getting filament and
plate power for the 6AH6 from the transmitter supply, and tapping off the
PLATE side of the Pi-net through a very small coupling capacitor.  The
suggested coupling cap was a piece of coax.  It's claim to fame was that it
had one of those "new-fangled" toroid transformers and was broadbanded, so
no seperate band switch as in the B&W unit.  Also, by putting it inside the
transmitter and behind the pi-net, the TVI problem was also greatly reduced.
I lived in a non-cable area at the time and had no problems with TVI.

The unit was very small, about 1.5" square and maybe 2.5" high, with the
tube sitting on top.  There was a "hole" in the top of the box, rather than
a connector,to insert the stripped end of a piece of RG-58 to run to the
station receiver.  Sort of like the internal coax connectors in the TR-7.

The disadvantages were that the transmitter pi-net had to be tuned when you
were just roaming the bands, sort of like a receive antenna tuner.  And of
course the shot noise from the final if it was not biased to cut-off in

I used mine in my Globe Chief 90, so didn't have to worry about noise from
mercury vapor rectifiers! :-)

This is all from memory, but I believe this switch was manufactured by a W3,
maybe W3DM??.  I also believe it was written up as a construction project in
QST, and the toroidal transformer was offered by W3??.  I know that at least
the schematic was in QST.  The basic circuit was the same as the Johnson T/R
switch, i.e., a tube pre-amp in front of the receiver that was biased off by
RF.  I'm pretty sure it was rated for up to a kw, with smaller coupling caps
as the power went up.

I have a couple of the Johnson T/R switches, and the key to using them is to
mount them right at the transmitter, (I used he-to-he UHF connectors,) and
to sure they get plenty of air around them as they _really_ get HOT.
(Although I've never had one fail or show signs of being "overheated".)

73, Garey - K4OAH

----- Original Message -----
Sent: Wednesday, March 07, 2001 9:22 AM
Subject: Electronic T/R switch

> In the 1960's timeframe of the ARRL handbook was a electronic T/R circuit.
> Johnson made one for a
> while and B&W made a copy of the handbook circuit.  Many complained that
> was a
> TVI generator and a dummy load  for the receiver.
> I have been looking at the circuit and thinking of how to modify it to
give a
> bit more amplification for the receiver and better filtering.
> Oh yea  could even sand state it but all the boatanchors would go belly up
> (GRIN).
> Anyone got any comments?
> Bill K0QK

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