[TowerTalk] Interaction and loaded elements

[Jim Lux]

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.