Hi John, it looks as though your JFETs in the preamp were not running in a
linear part of their characteristic. Devices of this vintage have a wide
range of threshold voltages and really need to be checked or the drain
resistor value tweaked. If the drain volts were only 1 volt then the
operating conditions for the emitter follower are also a bit dodgy. It also
occurs to me that Tony's preamp had a pair of diodes across the loop to
protect the preamp from the TX RF. This seems like more trouble. I have not
tried it but I would prefer to protect the preamp fets by shorting the loop
out with a small relay. This could be remotely switched by cutting the power
(assuming it is remotely powered up the coax) to the preamp on TX.
I was involved many years ago in protecting submerged repeater amplifiers
from over-voltage pulses with diodes. We uses a pair of diodes in series (in
'anti-parallel' with another series pair). The reason being that it is the
change in capacity with voltage that generates the products, and this is an
inverse square law. So 2 diodes in series reduce the capacity swing by four
times.
I must admit I have never been a fan of op-amps at frequencies above audio
maybe I am still living in the world of the 741 !! ) I would think that an
op-amp would need to have a transition frequency (unity gain-bandwidth
product ) of at least 15MHz to be any good at 136kHz. This is probably OK
now, but they also used to be terribly noisy (by RF standards). I am afraid
I cynically say that modern engineers would rather use 15 op-amps where one
transistor would probably do the job just as well (probably because they
dont have to work out the biassing of an op-amp !). Most things will
amplify....I've seen BC107s used at 2metres !! (they were not very good! but
they worked) ...but has anyone any real experience (and measurements)
showing op-amps have a better performance than a discrete transistor at
136kHz. (that should start the flames going)
Cheers de Alan G3NYK
[email protected]
|
