[sci.electronics] Commercial radio transmitters

roy@alanine.phri.nyu.edu (Roy Smith) (12/21/90)

	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 is just a small house which looks like it is probably big enough to
hold the actual transmitting gear, a couple of guys on duty watching it,
and maybe an emergency generator.  How do they get the audio signal from
the studio to the transmitter?  Plain old leased phone lines?  Private
microwave link?

	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.
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
"Arcane?  Did you say arcane?  It wouldn't be Unix if it wasn't arcane!"

joel@cfctech.cfc.com (Joel Lessenberry) (12/21/90)

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.
>--
>Roy Smith, Public Health Research Institute

	Yes, the studio and transmitter site are linked by either phone line,
	microwave link, or just radio link. (Probably two different methods
	for backup purposes. WEXL in detroit (I am Chief Engineer) uses phone 
	lines.  

	AM stations are at a very low freq. compared to FM stations, 1 Million
	Cycles per Second, compared to 100 million  Cycles per second.

	The frequency determines the antenna element size, the lower the 
	frequency, the longer the element.

	FM stations use antennas supported on towers, FM being more or less
	limited to line of site, the higher the tower, the farther the 
	antenna can see. Elements at these freqencies are only a few feet.

	At AM frequencies, this is houndreds of feet, therefor AM station 
	uses the tower itself as the antenna.

	Broadcasting two different signals on the same tower is easy in 
	principle, you insert into each trasmitters feed line tuned notch
	filters, which keep out all other signals except your own. I have
	never done this at AM freqs. but have installed FM sites.

	Yes, the shorter tower is probable a tuning stub, buy varying 
	the power split and phase relationship of the signals between the
	two towers, you can control the pattern.

	Is there any one out there interested in forming a Commercial
	Radio Mailing list, or starting some discussions??

	My current home is WEXL 1340 am in Detroit
				1Kw DA-D
				1KW ND-N

	I have worked at   WLDM 95.5 FM in detroit
				70KW
		    
		      and  WOMC 104.3



 Joel Lessenberry, Distributed Systems | +1 313 948 3342
 joel@cfctech.UUCP                     | Chrysler Financial Corp.
 joel%cfctech.uucp@mailgw.cc.umich.edu | MIS, Technical Services
 {sharkey|mailrus}!cfctech!joel        | 2777 Franklin, Sfld, MI

kludge@grissom.larc.nasa.gov ( Scott Dorsey) (12/29/90)

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

dbell@cup.portal.com (David J Bell) (12/30/90)

from Scott Dorsey/ Kaptain Kludge:

>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
          ^.... Hmm? Usually this was/is to allow ....^
           directional control of the transmitted signal,
           by feeding the two or more towers in particular
           phase relationships.
>fairly long wavelength (600 ft. or even longer), so the antennae are
>much larger than those for FM.
   ^.... True. The towers are normally 1/4 wave verticals.
    At 98 MHz, this is a LOT shorter than 1.1 MHz!

Dave     dbell@cup.portal.com

kludge@grissom.larc.nasa.gov ( Scott Dorsey) (01/03/91)

In article <37390@cup.portal.com> dbell@cup.portal.com (David J Bell) writes:
>from Scott Dorsey/ Kaptain Kludge:
>
>>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
>          ^.... Hmm? Usually this was/is to allow ....^
>           directional control of the transmitted signal,
>           by feeding the two or more towers in particular
>           phase relationships.

Absolutely true.  I didn't think about that here, far away from the major
markets.  But there are still a whole lot of horizontal AM antennae in
use... our local daytimer uses a dipole hung between two towers and does
get his signal out.  Again, with AM, polarization doesn't mean very much
because of the long wavelength (as opposed to FM where you'll lose 10 dB
with crossed transmit and receive antennae (and yes there are still some
FM stations that haven't gone to circular polarization yet)).

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

Disclaimer: Neither NASA nor Lockheed really know anything about what