Jim-NN7K . wrote in
t:

....
And, as a
reliatively good test as to HOW MUCH the preamp will improve your
reception, try this test: 1) attach a 50 ohm non inductive
resistor to your antenna connector, and note your NOISE LEVEL/
S-METER. 2) Now replace your dummy load with your antenna.
Did the noise /or S-METER INCREASE? If so, you are most likely
receiveing at your maximum senseativity. (IF the noise level stays the
same, you MAY make an improvement to your system, tho, HOW much,
is hard to say! Hope this helps-- Jim NN7K

It is a relatively simple matter to measure the ambient noise level. The
ambient noise level is one of the factors that is vital to understanding
the benefit of a given preamp.

A simple test along the line of what Jim describes above, but leading to
a quantitative figure for ambient noise is described at
http://www.vk1od.net/sc/anc.htm , including a calculator to perform the
calculations.

Not many hams can tell you what their ambient noise level is on 144MHz,
but lots will wax on about the relative importance of LNA gain, LNA noise
figure, and cable loss. If you don't know the ambient noise level, you
cannot answer any of the other questions.

Note that ambient noise may change by hour of day and with antenna
pointing.

Owen