"Radioman390" wrote

In a scan-set of utility frequencies, there are daytime freqs and night
time
freqs.

This wasn't what I was talking about.
Yes, DOD, CG and other HF users respond to different PROPAGATION
characteristics by having "day" and "night" channels. What I was referring
to
was that the noise level ("noise floor") changes over time.

Yes and my reply wasn't a response to you alone, but to the poster's
question in general.The noise floor changes often correspond to the fading
of a band when it's time is done. This is almost always true at the
extremities such as 4mhz and 20mhz. They will normally have the least
amount of background noise floor durnig their respective best propagation
times. Not a rule, just an observation.


For example on 11175 (which is a day AND night channel) I'd have to adjust
the
threshhold as the day went on, because an increase in background noise
continued until after dusk. And then around midnight, the noise floor
would
drop and world-wide signals would come in. With my R71, I'd make two or
three
adjustments a day, at least.

Hard to make a generalization about a transitional frequency. That
particular one is pretty quiet all the time, but I too have had loud and
clear copy from aircraft over Egypt and Saudi on it, and only during the
night here on the East coast. That is however also a feature of when
aircraft are most active there, during the daytime in that area.


The RF 505 NEVER needed adjustment because it "sensed" voice rather than
"sound". It, and my Scientific Radio Systems fixed-channel radios, hardly
ever
responded to lightning. The Icom would be triggered all the time.

Yes those were unique radios. But since that feature doesn't appear to be
available in modern receivers, then the tactic of applying a mixture of
pre-amp, no pre-amp and attenuation is a system that works well when a wider
band of hobbyist scanning than only clear channel (day) or clear channel
(night) is desired. The detection feature is used in transceivers today, but
it's different because they operate with squelch "off" (all static, all the
time) and still detect and hold on receipt of their address, its just not
voice they're looking for. If I were guessing, I would say the voice-detect
feature might have been abandoned because of its uncertainty in opening the
squelch for weak and barely readable voice-calls.

Jack