[BC] KKGM

[Phil Alexander]

On 18 Jul 2005 at 16:39, Dan Strassberg wrote:

> Resending this because the original bounced but it may have reached some of
> the addressees.
> 
> 1. What is the efficiency of a 263+-degree radiator SUPPOSED to be (in
> mV/m/kW @ 1 km), assuming a normal (120 quarter-wave-radial) ground system?
> It is presumably less than the efficiency of a 225-degree radiator with the
> same ground configuration. I have never seen theoretical efficiencies for
> standard series-fed radiators of heights greater than 225 degrees
> (approximately 440 mV/m/kW @ 1 km). Also, what does the vertical-plane
> radiation pattern look like?

The reason is found in the vertical plane. Above 180 deg the high angle
lobe begins to form, and above 225 deg the low angle lobe begins to
diminish. I recall seeing numbers to 240 somewhere, but anything above
225 is beyond the point of diminishing returns. In fact, when velocity
is taken into account the true electrical height will be more or less
10% greater than its physical height. The way I recall it, I believe the
high angle and low angle lobes should be equal in the vertical plane
at 360, and above that a third higher lobe should begin to form.

> 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?

The NAB Eng HB suggest possibilities including skirting and isolation
of the upper part.

> 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.

Theory says it can be done, but it seldom happens so there is not much
literature. Secondly, each tower will have its own dimensional peculiarities.
The best way would be a modeling exercise in NEC-4 to look at possibilities
before the field work. However, it is possible.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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