Short spaced TV stations...

[Colly0410]

A quote from Wikipedia about WESH TV channel 2 Orlando... "The stations signal was short spaced to prevent interference with WTHS channel 2 in Miami."... I'm trying to work out what 'short spacing' means & how it prevents co-channel interference?

[old_tv_nut]

I think this article was poorly worded. If you look up the definition of "short-spaced" it means to reduce the power or increase directivity to protect another station on the same channel at less than normal distance.

The article says:
"On that day, the station activated a new 1,000-foot (305 m) transmitter tower in Orange City. The tower was located farther north than the other major Orlando stations' transmitters because of Federal Communications Commission (FCC) rules at the time that required a station's transmitter to be located within 15 miles (24 km) of its city of license. The station's signal was short-spaced to prevent interference with non-commercial educational station WTHS-TV (channel 2, now PBS member station WPBT) in Miami."

I think it should say:
"On that day, the station activated a new 1,000-foot (305 m) transmitter tower in Orange City. The tower was located farther north than the other major Orlando stations' transmitters to prevent interference with non-commercial educational station WTHS-TV (channel 2, now PBS member station WPBT) in Miami. The tower had previously been located closer to Orlando because Federal Communications Commission (FCC) rules at the time that required a station's transmitter to be located within 15 miles (24 km) of its city of license. The shorter distance to Miami had required the signal to be short-spaced."

see:
https://en.wikipedia.org/wiki/City_of_license

[old_tv_nut]

http://rogersimmons.com/wesh-channel-2/

this site says the first transmitter was furhther north, in Holly Hill (near Daytona Beach), so maybe it was short-spaced from the time it moved to Orange City, and the location in Orange City rather than even further south was part of the short-spacing.

[old_tv_nut]

"On that day, the station activated a new 1,000-foot (305 m) transmitter tower in Orange City. The tower was located farther north than the other major Orlando stations' transmitters because of Federal Communications Commission (FCC) rules at the time that required a station's transmitter to be located within 15 miles (24 km) of its city of license." Still doesn't make sense, because Orange City is 22 miles from Daytona Beach.

Also, any of the locations are more than 200 miles from Miami.

[Colly0410]

In UK when they reused a low band channel that was near'ish to a high power vertically polarised transmitter (all high power low band TX's were vertically polarised) they'd use horizontal polarisation to try & cut down co-channel interference, then the next co-channel TX would use vertical & so on. AFAIK all american TX's use/used horizontal polarisation...

[zeno]

In the good old NTSC days WESH was the most common DX station
to be picked up in New England. Always in there with the slightest
sporatic-E. No other did this, it would be random from certain areas.

73 Zeno
LFOD !

[old_tv_nut]

Originally Posted by Colly0410

In UK when they reused a low band channel that was near'ish to a high power vertically polarised transmitter (all high power low band TX's were vertically polarised) they'd use horizontal polarisation to try & cut down co-channel interference, then the next co-channel TX would use vertical & so on. AFAIK all american TX's use/used horizontal polarisation...

You are correct. Either pure horizontal or elliptical.

[zeno]

In the Boston area we had 2 OTA pay TV stations in the late 70's
Ch 27 in Worcester ( Wistah in NE speak) used the Zenith system &
Ch 68 from Boston ( B-T system ??). IIRC Ch 68 used circular polarization. On
the biggest UHF antennas it was always very snowy even at +- 30 miles.
Both installed cut to freq antenna & the signal from CH 68 was rock solid.
Just before OTA pay died "someone" sold off boxes for a few hundred $$ each.
It lasted about a year then back to unscrambled NTSC. Gee I wonder who
was selling them

73 Zeno
LFOD !

[Colly0410]

I've actually experienced 'short spacing.' When Channel Five (Channel Five is the station name, it doesn't actually transmit on channel 5) started in 1997 in UK the only spare channel in the UHF (the VHF band was de facto not used since the 1970's) in most parts of the country was 37, so most high power stations used 37. I lived about half way between two Channel Five channel 37 TX's (Lichfield near Birmingham & Emely Moor near Leeds) when I lived in Kirkby-in-Ashfield near Mansfield, my antenna was pointed at Lichfield but was wiped out by interference from Emley Moor with severe patterning & beating effects making it unwatchable. I moved to Hucknall near Nottingham & received a good picture when the local'ish Waltham TX started TXing Channel Five on channel 35. I now get perfect Channel Five pictures from Waltham via over the air Freeview & Virgin free cable. (If you use Virgin media for broadband & landline phone you get free cable TV.)

