Lowest NF

Rod Maupin wrote:

I am wanting to buy some receive preamps and am wondering if I'm looking at
it the correct way. Here are two examples from two different manufacturers,
both for the same band.

#1) .55dB NF, 16dB gain
#2) .9dB NF, 20dB gain

So, do you go for more gain or less NF? Since this is for weak signal
VHF/UHF work, I would say go for less NF but you can tell me what you think.

Jim - NN7K wrote:

Generally speaking, the BEST NOISE figure (factor,
ect. is what you want (best sensitivity),
however, the Gain would be a consideration, IF you had excessive coax
loss. (in this case, in
excess of 16 dB, which would be quite doubtful.
The gain of a vhf (or any amp, for that matter),
only needs to exceed your coax (and your front end
I.E. Reciever's Noise figure, so as to allow the
preamps noise figure to take precidence (establish
your floor sensitivity) I hope I stated that
correctly. And, even then, as long as THAT noise
figure is below the cold sky noise (from other
sources), then the cold sky noise (about 1.8 dB
on 2 meters) is the limiting factor on your sensitivity, before
figureing the SELECTIVITY of
your reciever (half the bandwidth, gain 6 dB,
half again, now gain another 6dB). Probably
as clear as mud, but the best I can describe it--
Jim -NN7K

Great response, Jim. Only one nit to pick, that Noise Figure and
incoming noise energy are not equivalent or even really related. Also,
a .1 dB change in NF does not imply a .1dB change in signal to noise or
received signal level. It is a non-intuitive relationship to say the least.

tom
K0TAR