AM Aux sites / towers (WCBS)

[Dan.Strassberg]

Thanks, Dave! I had not checked CDBS itself for WCBS's aux CP; I was
relying on Bob Carpetner's AMSTNS program, which, before a few
database updates (there is usually an update every week), had shown
the CP. Bob did write to me that the FCC had done something funny with
the database that caused some records to disappear and I suspect that
the very record I wanted was one of those. I should not have assumed
that if AMSTNS did not show a record, neither would the record show up
with the other WCBS records at fcc.gov. That said, neither would
AMSTNS have provided the kind of detail you found in the body of the
application itself.

I am surprised, however, that at the very high critical angles in
question (WCBS is only 155 airline miles from WAMG), with a
non-top-loaded 300' aux tower (~96 degrees at 880), protection to WAMG
would limit WCBS to so close to half of the power it is allowed from
its 207-degree main tower. I had thought that half power into an
~60-degree tower produced high-angle radiation equivalent to that from
an ~200-degree tower. In WCBS's case, even the old top-loaded 200' aux
tower must have been equivalent to quite a bit more than 60 degrees.
Remember that the tower also served WFAN, so I suspect that the 200'
tower was top loaded to no less than 72 degrees at 880 in order to
achieve reasonable efficiency at 660, which would require an
electrical height of at least 54 degrees at 660. And as Glen Clark
pointed out yesterday, the base impedance for such a short tower is so
low that, with the combined power that WCBS and WFAN would pump into
it, the design and maintenance of the ground system must have
presented non-trivial problems.

-----
Dan Strassberg (dan.strassberg@att.net)
eFax 1-707-215-6367

----- Original Message ----- 
From: "Dave Doherty" <dave@skywaves.net>
To: <boston-radio-interest@rolinin.bostonradio.org>
Sent: Monday, March 17, 2008 10:30 PM
Subject: Re: AM Aux sites / towers (WCBS)


> Well, let's check out the actual app.  It is available online at
> http://svartifoss2.fcc.gov/prod/cdbs/pubacc/prod/app_sear.htm.
> Search for apps for WCBS and select BXP-20020816AAO.
>
> They proposed a new 300' non-top-loaded tower. Presumably, although
> I did not check this out, the 200' tower it replaced was top-loaded,
> or they would not have mentioned this.
>
> There is not a complete ground system for this tower. It was
> proposed to use a square ground mesh bonded to the copper radials of
> the main tower that pass through the area. Although not specifically
> stated, that would go a long way toward explaining the 35kW daytime
> maximum. The AM License app form 302 is still a paper app, so
> nothing is available online that would indicate what the final base
> resistance was.
>
> As for night operations, the two stations studied are a new 870 in
> Reading, PA (still unbuilt, as far as I can tell) and WBPS (now
> WAMG) in Dedham, MA. Both are first adjacents, and in neither case
> does the proposed operating power of 26kW increase the nighttime
> limit.
>
> The main night power limit appears to be Dedham. At 26kW, the
> predicted skywave at Dedham is 0.2373mV/m. The main WCBS facility is
> 0.2374mV/m, and the previous aux was 0.2373mV/m.
>
> According to 73.31, in the range from 10 to 50kW, you must specify
> power to the nearest kW, so there is no option for, say, 26.1kW.
>
> At 27kW, the nighttime skywave would be 0.2373 * (27/26) ^ 0.5 =
> 0.2418mV/m, which would be an increase in nighttime interference at
> Dedham.
>
> So that explains the 26kW night power limitation.
>
> -Dave Doherty
> Skywaves, Inc.
> 97 Webster Street
> Worcester, MA 01603
> 508-425-7176
>