Has anyone else looked at the input return loss on their ARR preamp? I had
one that, when put in place, was throwing off the tuning of a two-cavity
bandpass filter I had ahead of it. I swept it and found the return loss to
only be about 5 dB. I put an Angle Linear preamp in its place and all was
well. That was a few years ago.
Then, last week, a friend brought over his 440 duplexer (Wacom 678) and
asked me to tune it on the network analyzer as it had been victimized by the
golden screwdriver. He had an ARR preamp mounted to the duplexer bracket.
Once the four cavities were properly tuned, I hooked the preamp back up and
swept again, providing power to the preamp, terminating the preamp with a
good 50 ohm terminator, and sweeping at -50 dBm instead of 0. As I had
found years ago, the duplexer was severely detuned via the additional of the
ARR preamp. A sweep of the preamp alone showed about 6 dB. Yuck.
I'm starting to wonder if ARR tunes these using a noise figure meter only,
without regard to input return loss, gain, or anything else? Bad news if
you're terminating a cavity filter into one of these preamps. All of the
Angle Linears I've tried, on 2, 440, and 900, all had very good input return
loss, in some cases > 20 dB.
--- Jeff
