Bandpass tuning in the 75A4 was expensive to produce. The 75A4 and KWS-1
were intended only for the elite amateur, and priced accordingly. That
bandpass tuning was different than the band pass tuning on my Corsair.
There's only one filter in the A4 in use at a time, and the bandpass
tuning required ganging the BFO PTO and the main PTO and having both
have the same exact tuning rate so the received spectrum could be moved
with respect to zero beat. It never varied the width of the pass band,
just its edges simultaneously.
Bandpass tuning wasn't in the S-line because it was a "low cost" radio
for the masses and sold in large quantity. Art didn't especially like
that and the next radio (which took a long time and many proposals) was
to be gold plated again and only for the elite (e.g. rich) amateur.
The 2.1 KHz Collins mechanical filter was good for its day, but has a
lot of ripple and a lot of phase change across the pass band that made
it ring really bad on static and power line noise and probably didn't
help the audio though ears are supposed to not recognize phase changes
of harmonics.
The rules require limiting the bandwidth to that required for
communications. Interference or the lack thereof is not mentioned in
that rule.
As for 2.1 as the optimum bandwidth, its probably discussed in the book
on SSB by Papenfuse, Shoeneke and Bruene. I have a couple copies but
have been busy with other things today and haven't taken time to scan
either for that. The Collins yellow book, Principles of SSB, talks about
bandwidth but never gives a number. I find with my Corsair and its pair
of 2.4 KHz filters that I tend to slide the band pass tuning so I roll
off some of the highs, probably 300 or 400 Hz to get it down to about 2
KHz bandwidth on a crowded band. Always seems to help. And I'm doing
that with my Yaesu VHF multiband multimodes and adding a speaker low
pass filter to make signals have a bit better signal to noise ratio.
Vague rules are good. They allow room for experimentation and
discussion. Tight and very specific rules preclude experimentation. They
may still be discussed, but once tight rules are set, discussion has
little effect on their enforcement.
There is some bandwidth limiting of AM stations because of their 10 KHz
spacings. Ends up they are more used for voice than for fine music,
probably partly because of that.
For that particular voice of Marv Albert, 1500 HZ chosen right was
enough, another higher pitched voice might have needed it shifted a
little higher, a very bassy voice might have needed it shifted a lot
lower.
It is easy to use lots of spectrum, just run a class C final on a SSB
radio. Or turn up the drive all the way with a linear PA. Amplifier
distortion as well as oscillator phase noise can add greatly to the
occupied spectrum of a signal and the cleanest signal should take the
least spectrum, given the same bandwidth audio source and filtering. In
our unchannelized communications, using less bandwidth per QSO allows
more QSOs per band.
In many years of experimenting I've found that one can overlap SSB
signals better if they are opposite side bands than if they are the
same. It has to do with the lowered spectral content of the high
frequency part of each transmitted signal and the frequency inversion of
the monkey chatter is less annoying than splatter or guttural sounds
tuned off 1.5 KHz. For close to 30 years I worked my dad, K0CPN on 75
meters USB doing just that. It works. Confuses a few listeners, often
kept our QSO fairly private. Occasionally someone would switch over and
then accuse us of breaking rules. But we weren't so my response was, "I
feel right side up."
Try using the opposite sideband in the thick of QRM. Its a technique
that works and makes fuller use of the available spectrum.
73, Jerry, K0CQ
--
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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