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Re: [TowerTalk] Rohn 25 and stacking questions

Subject: Re: [TowerTalk] Rohn 25 and stacking questions
Date: Thu, 22 Mar 2007 07:04:30 -0400 (GMT-04:00)
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HFTA is fantastic software for its purpose.  I use it extensively here.  One 
thing that it DOES NOT do from my observation is model stacking distances.  It 
seems to assume that you will get a combined 2.8dB gain at the center of the 
stack spacing.  To confirm this, model 2 20M yagis with 5 feet of spacing on 
flat ground vs 2 20M yagis with 50 feet of spacing.  Intuitively (and by 
modeling) we know that the 50 feet spacing will beat the 5 foot spacing.  But 
on HFTA, the reverse is the case because it seems to think that the 2.8dB gain 
will occur at the center point of the yagis and that center point is higher at 
5 foot spacing than at 50.

I would try to understand the best heights for your antennas at the individual 
heights and then make the best compromise decision on tri-band stacking 
distance vs. information learned on HFTA for your individual location.  Good 
stacking distances should also equal balanced individual lobe coverage of each 
antenna.  A stack with one antenna at a bad individual height to the target 
area is not going to be a good stack.

"Guying Rohn 25 at 20/40/60/80" why?  Why not just buy 45G and guy it at 40 and 
80 with 1/4EHS guys?  Seems a lot simpler and probably cheaper.  You had better 
use Philly guys if you are going to have 4 guys over 80 feet of tower.  The RF 
reflecting off of that much close metal in the air is not going to help your 
"in practice" performance at all.

Let us know how the project progresses.

Ed  N1UR

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