On Sun, 17 Sep 2006 18:31:00 -0700, "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.

Rod,

The context in which you intend to employ the preamp is needed to
accurately answer your question.

The industry uses a single metric for fully capture the receive
performance of the entire station, it is the Gain to Noise Temperature
ratio, written as G/T. S/N is directly related to G/T, for every one
dB improvement in G/T, you get exactly 1dB improvement in G/T.

Calculating G/T brings to book the NF of your "receiver", transmission
line etc losses, feed system losses, the gain and noise of your
preamp, and the external noise.

I have just explored options for improving my 144MHz station after
polite on-air advice suggested I needed better feedline and a preamp.
In fact, they are quite costly for little improvement, compared to
stacking another beam. See http://www.vk1od.net/gt/144.htm for the
write up and links to a description of the G/T method and calculation
tool.

A further aspect to keep in mind is that it is easy to build a cheap
preamp that exhibits very low NF and good gain in a shielded room. If
the preamp has poor IMD, it might actually degrade the system
performance depending on your own RF environment. Minimising noise
from IMD means better active devices and better front end selectivity,
both of which are high cost items.

So, for a start, read the page, follow the link to the second article,
download the spreadsheet and start populating the model with your own
scenario.

I would encourage a "try before you buy" evaluation of a preamp to see
what it does in your situation, the IMD noise cannot be simply implied
from published specs.

Owen
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