[Antennaware] Equivalent Diameter of Triangular Tower
K9AY
k9ay at k9ay.com
Mon Feb 23 10:29:29 EST 2009
Bill and all,
If I change from 5" to 8" the reactance moves the right way, but the
resistance moves the opposite direction. The net result is that the model is
a little closer to the measurement, but still far enough away to look for
other issues.
The 5" number comes from a paper at one of the ACES conferences (Applied
Computational Electromagnetics Society). I'll look it up later today and
post the reference.
Gary
K9AY
_________
> K9AY:
>
> > This is easily explained by uncertainty in choosing an equivalent
> diameter "solid wire" for
> the 25G tower (I used 5 inches).
>
> That sounds too small. Here's the formula I used to model
> my 160 vertical made of Rohn 45:
>
> http://lists.contesting.com/archives//html/Antennaware/2002-05/msg00001.html
>
> Equivalent Cylindrical Diameter = 2 * CUBEROOT [(D * F^2)/2]
>
> where D = tube diameter and F = Face width. For Rohn 45, D = 1.25"
> and F = 18", so cylindrical diameter equivalent = 11.7446"
>
> This turned out to be very accurate in predicting both the tap point
> and capacitance needed for my shunt fed tower (and later a 3-el
> parasitic array using the tower as the driven element). I don't have
> the tube diameter for Rohn 25 handy to actually go through the
> formula but think the equivalent diameter might be closer to 8" (just
> mentally scaling the result above).
>
> 73, Bill W4ZV
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