[TowerTalk] Re: 20M Yagi Interaction with nearby objects

[Pete Smith]

At 02:05 PM 10/20/03 -0400, tongaloa wrote:

>corneliuspaul at gmx.net wrote:
>>...
>>
>>
>>I recently have been asked a similar question by a friend who is trying 
>>to put up a 3ele 20m yagi 6m (18ft) above a sheet metal roof.
>>
>>The resonance (point of lowest SWR he measured with his SWR bridge) moves 
>>down to 13.8 MHz.
>>The interaction seems to be much less pronounced when he moves the 
>>antenna to only 2m (6ft) above the metal roof.
>>
>>I was hesitating a bit to give the advise of shortening first the driven
>>element,
>>and afterwards shortening also the parasitic elements by the same percentage.
>>
>>Do you really think this will work?
>>OK, I understand your point, the roof is "capacitively loading" all the
>>elements
>>and therefore all the elements have to be shortened accordingly. True?
>>I am still afraid that even with the shortened elements, the antenna pattern
>>will not be restored.
>>
>>I have tried to model the situation with NEC2 but had no succes in making
>>a workable model (too many segments)

I think your concerns are fully justified, Con.  After all, electromagnetic 
radiation obeys the inverse square law, so the current in the roof will 
become greater the closer the antenna is to it.  Serendipitously, the SWR 
or resonant frequency may seem more normal, but it's unlikely that the 
pattern is improved by being closer.  I think Steve's wrong about just 
adjusting the elements and living with the thing -- I think that resonant 
frequency perturbations are a sure sign of bad things happening to the 
pattern.  Once in a great while, good things can happen through 
interaction, too, but it's a lot less likely...

If you are hacing segment problems, may I suggest you try MultiNEC.  It's 
an Excel front-end for NEC-2 that can be used with various public domain 
NEC-2 compilations with up to 5000 segments.  It comes with a 1500-segment 
version, and will also automatically produce a wire grid that can be used 
to simulate a metal roof surface.

You can download a fully-functional trial version at 
<http://www.qsl.net/ac6la/>

73, Pete N4ZR
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