wjc@ll-vlsi (Bill Chiarchiaro) (01/08/86)
Circular polarization gets some use in both the FM broadcast and TV broadcast services. Originally, both services had to use horizontal polarization, but in the 1960's the FCC authorized FM stations to use circular polarization. In 1977, the FCC authorized broadcast TV sta- tions to use right-hand circular polarization. Furthermore, the FCC allowed the TV stations to run the old ERP limit in both the horizontal and vertical planes; I don't know if the FM broadcast service received a similar ERP authorization. I found the above information in the 1984 edition of "The Antenna Engineering Handbook" by Jasik and Johnson (this is a great book). By the way, ERP limits (in each plane) for TV stations are: Channels 2-6 100 kW Channels 7-13 316 kW Channels 14-83 5 MW I don't know of any stations that actually run the limits. The most powerful station that I know of is Channel 68 (WQTV, I believe) in Boston at 1.3 MW ERP. Some special TV transmitters, such as translators and boosters, have much lower ERP limits. Bill N1CPK
dsi@unccvax.UUCP (01/10/86)
> > Circular polarization gets some use in both the FM broadcast and > TV broadcast services. Originally, both services had to use horizontal > polarization, but in the 1960's the FCC authorized FM stations to > use circular polarization. In 1977, the FCC authorized broadcast TV sta- > tions to use right-hand circular polarization. Furthermore, the > FCC allowed the TV stations to run the old ERP limit in both FM radio stations were always allowed to use "circular polarisation" in a sense, being permitted "any power divider you want" and the same ERP in the horizontal and vertical planes (actually, the vertical plane couldn't be more than the horizontal). No one, (except idiots who buy antennas out of the back of "Radio World") but no one, would dare not use CP for FM. Note that if the power divider and antenna combination was a Hilbert transformer (in effect) - 90 degree lag between the H and V antennas - CP would be obtained. Virtually all new builds for TV also use the RH circular polar- isation, at VHF this is typically the CBR (tm) - cavity backed radiator type of antenna. Three (or six) of these are put on the tower face(s) and fed with equal power; the pattern circularity obtained is excellent. Thus, the trend to higher power VHF television transmitters...one dis- advantage of the CBR antenna is that it has very little gain and is very expensive. In actual use, there is *no comparison* between the old Batwing antenna and a CBR installation at WFMY-TV (2) Greensboro, NC. The CBR (and other CP antennas for VHF) have unbelieveably good VSWR characteristics, usually less than 1.1:1 over the entire low or high VHF band (!). A properly pruned CBR installation can support two or three TV stations in a market. The break even point is about 2.2 TV stations on a CBR installation (and about four FM stations on a "panel radiator" thingie which is similar to the CBR). > I don't know of any stations that actually run the limits. The most > powerful station that I know of is Channel 68 (WQTV, I believe) in > Boston at 1.3 MW ERP. > There are more than a few UHF stations at 5 mw ERP: WGGT Greensboro, NC (48) - 1800 feet and 5 mw WNRW Winston Salem (45) - 1940 feet and 5 mw New TV Belmont NC (44) - 2000 feet and 5 mw The most "powerful" TV station in the US (i.e. the one running the limits with the least antenna gain) is WTIC Hartford CT (62?) which has - count 'em - 5 Varian klystrons at 220 kw visual. They are also 5 mw ERP. I'm sure the guy who changes the tower lights can feel this, and birds flying by probably drop to the ground. However, you guys in the civilized world (known also as Zone 1) are permitted the EIRP limits only to 500 feet, while we are permitted the same ERP up to 2000 feet. Thus, the Boston station probably has more than a 500 foot tower. I might add that ghost discrimination (the whole reason for circular polarisation) is still about 9 - 13 dB improved with CP over regular horizontal polarisation, even when the receiving antenna is horizontal only. However, your mileage may vary at UHF. Millions of people with wabbit ears like CP as well. David Anthony DataSpan, Inc. > Some special TV transmitters, such as translators and boosters, have much > lower ERP limits. > > Bill > N1CPK *** REPLACE THIS LINE WITH YOUR MESSAGE ***
wjc@ll-vlsi (Bill Chiarchiaro) (01/13/86)
After checking the 1979 TV Factbook, I found that quite a few TV stations are (or were) running the legal limits. Also, in the Boston area, WSBK (38) at 3.09 MW ERP takes first place instead of WQTV (68) (according to 1979 data). I believe all the Boston stations are over 500 ft AGL, with most of them at about 1000 ft AGL. Bill N1CPK