[BC] Two-tower array designed for 990 running on 1030

[Phil Alexander]

On 8 Jul 2005 at 11:31, Dan Strassberg wrote:

> Dale Cook observed that since WBZ's two-tower array in Hull MA was built
> (shortly) before the NARBA, the station's forced move from 990 to 1030
> afftected the cardioid pattern. I was aware of that. BUT, take a look at all
> of the two-tower arrays that create such patterns in the US and Canada. At
> 1030, the spacing of WBZ's towers is 86.5 degrees. The average spacing for
> arrays that produce simular patterns must be around 80 degrees, and most of
> those are decades more recent than NARBA. My assumption is that DA designers
> like the fact that the shorter spcing produces a bit more gain in front of
> the pattern and a slighlty narrower pattern without sacrificing much in the
> depth of the minima behind the pattern.

The shorter spacing may result from site topography or the desire to use a
smaller plot.

You can get more or less the same result with any spacing between 60 deg and
120 deg by altering the phase angle. What happens in the "null" (more correctly,
the minima) depends on the field ratio, essentially a function tower currents.

The principal problem with closely spaced towers is mutual coupling which
makes the array somewhat more challenging to manage.

Except in cases where there is no population on the back side of the pattern
there is generally some null fill unless there is a critical interference.

Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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