On Wed, 20 Aug 1997 01:45:22 EDT k6se@juno.com (Earl W Cunningham)
writes:
>Hello, All,
>
>In the thread about shunt-feeding Rohn 25, two things have been
>mentioned that cause me some concern about error in my modeling of the
>structures:
>
>1) Someone mentioned that an 8" diameter "wire" should be used in
>models to emulate 12" triangular Rohn 25. After giving this some
>thought, I decided to try using a wire diameter of about 8.91" because
>a circle that size covers the same area as a 12" per side triangle.
>In an attempt to verify this, I'm building a more accurate (I think)
>model of Rohn 25 tower by defining tower legs and cross braces in
>EZNEC. I did this once for my own towers and came up with accurate
>results, but thought I'd never do it again because of the complexity
>and pain involved in such modeling, hi.
Earl, would the model show a difference if you tried placing the shunt
rod directly opposite the 12" face and also standing off from a leg so
that it effectively "sees" a taper? I found that placing off the leg
was the better match. This was evidenced by better 2:1 bandwidth and FS
measurements.
>
>2) Someone also mentioned that Rohn 25 top sections come in 8'
>lengths. Are they all 8', or also 10'? What is the net length of
>such Rohn 25 top sections? Do they taper ( the last foot or so) down
>to accommodate 2" mast?
25G is actually 12.5" on a face and the legs are 1.25". There are 8
zig-zag braces per 10' regular section. Measured overlap is 3.5" per
section.
Top sections may be 8' which is the flat top version for use with a
thrust bearing.Part # 25AG4.
There are numerous tapered top sections with the 25AG2 and 25AG3 both
having a 2.25" top tube, one extends 12" above the apex of the top rails
and the other 18". The Rohn catalog does not give the total length of
these two but they are under 10'. They are the most popular since they
accept a 2" mast. Other tapered models have smaller tubes.
>When I get the answer to 2) above and determine an accurate "wire"
>diameter to substitute for triangular towers, I'll re-post the ELNEC
>results again for various heights of shunt-fed Rohn 25, including 2:1
>swr bandwidth. Also gain and angle of radiation figures for over all
>types of soil, including over fresh water and salt water.
Then you can start over again on 45G Earl!
Thanks for your effort.
73...Carl KM1H
>
>73, de Earl, K6SE
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