[TenTec] Overall system noise (was: Orion redefines the meaning of ... AGC OFF)

[Duane Grotophorst]

--- Sinisa Hristov <shristov at ptt.yu> wrote:
> The total noise power audible from the speaker is a
> sum of
>   * receiver's internal noise, plus
>   * external noise (man made, atmospheric,
> galactic...).

And once you get into the VHF/UHF spectrum you must
also consider that the overall RX system's noise
figure limit can in some cases be determined by feed
line losses alone. That is a situation that is not
often encountered on HF. In addition ordinary objects
like trees, buildings, or the ground itself can all be
sources of noise as well. Highly directional gain
antennas can be a good tool to help minimize the
effects of those kinds of external noise sources. 

> Real receivers add noise of their own, which must be
> low enough
> in order not to degrade the signal to noise ratio
> available from the antenna.

Our station's performance for those of us who play
with the VHF/UHF weak signal stuff live or die by this
concept. On the lower VHF bands it isn't too hard to
achieve an "externally noise limited" system with
today's technology. However by the time you get up to
432 Mhz it is a little tougher to achieve and the feed
line losses plus the antenna's own noise performance
become major issues to be dealt with. There is a good
reason that I have over 320lbs of coax cable in just
4-100 foot runs between the shack and the top of the
tower ;).

Duane
N9DG


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