> > > You might need to add a several DB attenuator
> > > between
> > > the pre amp and the receiver to keep from over
> > > driving
> > > the front end.
>
> Not if you use a good receiver, or not use a preamp with
> too much gain.
>
> Bob NO6B
What defines "too much gain" can vary wildly. One trick I learned in building
transverters for the microwave bands, and one I now apply to VHF/UHF preamps is
to check the overall noise figure of the system as a whole. You'd be surprised
at what just a few db too much gain can do, and it doesn't necessarily show up
with a quick sensitivity check.
A preamp can be placed in front of a receiver and, yeah, now the receiver is
more sensitive. But if it's a .5 DB NF preamp, and you're not careful, your
system noise figure can end up going from, say, 6 db for the barefoot receiver,
to 4 db with the preamp - an improvement to be sure, but not nearly as good as
the preamp may be capable of. If that preamp is driving the receiver front end
even just a little bit into compression, you've lost a lot of potential. Even
with a good receiver. Carefully balancing preamp gain with attenuation on the
output can be extremely useful.
Not everyone has a noise figure meter, though, and measuring NF on an FM
receiver is a pain in the neck. A sinadder can be used to the same effect, even
if the actual noise figure isn't known. It can be interesting to observe
insertion of a few db of pad between the preamp and a receiver, and watch the
sinad sensitivity of a receiver improve by a few tenths of a microvolt.
