Larry,
Back in the 60's I worked for a AM station in Minneapolis (KQRS) which used a
three tower directional array. All three towers were made electrically longer
and the radiation resistance increased which the adoption of this method of
jumpering all the top guys. (Triple guy wire insulators were used to overcome
the HV breakdown at the transition. Today long fiberglass HV guy insulators are
available from the power company suppliers. Even Phillystran would be easier
eliminating the need for insulators completely.)
The design engineer of this antenna was John Mulaney of Multronics, I believe
out of Rockville, MD, who was then well respected as an AM antenna design
guru.. The FCC accepted the filing of this design for a shortened radiator for
1440 Khz.
The 30 percent limit as compared to the vertical was a practical "rule of
thumb" as computer modeling was not yet common place in the early 60's. Nor
have I tried to run it through today. It would be interesting for someone to
subject the design to modeling criteria.
With respect to your question in the number of horizontal jumpers, I believe
that there would be less cancellation with fewer sloping wires but more
increase in radiation resistance with the increased top loading effect
comparing 3,4, 5 and even six horizontal jumper wires. Only modeling or many
measurements could say for sure. My suggestion is that with to many wires and
an acute slope you will eventually arrive at a point where you have constructed
a decoupling sleeve and this is going in the wrong direction. But I think the
rule of thumb still will hold, that it is better to make the sloping portion as
big horizontally as possible but limiting the coverage of your vertical portion
to less than 30 per cent. The modeling might possibly put the limit at less
than 25 percent before the benefits are reduced or eliminated entirely. In the
case of KQRS the guying angle was closer to 50 degrees downward declivity,
which I believe is closer to most commercial guyed towers.
Herb Schoenbohm, ex-KV4FZ
Larry Molitor wrote:
> I've never tried this because in every instance where I needed a "hat", I
> have been limited to only two drooping wires.
>
> Is this data from computer modeling or actual measurement? Is the 30
> percent you refer to slope or percent of vertical section covered? If
> slope, then how far can you carry this concept, with respect to percentage
> of vertical section covered? Is the effect proportional with respect to the
> number of wires, 3,4,5 etc?
>
> thanks,
>
> Larry - W7IUV
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