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From: dsi@unccvax.UUCP
Newsgroups: net.ham-radio
Subject: Re: FM & TV Broadcast Polarization
Message-ID: <415@unccvax.unccvax.UUCP>
Date: Fri, 10-Jan-86 08:23:30 EST
Article-I.D.: unccvax.415
Posted: Fri Jan 10 08:23:30 1986
Date-Received: Mon, 13-Jan-86 08:09:48 EST
References: <1340@brl-tgr.ARPA>
Organization: UNC-Charlotte
Lines: 71

> 
> 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

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