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Re: WBZ/KDKA (was: DX question)

At 07:40 PM 5/30/99 -0400, you wrote:
>Dan Strassberg writes:
>> broad. WBZ comes in relatively fade-free in Ohio. Even eastern Ohio
>> farther from Hull than Concord NH is from Philadelphia
>I can attest that once (the relatively new 1 kW daytimer) WBGS (1030
>Point Pleasant, WV) on the SE OH/WV line signs off, WBZ is clear,
>albeit not always with full quieting and with some fading.  An old
>vacuum tube radio relic at the in-laws in OH gives WBZ the full
>treatment.  I think that is about 760 crow miles (830 road).  Since
>sunset at that end of ET is an hour later (9:30 summer, 5 pm winter)
>and sunrise is later (8 am winter) you can benefit from AM drive on BZ
>and other clears.  Interestingly, and not sure why, but KDKA (1020
>Pittsburg) is not as L&C in SE OH as is WBZ.  (??)  Is KDKA a single
>stick at 50 kW?
Not merely a single stick, but as of a few years back (five? I'm sure that
Scott knows exactly how long), KDKA has had a sectionalized (Franklin)
antenna that produces greater groundwave efficiency than even a 210-degree
tower of conventional design. I haven't looked up KDKA's RMS field but it
must be well over 432 mV/m/kW @ 1 km. That's the efficiency of a
5/8-wavelength conventional tower. By conventional wisdom, you don't go
higher than 5/8 wave with a conventional tower because, above 1/2 wave, the
vertical radiation pattern develops a secondary upward lobe. At night (and
even for a couple of hours after sunrise and before sunset) the ionosphere
reflects the signals from this upward lobe. They return to earth within the
groundwave service area of the station and cause selective fading. Franklin
antennas squash the vertical radiation pattern and take the energy that
would have gone into the upward lobe and push it down along the ground,
further increasing the groundwave coverage.

The problem with Franklin antennas is that they are expensive to construct
and maintain and can be troublesome. As a result, a number of stations have
abandoned them. WOAI did so recently. WHO has one and the chief engineer has
been quoted as saying that it's a lot more trouble than it's worth. KSTP
uses one during the day but switches to conventional towers for its
directional nighttime operation. The only DA that I'm aware of that uses
Franklins (a pair of them) belongs to KFBK in Sacramento. The only one that
I know of that belongs to a former Class III station is at KELO in Sioux
Falls SD. Like KSTP's Franklin, This one is in use during the day only.
Since Class IIIs (now Class Bs) are now allowed powers greater than 5 kW, I
wouldn't be surprised to find KELO replacing its tall tower with one of the
towers in its night array. and increasing its day power to achieve the same
RMS field as it got with 5 kW into the Franklin.

One of the best known sectionalized towers belongs to KNBR. This is not a
Franklin but an unconventional design about which little is known. The
details were apparently lost in a housecleaning after Susquehanna bought the
station. The reason for the unconventional design is that it was necessary
to achieve high groundwave efficency (KNBR is a former Class IB) despite
restrictions on the tower height. The tower, at the edge of San Francisco
Bay in Palo Alto, is on the glide path to SF International Airport.

- -------------------------------
Dan Strassberg (Note: Address is CASE SENSITIVE!)
ALL _LOWER_ CASE!!!--> dan.strassberg@worldnet.att.net
(617) 558-4205; Fax (617) 928-4205