[BC] Shortening non-sectionalized "too-tall" AM radiators

[Dan Strassberg]

Craig, and all: Thanks for the info. A question about WNBH: Is the tower
grounded-base or series-fed and if ground-base is it shunt or skirt-fed?
According to the FCC database, your scheme shortens a tower that is
mechanically 240 degrees to an electrical height--based on the efficiency of
386 mV/m/kW @ 1 km, IIRC--of just a skosh more than 180 degrees.

I don't think that any scheme that involved sectionalization would work in
the case I'm interested in--and not only because the towers are in use by
another station (actually, two stations at night). I believe that the town
in which the array is located would raise hell at any suggestion involving
mechanical modification of the existing tower structures. I think that
skirting might fly with the zoning authorities, though.

The other case that I mentioned in central New York State is different. That
one IS a grounded, skirt-fed tower. Based simply on the tower's physical
height, it is 5/8 wave at the AM frequency and has more VHF and UHF antennas
on it than I was able to count (one or more FM translators, an LPTV, and
much other stuff). The skirt for the AM goes up just 1/4 wavelength, I
believe, but, of course, the entire tower is supposed to radiate the AM
signal. The guy who owns the tower and the AM is presumably making money
from his tenants but he's been stuck trying to get the AM to work even
moderately satisfactorily. Last I heard, he had hired as a consultant a
South American PhD college professor, whom he claims is the only expert he
could find with actual hands-on practical experience solving efficiency
problems in "super-tall" skirt-fed grounded AM towers. Even before he hired
this particular consultant, the owner recognized that the propagation
velocity was enough lower than the speed of light in free space to make the
tower electrically taller than 225 degrees. However, when I met him several
years back, neither he (that is, the tower owner) nor any of the other
consultants he had hired (I'm not a consultant; I'm just a friend of a
friend of the tower owner) had been able to explain why the efficiency was
as low as it was. Maybe by now the South American professor has fixed the
problem.

Received: Mon, 18 Jul 2005 07:31:56 PM PDT
From: "Craig Healy" <craig.healy at chowdanet.com>
To: "Broadcast Radio Mailing List" <broadcast at radiolists.net>
Subject: Re: [BC] KKGM

> Dan Strassberg wrote:
> 2. Is there a practical way to electrically shorten such a radiator? For
> example, can you attach a skirt to the bottom and drive it so that the
> skirted section (in this case something between about 40 degrees and 100
> degrees) does not radiate?
>
> I've been trying for several years now to find someone who can comment on
> electrically shortening AM towers that are too tall for a diplexed station
> operating at a frequency higher than that of the station that originally
> occupied the tower. KKGM is now the second case I've heard of of a tall
> tower that proved to be too tall and provided very poor efficiency. The
> other, in central New York State, did not, AFAIK, even meet Class C
minimum
> efficiency (241 mV/m/kW @ 1 km) and defied the ministrations of several
> well-known consultants. I don't know whether it is yet working as the
> original consultant intended.

WNBH in New Bedford, MA has a tower about 3/4 wave at 1340.  Has a six-bay
FM on top which probably provides a bit of top loading to boot.  What they
did was put a skirt on top connected to the tower below the FM bays and
going down somewhat less than 1/4 wave and insulated there.  That end is
connected to a motor-driven vacuum variable capacitor between the skirt and
the tower proper. The control is at the base.  There's also a sample loop in
the skirt section.  What is done is the capacitor is tuned to minimum field
on the sample loop.  That has that top section above the half or 5/8 wave
point not radiating much at all.  In practice, it seems to work well.  WNBH
has the full kilowatt into that tall tower, and covers pretty well for a
local station.

One of these days I'll get an FIM out there and tune the capacitor for
maximum field.  I'll bet it's close to where it is now.

I have often wondered how well an elevated feed point would work.  Find a
point where the tower is between 180? and 225? down from the top.  Put a
skirt feed above that.  Either use the existing radials as a counterpoise
(sans insulators) or a detuning skirt below that.  I wonder how much the
elevated feed would affect field strength?  It would be interesting to model
in EZNEC, though it would take a while to enter the data.

Craig Healy
Providence, RI

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