At 07:02 AM 6/30/2005, Dave Fuller wrote:
>Manufacturers of shortened and loaded 40 meter beams say there is less
>interaction with a 15 meter beam than with a fill sized beam. How is
>that so, since the elements are resonant on 40 meters whether they are
>shortened and loaded or not? Can someone explain this to me?
>
>Thanks
>Dave-WD7Z
>_
"Manufacturers say".. now there's an interesting statement. Ask them to
back up their assertion with some sort of analysis or measurement.
This would be a trivial modeling exercise.
As far as interactions go, it's mostly due to the fields from the currents
in the element, which are highest in the middle of the element (the classic
approximation is a cosine). A shortened radiator still has the high
currents in the middle, and may, in fact, have a current distribution that
looks more rectangular than half a cosine (particularly with end
loading). Whether this would improve or degrade interaction is kind of
dependent on which antenna is where and how close.
For very small antennas (like compact loops), the interaction IS less, but
that's because the antenna itself is less efficient. Two antennas that are
50% efficient will interact more than two antennas that are 5% efficient.
And then we get into a discussion of what is meant by "interaction". Does
it mean a shift in the resonant frequency? Does it mean a change in the
radiation pattern (like filling in a 20 dB null, or ruining the F/B)?
Take an example of a triband dipole for 10, 15, and 20, with the different
wires separated by 6" or so. There's huge interaction in the sense of
trying to get it tuned up so that it's resonant on all three bands at the
right places. However, there's very little effect on the eventual
radiation pattern. The radiation pattern of the combination is so close to
that of a single dipole in the same place that I doubt you could measure
any difference.
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