|[AMRadio] Re: AM and SSB history (was: Eliminate AM)|
k4kyv at hotmail.com
Tue Sep 20 20:23:29 EDT 2005
>...In Terman's book "Radio Engineering", the standard text for this subject
>in the 30s and 40s, he points out that (in 1932) "The only single side-band
>radio system in commercial operation is the long-wave transatlantic
>telephone...", and that SSB "... is extensively used in carrier-current
>communication over wire lines, but the difficulty of producing large
>amounts of single side-band power at radio frequencies and the difficulty
>of receiving the signals have prevented single side-band transmission from
>being standard practice in radio work."
Actually, amateur SSB would have been very practical in the early 30's. The
vast majority of ham operation in those days was on CW, and if a receiver
will copy CW it will copy SSB. I still have my 1935 model HRO, and when I
used to use it as my station receiver, it was stable enough to copy 20m SSB
for hours at a time. And it would copy it well with the simple BFO and
diode dectector, after I increased the size of the coupling capacitance
between the BFO and detector stage. The xtal filter wasn't the greatest,
but it wasn't great for AM, either.
As for the transmitter, a multi-conversion filter exciter, starting out with
a low frequency i.f., say below 50 kc/s and capable of generating an
acceptable SSB signal could have been built by knowledgeable amateurs of the
day, with components and test equipment readily available at that time.
Such an exciter, using L-C transformers to filter out the unwanted sideband,
would have been no more difficultt to construct than an early
Linear amplifiers capable of generating large amounts of power were already
in use before 1930. In fact, high power linears had been used for AM long
before high level class-B plate modulation was developed. The class-B audio
amplifier was invented when it occurred to someone that a linear amplifier
would amplify audio frequencies just as well as rf, if the proper
transformers were used, with pushpull stages that would allow the entire
audio cycle to be amplified. Single ended rf linears only amplify half the
cycle and depend on the flywheel effect of the rf tank circuit to fill in
the missing half. The same linear amplifiers used to generate AM would have
worked just as well on SSB. There were 50 kw broadcast stations on the air
in 1930 that used linear amplifiers.
The main problem was economics, just like cathode-ray television. Modern TV
was developed in the mid-30's too, and regular programming began in London
in about 1936. But this was in the midst of the great worldwide depression,
and most hams worldwide were lucky to be able to get on the air with a
crude, homebrew one-tube xtal oscillator on CW, using a simple regenerateve
receiver. High power transmitters, even on AM were extremely rare, as only
the well-to-do could afford one. The National HRO was a luxury receiver
only a few amateurs could afford, as well.
There was a brief article about SSB in QST sometime in the early 30's, but
the article was cut off right in the middle in mid sentence. There was
never any explanation why only part of the article appeared in the magazine,
and the follow-up articles that were promised were never mentioned again.
Reportedly, there were a few experimental amateur SSB stations on the air in
the mid-30's. But as in the case of TV, the technology existed but WW2 came
along and placed development on the back burner until after the war, when
both technoligies took off.
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