The AM Forum

A couple of years ago I designed a simple circuit for T/R switching, that delays activating the transmitter until the ant relay and all other change-over functions have been completed, and when switching back to receive, reverses the order and kills the transmitter immediately and delays all other functions.  This has completely eliminated sparking and arcing at the antenna changeover relay contacts.  The curcuit uses nothing more than a few dc relays, some resistors, one capacitor and 4 diodes.

I remember getting several requests for the circuit on this forum.  If you take Electric Radio, the circuit is described in an article on page 27 of the November issue.

Would be interested in hearing from anyone who builds the circuit and tries it out.  It has worked perfectly for me for nearly two  years now.  Feel free to send me your questions.

Without the sequencing circuit, the first time I tried the BC1-T on the air, it literally turned a dow-key coaxial relay to charcoal.

Thanks Don,
I read it and it is well written and should save some receiver front-ends from high RF.

Since I don't have a subscription to ER I can't take advantage of your circuit you mentioned.  I understand it can easily be done with cascading relays.  But it would be helpful if anyone could confirm the order and delays for the sequencer I am sketching.  This is what I have.

State Change: Rx -> Tx

   Trigger: close PTT sw

   Order of actions:

      1) mute Rx
      2) switch T/R sw to Tx
      3) enable VFO
      4) enable RF amp
      5) enable transmit

State Change: Tx -> Rx

   Trigger: open PTT sw

   Order of actions:

      1) disable transmit
      2) wait 0.5 sec
      3) disable RF amp
      4) disable VFO
      5) switch T/R sw to Rx
      4) unmute Rx

Did I miss any things I should do or delays I need?

What if we don't?

E-mail me and I'll reply with a copy as a pdf attachment.  On the Subject line, type in "ER Relay Article."

Use this e-mail address:

(my callsign with lower case letters as below)@hotmail.com

Don, k4kyv

TNX OM. Email head your way!

Me too!

Thanks Don.

The best interlock system is the one with the least number of parts.
extra functions are just asking for failures.
but then I learned a lot from a B17 driver who learned the hard way about electric bomb arming.

By trhis, do you mean to imply that thsi would be better:

State Change: Rx -> Tx

   Trigger: close PTT sw

   Order of actions:

      1) mute Rx, switch T/R sw to Tx, enable VFO, enable RF amp, etc. (all at once)
      2) enable transmit

State Change: Tx -> Rx

   Trigger: open PTT sw

   Order of actions:

      1) disable transmit
      2) wait 0.5 sec
      3) disable RF amp, disable VFO, switch T/R sw to Rx, unmute Rx, etc. (all at once)

Don,

I built a 12V version using DIP relays.   It works very well.   I've been recommending it to anyone who will sit still but I'm not sure any of them have followed my suggestion.   I suggested it to K3TUE in a thread about T/R switching in this very forum.

Jon
 

Which is why I headed over to this thread to get the word straight from the horses mouth,  :)
since I don't have an ER subscription.

If anyone requested a copy of the article by e-mail, pse let me know if you didn't get a response within a day or two.  I have sent out several already, but sometimes Hotmail's SPAM filter makes false positives with valid messages.