> > We've added a preamp to our local repeater and found that it > > had too much gain. It started picking up lots of garbage.
I would ask if there is such a thing as "too much gain" but there can be. Normally I would would not expect gain to be a problem in a well designed - constructed system. But I was not able to get one of Angle's 40 dB gain jobs to work without a whip and a chair. So yes I guess there can be cases of too much preamp gain. But probably not with a typical good repeater receiver preamplifier in many/most common repeater applications. You have a fly in your soup... is it gain fly or something else? > Elaborate on the term <garbage>. Intermod? Users from distant > repeaters? If intermod, it may be produced in the preamp or > receiver if not protected enough with selectivity ahead of the > preamp. A big time Amen... people forget the short distance a preamp travels to become a mixer. We really should know more about his receiver protection as requested below. > Adding attenuation ahead of the preamp very effectively > removes much of the sensitivity you've gained with the preamp. > I'd deal with that kind of garbage in other ways. A much better idea... why point a finger at the preamp when the problem might actually be part of a bigger system picture. > > Reducing the gain by about 6db seems to put it in a good > > operating place. We also found that by putting the 6db > > attenuator on the input of the preamp (right after the band > > pass can), it's real easy to add in the system. If we mount > > it after the preamp, we'll need an adapter on each side of > > the preamp to make it all connect up. Obviously it would > > be best if we could put the attenuator after the pre and have > > no adapters, but it's not convenient. > Attenuation placed ahead of the preamp is nowhere near the same > as placing it behind. Placing it behind reduces the gain of the > preamp while retaining the noise figure of the preamp. In my > experiments on 2m with an ARR preamp and Motrac receiver, up to > about 10db of attenuation after the preamp did not reduce system > sensitivity. Some of us are wondering why you need an attenuator in any position? It's not an automatic requirement... > > The question is how much trouble will be getting into by putting > > the 6db attenuator on the input side of the preamp? Would it > > still be better to put it after the preamp even though it would > > add two adapters? Forget the attenuators and the adapters and give us more information about the system. What type of preamp, bandpass cavity, duplexer and receiver are you using? Gasfet?, Phempt?, Bipolar? Nuvistor? (I said Nuvistor for some of you older radio guys) > In most cases yes. I've read about cases on this group where > attenuation ahead of the preamp is the proper thing to do, but the > reasons escape me. Depends on the equipment but the first answer I would say is for reasons of stability. Some of us actually use more than one series preamp with specific value attenuators in specialized antenna combiner systems... very "hot" high performance receiver antenna systems. cheers, s.
