a) | b) |
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S S
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Fig a) above is an attempt to portray a colinear vertical over infinite
ground with a source at "S". The configuration is easy enough to model in
NEC with sensible results.

The common explanation for operation of a) is that the U shaped section
is a quarter wave s/c stub, that it is responsible for delivering direct
in-phase drive to the upper section, and that it plays no part itself in
radiation ie, that the common mode current on the pair of conductors is
zero at all points. Notwithstanding the conventional wisdom, it seems
unlikely that there is no common mode current on that section, and NEC
models suggest that there is, and that it accounts for some small
asymmetric distortion of the pattern.

Fig b) above is an attempt to represent a coaxial arrangement of tubes
where the lower end of the tubes are connected together, and that is fed
at S against an infinite ground.

My questions a

1. To what extent is b) equivalent to a)?

2. How is b) modelled in NEC?

Thanks
Owen