Our Usenet feed went away for several months and I have only recently
discovered that it is back.
I ran across the thread from this past March about Channel 1 TV and
the discussion of the BBC1 405-line service that shut down in 1985.
That brought back a bunch of memories.
Every eleven years, the Sun spews lots of charged particles in the
direction of Earth via Sun spots and Solar flares and these events
cause the ionosphere to become more reflective of higher-frequency
radio waves at various times.
I was too young to have known anything about the grand daddy of all
Solar cycles called Cycle19 in the late fifties, but I do remember all
the rest from the late sixties to the one that is just now winding
down.
In November of 1970, I heard BBC1 TV audio in the 41.5 MHZ range for
the first time here in the very center of the contiguous 48 United
States.
When Solar activity began to peak in the late seventies again, the BBC
was back during our Fall and Winter months. We would start to hear
some of the transmitters begin to fade in around 08:00 or so on a good
day. If it was a really good day, the 45-MHZ video buzz would be
there, also. The 30-50 MHZ band is used for two-way radios and paging
services in North America so the video was _NOT_ amusing to users of
that spectrum such as state police agencies. The Oklahoma Highway
Patrol which polices primarily traffic regulations on those parts of
roads that are not part of some municipality suffered withering
jamming for hours at a time by video sidebands off the 45-MHZ center
carrier as the Highway Patrol used several frequencies between 44.7
and 45.22 MHZ.
The signals usually came and went, sometimes reaching what one might
call local quality until about 1 or 2 in the afternoon. By that time,
it would be well after dark in England as we are UTC - 6 in the
Central Standard time zone.
Due to the fact that F2 Layer propagation above 30 MHZ is almost
exclusively a daytime-only phenomenon, the reception of BBC1 audio was
always a mid-morning to early afternoon experience for us. One could
hear programs aimed at children coming home from school and the
evening news and weather among other BBC programs.
A few other notes are in order. The BBC wasn't the only television
service audible during those exciting times. The French also had a
Band-I service whose audio was at 41.250 MHZ plus or minus offsets.
More often than not, both the British and French television audio were
simultaneously receivable so it is good that they weren't on the same
exact frequencies.
If you consider the distance between Oklahoma in the middle of the
continental US and the UK or France, one would expect the propagation
to be almost identical, but the path tended to favor the French
system. Either their transmitters were run at a higher power level or
the angle of propagation for the signals was more favorable to France
than to England. Also, various transmitters in the UK would be
booming in loud and clear while others were barely audible.
Both the French and British television systems made extensive use of
offsets to minimize video interference to television sets near enough
to receive signals from two transmitters at the same time such as what
would happen if someone lived in the countryside between two towns
whose Band-I transmitters were on the same channel. On the BBC1
system, the offsets appeared to be about twelve KHZ or so which meant
that with a selective AM receiver, one could tune in the London area
or the Northern Ireland transmitter.
One slightly amusing thing I remember was that one of the BBC
transmitters must have had a crystal oven that wasn't too accurate.
Its signal would whistle or heterodyne against another BBC transmitter
in another part of the country, but the pitch of the note would begin
to rise or fall through zero beat within 20 or 30 seconds. A few
minutes later, it would shift back the other direction. I am guessing
this coincided with the cycling of the thermostat and the heating and
cooling of the crystal.
The audio, by the way, for both the British and French systems was AM
or amplitude modulated. The video for at least the British system was
405 lines at 50 fields per second. A few hobiests in the US actually
cobbled together modified monochrome television sets and tuners and
were able to get scratchy images. I was told that without modification
to the video circuits, the images were reverse polarity because the
405-line system used the opposite signal levels for black and white
than do modern PAL or NTSC systems.
If one reads the book, The History of RADAR, there is an interesting
footnote to the British Band-I system. Since the UK figured that war
with Germany was imminent in the late 30's, the decision was made to
pump money in to the fledgling television equipment manufacturing
industry in England because the manufacture of TV picture tubes and
VHF transmitting and receiving equipment fit perfectly in to the still
top-secret efforts to develop useful radar systems for defense.
Martin McCormick WB5AGZ Stillwater, OK
Information Technology Division Network Operations Group
[TELECOM Digest Editor's Note: Ah yes, the grand old 1970's when we
could use our 11-meter (Citizens Band Radio) gear and 'work the skip'
all over the world on a good hot summer night, particularly if your
antenna was on top of an eight story apartment building and you had
a 'little help' downstairs at your base, hahahaha! Truely, from the
base on the first floor to the antenna on the ninth floor (the
elevator machine room on the roof) the signal did dissipate a little,
but the 5/8 wave directional antenna picked up the slack. I talked
to guys in England and France also, but more often than not guys on
the west coast in California (from my former home in Chicago) in the
early morning hours before the sun had come up out there. PAT]
Martin McCormick (martin@hydrogen.cis.okstate.edu)