At 11:28 PM 3/20/02 -0500, jljarvis wrote:
>
>Tennadyne has modelled stacked lpda's, and offered
>a few words of wisdom today.
>
>1) you can expect 2.2dB from stacking any pair of
>antennas. if you're not careful about matching the
>common point impedance, you can lose 1 dB of that.
>It almost doesn't pay to bother.
>
>2) By stacking...depending on height....you will
>clean up the pattern, and suppress high angle lobes.
>You will also RAISE the lower lobe of the in-phase system
>by a couple of degrees.
>3) It was their opinion that getting a single antenna higher
>would almost always be better than a stack. Until you want to use it for
>sweepstakes.
>
>4) My conclusion was that a single antenna, with the ability to
>modify its height, was probably a better solution.
>From my modeling and several contest seasons' experience with a tribander
stack, I think Tennadyne's analysis misses several points. Most important
is the beneficial broadening of the first vertical lobe in the stack, which
makes a big difference when you compute the average gain weighted by the
arrival angles of signals of interest to contesters. Typically, while the
peak of the first lobe may go up a degree or two, the broadening gets most
or all of the low-angle gain back, and does a lot to raise the first null.
Next, of course, is the flexibility that results from being able to switch
effective heights instantly, and even to point the two antennas in
different directions for special situations, like Sweepstakes, where you
need different takeoff angles simultaneously in order to be effective to
different target areas.
And finally, given that any stacking of multi-frequency HF antennas will be
a compromise, even 2 dB of additional gain in a single high antenna would
require a considerably longer boom and probably additional elements. The
resulting pattern would not only be narrower in the vertical plane, but in
the horizontal plane as well, not usually a good thing from a contester's
perspective.
Substituting two shorter booms makes a lot of sense to me, particularly
when you also gain the other benefits outlined above.
73, Pete N4ZR
Check out the World HF
Contest Station Database at
www.pvrc.org
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