Your forgetting that the front end is still broadband and wide open and
being blasted into intermod many times.
Someone would need to have grave noise level issues if noise overloaded the
front end. They would not hear much of anything with or without a loop. Look
at receiver specs for 20 kHz spacing at this link:
http://www.sherweng.com/table.html
These days not everyone is blessed by the latest gigabuck rig which still
is wide open until it hits the roofing filter which wouldnt be there if
there werent problems.....
It doesn't work that way. Outside of exceptional very-rare cases, almost
anything from the 1970's onwards would not have an issue with noise
intermod.
My first experience was with a modified R4C in the early 80's which at
least had a front end tuneable preselector. Ive since used it with the
various xcvrs here and easily hear the difference plus it can be tuned
either side of the wanted signal. Does wonders for BCB rejection also.
Unless someone has something exceptionally terrible, or has done something
wrong (like back-to-back diode limiters), a loop won't add to selectivity.
Most of the time the improvement from a loop is either from nulling noise
more than signal, or from loss of level. Loss of level can get signal levels
down on the AGC slope where the AGC does not "equalize" the volume between
noise and signals.
The Drake R4C had exceptionally poor AGC, with terrible overshoot. It helps
a great deal to not have noise pumping the AGC, but the pumping is 100%
inside-passband signal level related, and not bandwidth related.
Dropping down below the AGC threshold is actually a "trick" that makes
people think one receiver is "quieter" and "hears less noise" than another
on HF. When we hear people talking about a certain receiver being "quieter"
or "less noisy" when connected to an HF antenna system, it is virtually
always because the "quiet" receiver is falling out of AGC, causing the
no-signal background noise volume level to drop. This gives the illusion
weak signals have better S/N, and copy actually can be better if an operator
sensitive to noise between signals is not distracted by noise. This is why
attenuators and RF gain controls are useful to many people.
All good topband ops know how to put up a beverage at night.
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Topband Reflector
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