Your FT-747 only has one antenna input. It does not have a second
receiver. It doesn't even have a receive-only antenna input. You cannot
transmit and receive at same time. Why do you need a "front end saver"?
It needs a preamp saver or a receiver antenna saver, not a front end saver.
"Any old relay" will not work.
Here is what the sequence is:
1.) The radio antenna port is connected through a small relay to the receive
preamp output, or to the receive antenna
2.) The key is closed
3.) At time X, after the key is closed, something tells the receiver relay
to release
4.) At the same time as the key is pressed, something tells the transmitter
to transmit. Let's call this delay time Y.
Now this is where the problem is. Many radios, especially the less expensive
radios with a single antenna, have a Y time as short as X time. Some have X
a little longer than Y, some have Y a little longer than X. There is no
guarantee without looking at the radio on ALL modes if X time is shorter
than X.
Almost all radios are not 10mS, the relay time you suggested as a limit.
Almost all radios are shorter than that, and some actually transmit while
the external relay line is held low.
There is an additional problem in a few radios. There are a few radios that
tell the relay control line to release while they are still transmitting. At
the end of a transmission, when you stop transmitting, a few radios will
actually turn the external circuits off **before** they stop putting out RF
power. I actually had to add a RF interlock in T-R relays for amplifiers
just for those radios.
Any relay used for this application should be as fast as possible. It should
NOT have a diode across the coil, because that slows the release time down
considerably. I would say the safe minimum speed for most radios would be
about a 5 mS relay transfer, including bounce. A few radios will be worse
than that, and have almost no delay. They would require a very fast relay,
or a sequencing system.
Some radios are designed so poorly they tell the external things to transfer
while RF is present. Those radios cannot be fixed without external interlock
systems.
By the way, if this does not damage the RX system, it will cause contact
spark clicks. It will also fold some radios back into SWR protect because
the relay transfers with TX RF applied.
The crummy interface timing in radios has been a nightmare ever since the
first transceiver came out, and continues to be a problem today.
73 Tom
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|