[TowerTalk] Two 15m yagi stacking...and SteppIRs

[Jim Lux]

At 09:17 AM 2/24/2007, Cqtestk4xs at aol.com wrote:
>
>In a message dated 2/24/2007 4:25:59 A.M. Greenwich Standard Time,
>jimlux at earthlink.net writes:
>
>The best  way to answer these questions is to get a copy of HFTA,
>which comes with  the ARRL Antenna book. It lets you model the pattern
>of stacked antennas,  varying spacing, etc., and factoring in your
>terrain, as well.  You  just select the kind of antenna (e.g. 5
>element beam),
>
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>But there is a rub to doing that, isn't there?  I think the program  assumes
>a standard length boom for 3 el and a standard length for a 4 el and so  on.
>If one uses the 4 element STeppIR for 10 will it not behave more like  a 6
>element in a stack since it has a boom which is what a usual 6 
>element  has?  I
>believe the boom on the SteppIr is around 32 feet and the usual 
>6  el 10 meter
>is around 27.
>
>What this means to me is the antenna will "behave" more like a 6 el in
>stacking since the boom length is a primary consideration in 
>stacking  distance.


Is it?  There's two factors at work in stacking: 1) the interaction 
between the antennas and 2) the pattern of a single antenna.  Boom 
length has an effect on this, but so does the particular design for 
the antenna.

With HFTA, since it doesn't actually model the antennas, you'd 
probably want to pick am "antenna type" based on the directivity of 
the antenna, independent of element count.  And then hope that the 
"antenna interaction effects" aren't going to dominate.


>The same concept is true for 15 for the SteppIR, with the boom  somewhere
>between what a 5 or a 6 element standard Yagi would have ...32  ft.
>
>Because of the above reasoning, I have input the 4 element on 20, a 5 el on
>15 and a 6 el on 10 on the HFTA program in computing stacking 
>distances...not
>the standard 4 element for all bands.

That's probably a reasonable assumption, since, to a first order, the 
directivity of the antennas will be more strongly affected by boom 
length than number of elements.