[TowerTalk] Noise figure in cascaded networks

[Tom Rauch]

Hi Gerald,

> At 28 Mhz, the noise levels are approximately 25 times stronger (freq
> squared) than at 144 Mhz. This is equivalent to a level of about 12 times
> that of a 50 ohm dummy load at a minimum and a much higher level as a
> maximum. The actual amount for the higher level depends on the beamwidth
> of the antenna as the Milky Way occupies only part of the sky and is
> probably not filling the entire antenna patern with noise at h.f.. This is
> a whole 'nuther subject to be discussed seperately.

Everything you say is true for VHF and higher, but the scaling only 
works when the ionosphere is transparent. If the ionosphere 
absorbs, deflects, or blocks signals (which it certainly does at HF), 
galactic noise is reduced.

That reduction is more than compensated for by propagated and 
local noise sources, which are often the dominant noise source at 
upper HF and always the dominant noise source below 15 Mhz or 
so.

HF noise level varies widely from location to location and even over 
short periods of time for many reasons. Noise at quiet rural 
locations easily varies more than 10 dB during times of poor 
propagation to times of exceptional propagation..especially if a 
tropical path is involved. The change in level from a quiet rural 
location to a city location can be staggering, 30 dB or more noise 
in the city isn't unheard of!

On 30 MHz at the quietest rural location during the quietest time of 
the day, night, and season a five dB noise figure is generally more 
than enough at 30 MHz in a system with reasonable antenna 
efficiency and modest directivity. Of course much higher noise 
figures can be tolerated on lower frequencies.

I chuckle every time I see a 1 dB noise figure amplifier for HF, or 
fussing about a few dB system loss on receiving.

Tune to a clear frequency and swap your antenna for a dummy 
load. If you clearly hear the noise drop, you are all set.
73, Tom W8JI
w8ji@contesting.com

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