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[TowerTalk] Feeding one antenna in a stack

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Subject: [TowerTalk] Feeding one antenna in a stack
From: (Pete Smith)
Date: Fri May 30 22:04:39 2003
I've been doing some modeling of various stack configurations, to explore 
the effect of various feedline options, and I thought the results might be 
of interest.

The basic stack is a pair of Force 12 C-3Es at 97 and 69 feet over average 
ground.  At 14.1 MHz, using NEC-2, the stack shows gain of 14.43 dBi at 11 
degrees takeoff angle, when the two Yagis are fed in phase.

If you feed the bottom antenna only, with no feedline on the top one, the 
model shows 10.38 dBi at 17 degrees.  This result is strikingly less, and 
at a higher takeoff angle, than the same antenna shows, at that height, 
without the second antenna in the near field (12.06 dBi at 14 degrees).

Next I simulated a 20-foot 50 ohm feedline connected to the top 
antenna.    With that feedline shorted at the far end (for example, at a 
stack switchbox), the bottom antenna showed a gain of 11.81 dBi at 15 
degrees.  With the same length of feedline open at the switchbox end, 
the  bottom antenna showed a gain of 12.24 dBi at a takeoff angle of 13 

Now clearly, this result can't be extrapolated to other antennas.  The 
C-3Es have an open cell feed, and are short-boom two-element designs.  What 
it DOES suggest is that it isn't sufficient to consider one antenna of a 
stack as if it's alone up there, even if you're only feeding that one 
antenna.  What you do with the feedlines of the other antennas can make a 
real difference.

I'm planning to model some monoband stacks next, and will report results, 
if there's interest.

73, Pete N4ZR
The World HF Contest Station Database was updated 9 May 03.
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