[TowerTalk] Feeding one antenna in a stack
n4zr at contesting.com
Fri May 30 23:03:58 EDT 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
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
I'm planning to model some monoband stacks next, and will report results,
if there's interest.
73, Pete N4ZR
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