Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!umich!sharkey!cfctech!joel
From: joel@cfctech.cfc.com (Joel Lessenberry)
Newsgroups: sci.electronics
Subject: Re: Commercial radio transmitters
Summary: The towers are the antennas.
Message-ID: <1990Dec21.135235.22887@cfctech.cfc.com>
Date: 21 Dec 90 13:52:35 GMT
References: <1990Dec20.172920.7325@phri.nyu.edu>
Reply-To: joel@cfctech (Joel Lessenberry)
Organization: Chrysler Financial Corporation, Southfield, MI.
Lines: 63

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



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