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]