[Boatanchors] Electronic TR Switch?

[Garey Barrell]

The best "electronic" T-R switch I ever used was the W3DM design from 
June 1957 QST.  The same unit was later sold commercially as the Lynmar 
TRS-1.  It was a simple 6AH6 with a toroidal (rare in those days) 
"broadband" output transformer to drive the receiver.  The switch was 
designed to be mounted INSIDE the transmitter, (plenty of room in there 
back then,) and the grid of the tube was coupled to the INPUT side of 
the transmitter Pi-net via a very small capacitor, typically 1-3 pF.   
This method of connection was vastly superior to the more typical 
connection to the transmitter output connector.   Of course it DID 
require that you were not afraid to open the cabinet of your transmitter 
to install it.   It obtained filament and B+ voltage from the 
transmitter, so no additional power supply was needed.

The advantages of this method were that 1, the grid was tapped at a MUCH 
higher impedance point, allowing much lighter coupling; 2, any harmonics 
generated by the grid of the 6AH6 being driven overdriven had to get 
past the Pi-net of the transmitter and any other LP filter applied 
externally; and 3, some gain on received signals was provided by the 
step-UP of the "reverse" Pi-net.  It did require that the Pi-net be 
tuned at least close to the receive frequency, usually noticed when the 
band you just switched to was "dead".   A quick peaking of the received 
signal with the transmitter controls soon restored order, and left the 
transmitter at least near where it needed to be for transmitting.

I used one  _heavily_  for a couple of years, (you can't beat full QSK 
for handling traffic,) in a suburban area with a TV antenna on every 
house with not a single TVI complaint.   Transmitter was a Globe Chief 
90 with an HQ-129X receiver to a "long wire" (untuned) antenna on 80 
through 10 M.

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
<http://hr99.home.mindspring.com/R-4C_Servicez/>

StephenTetorka at cs.com wrote:

>Good Observation about TVI.
>
>The 1970 ARRL handbook - which does not have my solid state version - does 
>clearly say " the preceding t.r. switches generate harmonics when their grid 
>circuits are driven positive, and these harmonics can cause TVI...."
>
>In order to work, the grid are driven positive when the transmitter is 'on'!!
>
>R,
>Steve
>
>  
>