[TenTec] Quietness/Apparent Sensitivity

[Dr. Gerald N. Johnson, electrical engineer]

There is much value in the receiver that can hear 20 or 30 dB better
than the AVERAGE ambient noise level. You say it "hovers" and it does
exactly that but noise varies. Sometimes it gets quieter, sometimes
noisier. Amateur radio communications can be quite intermittent, once in
a year or once in a lifetime possibilities, sometimes just when the
vagaries of noise (goes down) and propagation (goes up a little), the
S/N gets to the ear strong enough to be copiable. But if the receiver
noise level sets the limit too close to the AVERAGE ambient noise, its
possible for that excess receiver noise to prevent copying that elusive
signal in that short interval when the ambient noise has dropped for an
instant. You'll never notice the missed signal with the noisy receiver.
While that may be valid criteria for a commercial circuit that demands
99.999% reliability and where the signal is always going to be way above
the noise, for amateur weak signal copy, its a bad criteria. Too much
equipment is made that way these days.

Then sometimes one can modify that ambient noise with direction
antennas.

As for oscillator phase noise, the worst case is when listening for a
weak signal with a strong signal outside the passband but close enough
the oscillator phase noise (and I generally presume the receiver local
oscillator set is represented fairly well by the ARRL's transmitted
noise test) that phase noise sideband of the oscillator will mix
artifacts (as noise) from the strong adjacent frequency signal into the
IF input hiding the weaker signal. That's one of the Tentec secrets to
hearing weak signals amongst strong signals, often ignored in rice
boxes. And why the venerable S-line still copys a lot of DX. It has low
phase noise.
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.