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

[Dan Strassberg]

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.

In the Boston area today, I can think of at least three stations with such
setups: WKOX, WRCA, and WXKS (AM). WUNR's 5/9 wave towers are spaced 105
degrees apart to produce a broadened cardioid with a small lobe in back. I'm
too lazy to check the database, but I seem to recall that all of the first
three use a spacing of 79.25 degrees, suggesting to me that there may be
something "magical" about that spacing--although I can't imagine what it
might be. WBIX, which is diplexed with WKOX during the daytime, had to
accept the spacing between the already existing WKOX towers, with the result
that the towers are only about 70 degrees apart for WBIX. That is scarcely a
record for close spacing in two-tower cardioids, though. There are at least
a couple with spacings of 60 degrees or less. However, maybe WBIX holds the
record for a low ratio of distance between the towers to tower height. For
WBIX, WKOX's towers are top-loaded to an electrical length of 189 degrees.
The 70-degree spacing is thus only about 37% of the tower height, which I've
heard is uncomfortably close--although in WBIX's case, I haven't heard of
unusual array-stability problems. WBIX's day pattern has no augmentations
and the station's CH pattern has only one augmentation, which I believe is
required because of the effect of the chimey at the Framingham Municipal
Incinerator across Mt Wayte Ave from the WKOX/WBIX/WSRO towers.

--
Dan Strassberg, dan.strassberg at att.net
eFax 707-215-6367