> Looking for a starting point on where to attach a gamma rod for a shunt
> fed 120'tall tower of Rohn 25 with a 10'tall 2"od stinger out the top.
120' should place you comfortably below half wave resonance on Top
Band (not so on 80). That being the case, I can share my rules of
thumb with you. These were developed from reading all the literature
on the subject a decade ago, and worked well on the two towers that I
have shunt fed. The one tower was the best low band antenna I've
ever had.
I must emphasise that these rules were obtained extremely
empirically, so I guess there would be better wisdom available on this
Reflector.
1. Attach the gamma as high as possible.
2. Make the gamma diameter as close as possible to that of the
tower itself. This objective can be achieved by using a cage
for the gamma rod.
3. Space the gamma rod as widely as you can. 1 m (36") should be
adequate.
Using this arrangement, you should end up with a manageable
impedance, and should require only series capacitance to bring it to
resonance. The SWR should be manageable (below 2.5:1).
In my case, the two towers were readily fed with only a series
capacitor made from less than 3 m (10') of RG213. Both presented an
SWR of less than 3:1, and could also be used on 80 with very good
performance. The SWR on 80 was somewhat higher.
Chris R. Burger
ZS6EZ
(also N3EZ these days)
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