Re: Let's Talk About Transmitters, part 2

["Dan Strassberg" <dan.strassberg@att.net>]

WLAW-FM 93.7 with antenna atop the center tower of what is now WRKO's array
in Burlington went dark. The license for WLAW-FM was not transferred to
WCCM. The FCC table of FM allocations allocated 93.7 to Lawrence, and when
WLAW turned in its FM license, the frequency went unused for quite a few
years until WCCM applied for it.

I'm not sure why the FM antenna wasn't allowed to just remain atop the
center tower. Most stations don't bother removing unused FM antennas from AM
towers. In any event, the top section of the center tower, which was
designed to be removed for installation of a top-mounted FM antenna, was
never replaced. According to a WRKO engineer many years ago, the effect on
the efficiency of having one 96-degree tower and two 109-degree towers vs
three 109-degree towers was so slight that it wasn't worth the effort to
replace the top section. Now, in most three-tower in-line arrays, the center
tower carries roughly twice the current of either end tower, so one would
think that those 13 degrees could be significant, but I remember looking up
the parameters of WRKO's array and I don't believe that, in either pattern,
the center-tower current is as great as twice the current in whichever end
tower carries the higher current.

WRKO's patterns are variations of the classic three-tower side-fire
figure-eight. The classical implementation uses towers 180 degrees apart
(WRKO's are 155 degrees apart), current ratios of 1:2:1, and zero-degree
phase in all towers. By using slightly shorter spacing, different current
ratios, and non-zero phase, patterns were created that bend each signal
maximum 15 degrees toward the azimuth of what, in a classic figure eight,
would be a null at 75 degrees. The null at 75 degrees is substantially
filled (somewhat moreso at night) and the day pattern also has a pair of
small lobes on either side of the null at 255 degrees.

If you take a look at WEEI's array in Needham, you will note that the top of
the center tower was also designed to be removed and replaced by a top-mount
ed FM antenna. The WLAW and WEEI arrays are of similar vintage, so that must
have been a common strategem at the time. Certainly the radiation
characteristics of top-mounted VHF antennas are better (that is, more
uniform) than those of the more common side-mounted antennas. In the case of
the WEEI (ex WHDH (AM)) array, the top section was never removed. WHDH-FM
originally transmitted from the top of the old John Hancock building. It
wasn't until the former Channel 5 tower (now FM-128) was constructed that
WHDH-FM transmitted from a site southwest of Boston.

--
Dan Strassberg, dan.strassberg@att.net
617-558-4205, eFax 707-215-6367

----- Original Message -----
From: A. Joseph Ross <lawyer@attorneyross.com>
To: <boston-radio-interest@khavrinen.lcs.mit.edu>; Donna Halper
<dlh@donnahalper.com>
Sent: Monday, July 23, 2001 1:12 AM
Subject: Re: Let's Talk About Transmitters, part 2


>
> We know what happened to the AM, but what happened to WLAW-FM?