TopBand: Rohn 25

Earl W Cunningham k6se@juno.com
Wed, 20 Aug 1997 15:37:58 EDT


You wrote:
>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. 

=====================
Interesting finding, Carl.  The way I shunt feed my towers is to
horizontally attach (via SS hose clamps) a 1" OD piece of aluminum tubing
about 3' long to one of the horizontal rungs of the tower's zig-zag
bracing.  I attach the gamma wire (I use old discarded RG-58 coax) to the
outer end of the tubing.  If needed, it's easy to loosen the clamps and
slide the tubing in or out to adjust the spacing.  For standoff
insulators every 15' or 20' below that, I use 3' lengths of 1" ID
schedule 40 PVC with a 1/4" hole in their outer end through which passes
the gamma wire.  These are similarly attached to the tower for easy
adjustment.  Anyway, it looks like this from above:

                       |o
    1" tube -----> |      o <------ tower legs
                       |o
                       |
                       |x <------- gamma wire

When you say "shunt feed directly opposite the 12" face of the tower", I
assume you mean like this:

                         o
                                   <------- tower legs
                    o        o

                         x     <------- gamma wire

When you say "standing off from a leg so that it effectively 'sees' a
taper", I think you mean like this:

                    o        o
                                   <------- tower legs
                         o

                         x    <------- gamma wire

When I have modeled shunt feeds with a triangular structure, I've always
place the gamma wire as in my first illustration above, with the 1"
tubing.  I shall try the other two scenarios and let you know the
results.  Of course, when using a circular "wire" to emulate the
triangular tower, this doesn't make any difference.  

BTW, preliminary results of comparing a 12" triangle structure in EZNEC
vs. a circular "wire" show that a 10.352" OD "wire" is very close to the
same as a 12" triangle.  This was not with a shunt feed, but merely
base-feeding the equivalent of a 13-section (126' 9-1/2") Rohn 25 tower
and comparing it with a 126' 9-1/2" round "wire".  I used #8 "wires" to
model the tower.  When I changed all of these "wires" to 1" OD, I
expected a large change, but the delta was hardly perceptible, which
encourages me.

Anyway that structure was 1/4-wave resonant at 1850.83 kHz, with an input
Z of 35.95 + j0 ohms.  A round "wire" 10.352" OD was resonant at the same
frequency with an input Z of 35.96 + j0 ohms.

===============

>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.

===========
Thanks for the 12.5" info.  I'll use it instead of 12".
===========

>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.

===================
No one has come up with any info on exact lengths of such top sections
yet, so I'll continue to model Rohn 25 with regular 10' (9' 8-1/2")
sections (using 10.783" OD "wires" if the same ratio applies for a 12.5"
face).
===================

>Then you can start over again on 45G Earl!

===========
Yes, I will, Carl.
===========

>Thanks for your effort.
>73...Carl   KM1H


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