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From: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
Newsgroups: sci.electronics
Subject: Re: Commercial radio transmitters
Message-ID: <1990Dec28.203208.2631@news.larc.nasa.gov>
Date: 28 Dec 90 20:32:08 GMT
References: <1990Dec20.172920.7325@phri.nyu.edu> <1990Dec21.135235.22887@cfctech.cfc.com>
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Reply-To: kludge@grissom.gatech.edu ( Scott Dorsey)
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Lines: 49

In article <1990Dec20.172920.7325@phri.nyu.edu> roy@alanine.phri.nyu.edu (Roy Smith) writes:
>
>	In New York, two radio stations (I think it's WCBS and WNBC, both
>50 kW AM stations) transmit from the same tower.  How do they do that?  Are
>there two independant antennas supported by the same physical structure, or
>do they somehow mix their signals to feed the same antenna?
>
>	As far as I know, there are no studios at the transmitting site;
>	There are actually two towers, one rather smaller (1/2 as high?)
>than the other.  I've always assumed that the smaller tower supported a
>second antenna element to provide some directionality to the signal, as the
>tower is at the extreme NE corner of New York City (actually on a tiny
>island in Long Island Sound).  Is that likely the real reason for the
>second tower?  My knowledge of antenna theory is pretty weak, so I'm not
>really sure if my guess is even reasonable.

Okay.  Two stations may transmit from the same antenna by using a mixer
system (a diplexer) which isolates the transmitters from one another.  
I have seen older AM systems which use a device like a telephone hybrid
with a crossed coil, though I don't know how the FM stuff works these days.
Of course, they may also have two antennae, which is more expensive, but
provides some redundancy.

Now, most stations these days have their transmitter and studios seperated,
and connected either by a telephone or microwave link.  (Actually, UHF
stuff can be found also).  This is mostly because running a studio with
huge amounts of RF beating down on you is an unpleasant thing, but also
because nobody likes to drive to work to the sort of places that are 
optimal for transmitter placement (usually pretty high up), and because
of real-estate costs.  Usually the STL (Studio to Transmitter Link) 
antenna is a small dish fairly low down on the tower.  Sometimes you'll
see small white boxes that contain the antenna.

If there are two towers for an AM station, I would expect it is because
they are using a long antenna sling between the two towers.  AM has a
fairly long wavelength (600 ft. or even longer), so the antennae are
much larger than those for FM.  If there are two towers for an FM station
I would have no idea why... unless it's because one of them is unsafe
and can't hold an antenna but the station management is too cheap to
take it down (like a particular station in Atlanta).
--scott
   (formerly chief engineer at a large commercial station, now happy
    to be the engineer at a small college station (WCWM, 100 watts of power)

--
Scott Dorsey/ Kaptain Kludge
NASA Langley Research Center, Aircraft Guidance and Control Branch

Disclaimer: Neither NASA nor Lockheed really know anything about what