[TowerTalk] Interaction and loaded elements

[TexasRF@aol.com]

The likely thinking is that a full sized 40m antenna element also resonates  
on it's odd harmonic frequencies; third being at 15 meters.
 
A loaded element is going to have different odd harmonic resonances due to  
the loading device being a different reactance on each of these harmonic  
frequencies.
 
It is very unlikely that there will be a resonance near 15meters therefore  
the lowered influence on the 15meter antenna.
 
73/K5GW
 
 
In a message dated 6/30/2005 11:59:14 A.M. Central Standard Time,  
jimlux at earthlink.net writes:

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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