AM Aux sites / towers (WCBS)

[Dave Doherty]

Hi Dan-

At 155 miles (250km), the departure angle ranges from 25 to 37.5 degrees for 
10% skywave calculations. That is right in the area of the vertical null of 
a tower in the 200-225 degree range. See the AM engineering formulae and 
charts at http://www.skywaves.com/fcc/47cfr73.190.pdf, and in particular 
page 7. Compare the 180 degree tower (low vertical radiation and no higher 
lobe) with the 225 degree tower (still lower vertical radiation, but with a 
high lobe at about 60 degrees). Then look at the 90 degree tower. The 
radiation in the 25 to 40 degree range is far higher - about triple at 30 
degrees elevation.

-d


----- Original Message ----- 
From: "Dan.Strassberg" <dan.strassberg@att.net>
To: "Dave Doherty" <dave@skywaves.net>; 
<boston-radio-interest@rolinin.bostonradio.org>
Cc: "Garrett Wollman" <wollman@khavrinen.csail.mit.edu>; "Glen Clark" 
<glen@clarkcom.com>; <rcarpen1@verizon.net>; "Peter Haas" 
<PeterH5322@rattlebrain.com>
Sent: Tuesday, March 18, 2008 7:07 AM
Subject: Re: AM Aux sites / towers (WCBS)


> 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
>>
>
>
>