AM Aux sites / towers

[Dan.Strassberg]

Certainly a shorter tower dramatically increases the vertical
radiation. No argument there! BUT the increased vertical radiation is
really an issue only at high crtical angles. High-angle skywave is
important only for stations close to the interfering station. Garrett,
in a private communication, said that the limitation on WCBS's (and
WFAN's) aux power is imposed by the power-handling capacity of the
short auxiliary tower that serves both stations. However, my
recollection is that WCBS's (but NOT WFAN's) aux power is restricted
to a lower value at night than by day. If so, I speculated that WCBS's
problem might be WAMG, which is definitely close enough to be
affected. There are a couple of problems with this theory, however.
The high-angle skywave should be no greater with 25 kW into a
60-degree tower than with 50 kW into a 207-degree tower. Also, AFAIK,
the only contributor to WAMG's 12.5 mV/m NIF is WLS. I find it hard to
believe that WCBS would have to reduce aux power from ~25 kW D to ~19
kW N (IIRC) to protect WAMG, which is unlikely to receive a bigger
interfering skywave with 25 kW into the 60+-degree tower than with 50
kW into the 207-degree tower.  So the mystery is not resolved.

BTW the high-angle skywave effect is probably nowhere more important
than with interference from WAMG to WCBS. Glen Clark, who designed
WAMG's new 6-kW night pattern, used a supercomputer to analyze the
skywave from WAMG toward WCBS and demonstrated that, because of WAMG's
tall towers, radiation is so low at high angles that no consequential
inteference occurs to WCBS until you are over open ocean south of Long
Island's south shore. Interference over open water doesn't count.
Interestingly, the reason that WAMG has such tall towers is that the
array was originally designed for what is now WBIX. WBIX has to
protect KYW which is only about 275 airline miles away. (Its protected
contour is even closer.) The array design required the tall towers to
suppress high-angle skywave.

-----
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>
Cc: "Dan.Strassberg" <dan.strassberg@att.net>
Sent: Sunday, March 16, 2008 9:14 PM
Subject: AM Aux sites / towers


> Hi Dan-
>
>>Note that WCBS, whose relatively short aux tower is on the same
>>little island off the Bronx as its 207-degree main tower, is limited
>>to substantially lower aux power than WCBS's normal 50 kW. I've
>>never heard an explanation of this quirk in FCC regulations.
>
> With a shorter tower, vertical radiation increases dramatically. It
> is not unreasonable to expect that the nighttime power limit for a
> short tower would be much lower than a tall tower.
>
> You are of course referring to the City Island site shared by 660
> and 880 - WFAN and WCBS - for many years. I had occasion to look
> into the history of the City Island site many years ago. As I
> recall, the WCBS aux antenna at that time was a section of one of
> the guy wires, or might have been a vertical wire hung from a guy
> wire.  In any event, I doubt that WCBS would have applied for more
> aux power than they could reasonably produce, given that
> arrangement.
>
> I have vague memories of the original WNBC site at Port Jefferson -
> two or three towers, with a pond in front of the building sporting a
> fountain to help cool the transmitter. My dad worked at NBC TV in
> those days, and all the NBC staff was invited to bring their
> families to an annual picnic.
>
> -d