On Mar 14, 2:33*pm, Owen Duffy wrote:
* *a) * * * * | * * * * * * * * * * * *b) * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * | * * * * * * * * * * * * * * * * * * *|
* * * * * * * -------| * * * * * * * * * * * * * * * |
* * * * * * * -------| * * * * * * * * * * * * * * *|||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * |||
* * * * * * * | * * * * * * * * * * * * * * * * * * ---
* * * * * * * S * * * * * * * * * * * * * * * * * * *S
* * * --------------- * * * * * * * * * * * *-----------------

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

Hi Owen,

I suppose that R.W.P. King disagrees with the "common explanation."
He makes it quite clear that there is interaction of the antenna field
with the stub perpendicular to the axis of the antenna wire, and that
the coaxial stub does not interact in the same way and the antenna
performance is therefore different. (Antennas chapter of Transmission
Lines, Antennas and Wave Guides, King, Mimno and Wing.) This is why I
like using a feedline to guarantee the phasing. It can be done by
driving collinear dipoles with equal lengths of transmission line, or
by using an arrangement like the "coaxial collinear," where the
radiating elements are outer conductors of coaxial transmission lines
used to insure that the multiple feedpoints are at least fed in-phase
voltages (and you have to consider that the currents are not exactly
in phase).

Cheers,
Tom