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

In most phased arrays, the objective is to get the fields from the
elements to be in some particular ratio. Driving them with currents in
that same ratio doesn't always accomplish the desired field ratio,
though, when elements have different current distributions as they often
do. (See http://eznec.com/Amateur/Articles/Current_Dist.pdf.) The
difference between field ratio and feedpoint current ratio is
particularly great when base feeding half wave elements. As it turns
out, you'll often get better field ratios by feeding with voltages
having the desired magnitude ratio and phase difference than feeding
with properly ratioed currents, when dealing with end fed half wave
elements. The coaxial collinear requires a pretty delicate balance of
outer and inner velocity factors as well as the effects of mutual
coupling, particularly when there are more than a couple of elements. So
I suspect that the current distribution can either help or hinder
depending on how the factors are traded off. I wouldn't be surprised,
though, if ratioing the voltages rather than currents is actually helpful.

As an illustration, open the EZNEC example file Cardioid.EZ. Change the
number of segments to 10 per wire for better accuracy. (It can still be
run with the demo program.) Click FF Plot and note the nice cardioid
pattern. Then change the Z coordinates of End 2 of the two wires to 0.47
m to make them nearly anti-resonant, and click FF Plot again. The
pattern deterioration is due to the elements having different current
distributions. Finally, change the source types from I to V. This will
force the voltages, rather than currents, at the antenna bases to be in
the desired ratio. Run FF Plot again. You still won't have the nice
cardioid back, but it's quite an improvement over the pattern with
"correctly" ratioed base currents. The bottom line is that the element
currents are more closely related to the base voltages than the base
currents, when the elements are near anti-resonance (parallel, or half
wave, resonance).

Roy Lewallen, W7EL