Re: Top o' the hour

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

At 02:39 AM 6/5/97 +0000, you wrote:
>
>They're going to replace their towers?  Are they finally going to put up
>towers made for transmission at 1030?  If they change their pattern, don't
>they need FCC approval? 
>
According to the database, WBZ's towers are 188.5 degrees, (0.524 wavelength
or 500 ft even). The rms field is 244.5 mV/m/kW unattenuated at 1 mile. Yes,
some stations make it up over 250 mV/m/kW, but what is a 2% increase in
signal strength worth? Zero, I submit. There could easily be FAA
restrictions that limit the antenna height to the existing 500 ft. BTW, the
spacing between the towers is supposedly exactly 90 degrees, although the
phasing is not quite 90 degrees. That means that the null to the east isn't
theretically perfect. (The phasing should be 90 degrees to form a
theoretically perfect cardioid pattern using towers 90 degrees apart.)

The thing not to miss here is the 90-degree spacing. Unless a tower was
moved, which seems unlikely, the implication I draw is that Westinghouse
knew that NARBA was coming when they built the plant and they figured that
WBZ would move to 1030, so they set the tower spacing at 90 degrees at 1030.

As a practical matter, a spacing of 86 degrees instead of 90 is just as
good. (A distance equal to 90 degrees at 1030 is 86 degrees at 990.) A lot
of stations with patterns that look like cardioids fudge the tower spacing
downward. Some use tower spacings as short as 70 degrees and adjust the
phasing accordingly. Such arrays seem to produce patterns that are
marginally wider than a true cardioid with slightly lower signal strength
dead ahead than at about 45 to 60 degrees off axis. A true cardioid has its
maximum signal strength dead ahead, but the drop-off in signal strength with
angle is very gradual.

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Dan Strassberg (Note: Address is CASE SENSITIVE!)
ALL _LOWER_ CASE!!!--> dan.strassberg@worldnet.att.net
(617) 558-4205; Fax (617) 928-4205

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