In article ,
"Pete KE9OA" wrote:

http://www.conknet.com/~b_mobile/NoiseStuff.html This link should
clarify things.

In summary, the theoretical value of noise is: [E^2 - tRkTB] where :

E is the RMS value of noise, R is in ohms T is in absolute
temperature, B is the bandwidth in Hz k is Boltzmans constant (k =
1.38 x 10^-23 joules/degree K)

At room temperature (290 degrees K) with a 50 ohm system with a 1kHz
bandwidth, this converts to a noise level of -144dBm. 15dB excess
noise from the environment converts to a level of (-144 + 15) or
-129dBm. This translates to a level of slightly less than 0.1uV. This
is the noise level that you might find in a very quiet rural area. In
my area just north of Chicago, the excess noise is around 4uV at
30MHz, increasing to higher levels on the lower frequencies. At
frequencies below 100MHz, the predominant noise is man-made noise and
atmospheric noise. Above 100MHz, the noise density decreases sharply
with galactic noise being the dominating factor. This is the reason
that a receiver with a 12dB NF is acceptable for reception below
30MHz. When you listen to receivers that seem to be quieter than
others on the same signal, this can be because of noise in the audio
amplifier or because of excess phase noise in the LO (synthesizer).
Excess phase noise manifests itself in a couple of ways; first of
all, it can cause that "white noise" that is sometimes experiences on
signals, but more often it is because of signal feedaround in the
I.F. filter chain. The second way that excess phase noise manifests
itself is by the rising of the system noise floor when tuning very
close in frequency to a very strong adjacent signal. Two ways that I
know of to clean up this problem a use a selective crystal filter
(500Hz BW) at the Phase Detector's reference oscillator input in the
synthesizer. The second way (mainly to improve close-in IP3) is to
use a multi-pole filter with good shape factor as the roofing filter.
The only problem here is when you use a multi-loop synthesizer to
tune the minor loop at the 2nd LO frequency. The Racal 6790/GM
addresses this problem by using a single loop Fractional N
synthesizer. That is why they can get away with using that (8 or
10-poles?) filter at the 45MHz 1st I.F. As far as receiving higher
frequencies, the benefit of a system NF of 2 to 3dB becomes apparent.
There are GaAsFET devices that have a NF of a fraction of a dB,but
those are more useful for EME communications, etc. Anyway, I hope
this clarifies things.

Snip

-144dBm NF

Well, that's the way it is, even for expensive spectrum analyzers and
other test equipment.

I see the main problem as reducing the nearby man made noise since you
can't do much about propagation other than using directional antenna.
The directional antenna is difficult for most people to achieve with
full size antennas. The best thing to do is use an antenna that is not
sensitive to the local man made noise. Lower frequencies will always be
a bigger problem for the reasons you noted.

--
Telamon
Ventura, California