At 2/9/2007 13:32, you wrote: >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.
Preamps are usually designed/tuned for lowest possible NF, even at the expense of some gain, & in this case, input match. Another reason why it's best to tune everything as a system. If you really want to isolate the preamp from the duplexer, you need to put an isolator ahead of it. Bob NO6B
