Brian Oakley wrote:

Well said Roy, however, can you explain why this is not so at VHF and
above? I would think that would have to do more with mode than with
antenna/propagation. When I turn down the squelch on my vhf rx i get
lots of noise. Let me know.
TIA,
B

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.

The noise you get from your VHF radio when you turn down the squelch is
receiver noise. You can prove it by disconnecting the antenna and
noticing that the noise doesn't change. Disconnect the antenna from an
HF receiver and the noise will drop, because it's coming from the other
side of the antenna.

Roy Lewallen, W7EL