Topband: KD9SV-OK1RR relays ???

Tom W8JI w8ji at w8ji.com
Sat Aug 29 18:14:04 EDT 2015


>  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 



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