Ran into something interesting this morning. A friend on one of the
reflectors had the idea of building a 3-high stack of C-3s with 19-foot
spacing (42-61-80), thinking that he could feed the 3-stack on the two
higher bands and only the upper and lower antennas on 20, thereby avoiding
the spacing compromises that tribander stacks otherwise face.
Anyhow, I woke up very early this morning, and decided to model the stacks
with EZNEC. Performance on 15 and 10 looked very good, as you might expect
(although the F/B ratio was degraded a bit on 10). But with the upper and
lower antennas fed, and the middle one passive, the performance of the
stack on 20 was worse than a single yagi. I looked at the currents, and
for some reason the energy coupled into the passive yagi appears as
currents that are wildly out of phase with those on the driven antennas.
It was actively fighting the stack.
Is this a well-known phenomenon in stacks, and I'm just coming late to the
party? Or is this maybe part of the reason why people with stacks report
they use them in the all-fed configuration most of the time? Has anyone
modeled a 2-high stack with the top one fed and the bottom one passive, or
vice versa? Maybe if I have another sleepless night...
73, Pete Smith N4ZR
In wild, wonderful, fairly rare WEST Virginia
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