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

[tongaloa <tongaloa@alltel.net>]

corneliuspaul@gmx.net wrote:

> VE6WZ  wrote:
>
>  
>
> > Put both elements back pointing the same direction.
> > Shorten all the elements on the 20 the same amount to bring it back to
> > resonance.
> >
> > The fact that the 20m Yagi has been "loaded down" because of the 40m
> > does not mean that there will be a problem with the pattern AFTER it is
> > re-tuned.  There IS interaction....but only to the extent that the 20m
> > elements now resonate lower in frequency.  
> >    
>
>
>
> 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)
>
> Any more comments?
> Thank you
>
> 73s Con DF4SA
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>
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>
>  
Had to shorten 20 and 40m sections of the 4BTV trap vertical when
I moved it to the KH6 house with the metal roof. It's ground mounted and
about 18" from the house with 40m trap at roof edge level and a fairly
shallow pitch to the roof :-)

Pattern? Happy to get any antenna up at that house~

-Bob



_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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