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 <[email protected]> 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 > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > _________________ Topband Reflector Archives - http://www.contesting.com/_topband
