The AM Forum

I am in the process of piecing a T/R switch together.  I was planning on using a Dow Key RF Relay, since I like the shielding and position sensing contacts.  But the more I think about it, the more out of hand it seems to get, and I just thought I would ask for some voices of experience to evaluate the extremity to which I believe it is going.

I started out the design with one simple
   antenna relay:
      pole: ant
      n/c: rx
      n/o: tx

Then I was thinking about the ill effects of an accidental transmission to an open circuit, so I thought I could add a failsafe to protect against this, as well as the ability to disable it in case I want to send my tx rf to a dummy load:

   ant relay:
      pole: ant
      n/c:  rx
      n/o:  tx relay n/o

   tx relay:
      pole: tx
      n/c:  dummy load
      n/o:  ant relay n/o

Then I was thinking, I could integrate a receiver antenna shorting mute into it as well:

   rx relay:
      pole: rx
      n/c:  ant relay n/c
      n/o:  short to ground

   ant relay:
      pole: ant
      n/c:  rx relay n/c
      n/o:  tx relay n/o

   tx relay:
      pole: tx
      n/c:  dummy load
      n/o:  ant relay n/o


Note: all relays are in parallel to the same signal line, only the tx relay can be disabled to send tx rf to the dummy load

Looking for opinions.  Is this too much, or is it well reasoned?

Here's the schizmatic:

It all makes sense, but is over engineered for my tastes. I would use a manual coax switch to cut in a dummy load. You can put other antennas on this switch (multiple bands) too, if you use a multiple position switch (B&W and others make these).

As far as the RX muting contacts, many Dow-Keys come with these, in the form of external contacts. Get one of these and you are pretty much set.

Dave,
        It seems like you may be taking the long way around the block. A DK-60
Dow-Key relay has receiver antenna shorting contacts built in to protect your receiver.
They also produce a very minimal swr bump in the transmitter line. The RF part is usually pretty good, but I have had some problems with the external muting contacts not being very good. What I usually like to do is parallel a second relay (your basic 3 or 4 pole ice cube) with the same coil voltage as the dow key.

Let the Dow-Key handle the the antenna switching and the ice cube handle the rx muting and whatever else you want to happen when you transmit. If it makes you feel better you can use one set of the ice cubes contacts to inhibbit the transmitter until it pulls in. (it would have to be something in the oscilator or buffer circuit.)

just my $.02 worth ;D
                                         The Slab Bacon 

..I JUST USE SIMPLE AC RELAYS...DONE THIS FER YEARS,WERKS FB FOR ME...ESPECIALLY CONSIDERING THE COST OF DOW KEY RELAYS,I CAN BUILD A RELAY FER UNDER 10BUX...TIM....SK..

...MAYBE THIS ATTACHMENT SHUD GO INTO ONE OF THE PAGES,THAT DEALS WITH BUILDING,OR THE TEACHING SECTION...TIM....SK...

You are right, I am way overthingking this.  It is because I was originally trying to account for conditions (like TX into an open citrcuit) that will actually be prevented by the sequencer circuit I am planning on employing (typical, first mute rx, then switch ant to tx, then power vfo, then tx (this order may be wrong or incomplete, still researching), (and undo in reverse order, and all with appropriate delays)).  But this comes later.  I'll keep it simple and build appropriate complexity as I go, like I was planning.

Thanks for helping point me back on track.

Life is better when the antenna is changed over and the receiver muted before the transmitter is keyed and the reverse order is observed when the transmitter is unkeyed.   I'm a big fan of Don, K4KYV's simple and elegant relay sequencer described in a recent ER.   I built a 12V version with DIP relays for the Class D transmitter.  It's simpler and just as effective as the fancy schmanz circuit I put in the big Class E.

Jon

I am making up some switching relay boxes  for some new boatanchors.  I found that a closing  Radio Shack store is selling out parts at 50% off.  Got a bunch of 110VAC relays for future use.  If adding PTT make sure this relay  doing the PTT switching is a low volage DC relay.  The easiiest way is using a half-wave diode rectifiier fed  off of a 5 or 6.3 AC filament winding of the power transmormer filtered with a large capacity, low voltage electrolyictic capacitor.