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
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