[BC] KKGM

[Craig Healy]

> 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