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

[tongaloa]

corneliuspaul at 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.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>  
>
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