>
> Where did this business about high bands being USB, and low
> bands being LSB come from, anyway ?
>
Steve:
Back in the "olden days" [1955] when I first started using SSB on the ham
bands, the rigs were almost exclusively home brew and used the 'phasing'
method to eliminate the carrier and audio filtering and phasing to get one
sideband. A frequency of 9 MHz was 'optimum' for doing the RF phasing based
on the technology and test equipment easily available to hams at the time.
At the same time there was a glut of WW II surplus radios around with good
stability on some of the VFO's. In particular there was a transmitter that
covered 5-6 MHz as I recall --- and it was "cheap"!!
The 9 MHz signal was mixed with a 5 MHz signal and the result was the sum or
difference of the two frequencies 14 MHz and 4 Mhz. The mixing produced
upper sideband on the 14 MHz frequency and lower sideband on the 4 MHz
frequency. If you wanted to be a SSB "Donald Ducker" and be understood by
other SSB'ers [there were at least 7 of us in Minneapolis-St. Paul in
1955-56] you adopted the convention. Actually, your equipment established
the convention. On 75 meters SSB ops were hassled by the "Phone" ops with
the big AM signals that used 3 kHz + of bandwidth, If we operated in the
range 3994 to 4000 kHz they left us alone. Being on LSB made it a lot easier
to crowd up close to the band edge. As I recall, in 1955 the lower edge of
the 20m phone band was 14.2 MHz and being on USB meant that you could crowd
down close to 14.200 MHz.
Not too much later Collins, E.F. Johnson and B&W equipment and Heathkit and
Central Electronics kits became available and one could actually select the
sideband you wanted to use on a particular frequency. But by that time the
pattern had been set --- in part because 75 meter SSB was at the top end of
the band and 20m SSB was at the lower end of the band.
The selection of 7MHz and LSB almost certainly came from having a 9 MHz RF
SSB signal that was mixed with a subtracting frequency to get 7MHz. The 21
and 28 MHz signals were created from mixing that added to the 9 MHz
frequency.
Not really a surprise that the initial technology used established the
convention.
> Are there any technical or legal reasons why we shouldn't
> switch to USB on 40 meters ?
>
As far as I know there is no reason to select USB or LSB for any frequency
in the phone band except to avoid sending RF over the band edge. I have
heard JA's using USB on 80m [not 75m] when talking among themselves. They
are a long way from the band edge so it seems to be simply a matter of
'operators choice'. The most recent phone and CW band plan for 160m was
deliberately set the way it was because LSB is the sideband of choice on
160. [SSB carrier is supposed to be no lower than 1843 kHz to avoid RF
getting below 1840 kHz and affecting CW contacts]. I expect that if one was
on USB you could place the carrier on 1840 kHz.
Again, in the "olden days" , we would sometimes have one conversation on LSB
and another on USB with the carrier frequency being the same for both. In
this case we would be using 3 KHz of so for two conversations --- that was
in distinction to one AM conversation using the same band width. This meant
that we could have four groups using the 3994 to 4000 kHz 'unofficial' SSB
band plan.
73, Tod, K0TO
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