[NowhereMan 1966]

I know back in 1948/49, WDTV of the Dumont TV network went on the air on channel 3. They were later shunted down to channel 2 in the early 1950's because Cleveland, OH had WEWS in channel 3 and there was concern that in the middle of the two cities, you would have co-channel interference. Later on, WDTV became KDKA-TV when Dumont went under and Westinghouse bought the station. There was a channel 6 in Beckley, WV that ran at halff maximum power because there was WJAC in Johnstown, PA on channel 6 also. Maybe not exactly co-channel interference, but when WTAE-TV, channel 4 in Pittsburgh signed off at night, there were times WRC-TV from Washington DC came in.

[KentTeffeteller]

And to add to this, co channel interference in that era was a very real problem, especially for viewers in between the two affected station signal. Those viewers had more interference issues than people living closer to either station. Short spacing is not good band planning, the recent years FM band debacle is a major reason why. Stations on a frequency need 50-100 miles or even a bit more depending on ERP, and also there needs to be reasonable interference protection. I think directional antennas are going to be a reality in practical terms for the FM broadcast band to have a chance at being usable. Especially for listeners who live between the two signals equally apart.

[NowhereMan 1966]

Originally Posted by KentTeffeteller

And to add to this, co channel interference in that era was a very real problem, especially for viewers in between the two affected station signal. Those viewers had more interference issues than people living closer to either station. Short spacing is not good band planning, the recent years FM band debacle is a major reason why. Stations on a frequency need 50-100 miles or even a bit more depending on ERP, and also there needs to be reasonable interference protection. I think directional antennas are going to be a reality in practical terms for the FM broadcast band to have a chance at being usable. Especially for listeners who live between the two signals equally apart.

I know, I live 8 miles north of Wheeling, WV on the Ohio side of the Ohio River and one station that plays a lot of 70's and 80's music, I love the 80's, is on 105.5 Mc, WUKL Bethlehem, WV, but when I drive to work in the Pittsburgh area, I get about 4 or 5 miles into PA and the translator they have on 105.5 Mc in Pittsburgh takes over and it is from 104.3 Mc, a Froggy country music station.

When Mom and I went to California in 1987 and drove from Frisco to LA on I-5 in the San Jouquin Valley, there were many FM stations going back and forth on the capture effect, the FM band was a pain in the butt to listen to.

[Colly0410]

In Britain a sort of solution to co-channel interference was frequency offsetting, but this only worked well when the interfering signal used the same system i.e. 405 lines. Problem was a lot of the interference was from foreign stations using 625 or 819 lines which used varying bandwidths, most European 625 systems used negative video modulation & FM sound, but Belgium had some TX's that used positive video & AM sound, French 819 used positive video & AM sound. The British TV set working on 405 lines, positive video & AM sound was totally confused when these signals came in & you'd get a horrible broken up picture & a warbling distorted sound. European viewers I presume had the same sort of problems when 405 line signals came bounding in..

[ppppenguin]

In the UK precision offset was used on the UHF 625 analogue TV network to minimise co-channel interference: https://en.wikipedia.org/wiki/Frequency_offset The UK UHF network was meticulously planned to allow for 4 national channels covering 98%+ of the population. Channel 5 was shoehorned in much later and was always a bodge.

I'm not convinced we used precision offset on VHF in the days of 405 transmissions. I could easily be proven wrong if somebody finds a BBC or other document saying that it was tried.

Frequency planning in the UK has always had to be co-ordinated with France, Belgium and Holland. Again back in the says of 405 lines on VHF, viewers on the south coast of England were often troubled by interference from French stations. Usually in periods of settled weather when the troposphere would happily propagate Band I frequencies further than usual. Because the French 819 system used almost exactly twice the line scan rate of 405 you could resolve them on a 405 set but with 2 pictures side by side.

I assume other countries used similar offset methods. All now irrelevant in this age of digital TV.

PS: Searched BBC R&D reports and found these:
https://www.bbc.co.uk/rd/publications/rdreport_1962_19
https://www.bbc.co.uk/rd/publications/rdreport_1968_21
Pluse several more possibly relevant papers.
So the method was defninitely known in 405 days but I'm still not certain it was used in practice.

[Electronic M]

There is still band planning in digital transmission systems. Sometime back I was reading about the band planning around ATSC 3.0 and how channels had to be spaced and how close cellular data channels could be placed to prevent interference.