On Mon, 29 Dec 2008 20:32:58 -0800, Roy Lewallen
wrote:

It's purely because of where the dominant noise comes from, more
specifically whether it gets into the system before or after the
antenna. Atmospheric noise gets greater as you go down in frequency. At
VHF and above, it's less than receiver noise, so receiver noise
dominates and masks whatever atmospheric noise there might be. At HF and
below, it's usually greater than receiver noise, so atmospheric noise
masks the receiver noise. Obviously there's no precise line, so
somewhere typically near the upper end of HF either one might dominate,
depending on conditions, antenna, and receiver.
(...)
Roy Lewallen, W7EL

This might help:
http://en.wikipedia.org/wiki/Atmospheric_noise

If you extend the red line showing man made noise, at greater than
about 30Mhz, the man made noise (ignition noise, motor noise, etc)
predominates over atmospheric noise.


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