Tom -
Maybe we should say "backwards T/R relay" when we mean the reverse, of
what we did with real T/R relays when I was a kid. All of my rigs used
120VAC outboard relay coils!
FT-747 is indeed notorious for T/R timing issues, its semi-break-in so
severely truncating the leading element on CW like many rigs of its era,
such a great delay between key down and RF out that the ARRL reviewer
recommended against using the rigs semi-break-in and instead recommending
for manual T/R changeover.
Tim N3QE
On Sun, Aug 30, 2015 at 12:27 AM, Tom W8JI <w8ji@w8ji.com> wrote:
> It seems to me that a very fast operating preamp protection circuit could
>> be
>> constructed employing a good fast saturating NPN switching transistor
>> across
>> the antenna path. In receive mode the collector-base junction would have
>> substantial reverse bias and the transistor can be chosen for low
>> collector-base capacitance. With a fast switch like a 2N708 or something
>> similar the switching time will, of course, FAR outperform a relay closure
>> time.
>>
>
>
> This thread might have splintered. I was responding to this:
>
> <<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"?
>
>
> If it is a transceiver without an RX antenna point, the problem is adding
> a receive antenna to a transceiver that does not have a receive port.
>
> If it is a transceiver with an RX port, the requirement for an external
> "front end saver" and what will work depends on the antennas, the power,
> the transceiver, and the antenna spacing.
>
> A front end saver can be very simple with some radios, more complicated,
> or not needed at all.
>
> An external switch is never easy to do correctly, unless the radio has
> good TX RX switching time sequencing.
>
> I'm unclear what the application is, but a 10 mS relay is really too slow
> for either application. The sequencing issues I pointed out apply to both
> systems.
>
>
> 73 Tom
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