At 10:28 AM 7/19/2005, Richard (Rick) Karlquist (N6RK) wrote:
> > With close elements you can design an antenna to have increased bandwidth.
> >
> > John KK9A
>
>That's news to me. I always thought
>wide spaced Yagi's have greater bandwidth.
Rick--
It is all in the tuning. I like to think of Yagis as band pass filters and
the more elements you have the more "poles" you have that you can
play with to broaden out the bandwidth.
There are lots of tradeoffs. Longer booms mean more gain of course,
but the minimum number of elements to realize that gain will give you
little leeway to tweak for other factors like gain-BW and F/B-BW and
VSWR-BW. So, by adding extra elements you can tweak things a bit
more. Of course this results in something that looks like it is closer
spaced. The 4L OWA is one example where that first director is so
tightly coupled to the DE that it is more like a dual-driven Yagi. By
adding this extra element it makes the antenna more "closely spaced"
but it allows more parameters to be optimized over a broader BW.
The tradeoffs come into play when you add more elements for a given
boom length you end up buying more aluminum and have more square
feet of array to deal with.
73 John W0UN
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