On Mar 16, 12:21*am, Roy Lewallen wrote:
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. (Seehttp://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

Thanks for the clarifications, Roy. Indeed, with my last slightly
cryptic comment about considering that currents might not be in phase,
I was wanting to communicate that you always want to check the
currents on the elements to make sure they do what you want. That's
true no matter how you feed the antenna, though as you say the feed
you use may aid in insuring that the currents stay the way you want.

I'm a bit surprised about your comment about the coaxial (fed)
collinear requiring a "pretty delicate balance" between coax
propagation velocity and (presumably) radiating element geometry.
What I've found in my simulations is that I could change the coax vf,
keeping the elements a transmission-line half wave long so that the
feedpoints were all the same in-phase voltage, and the net gain of the
antenna for a given physical length was only slightly affected. I'd
typically see a couple of the elements in a ten element array with
considerably lower current magnitude, but the currents were nearly in-
phase on all the elements, and the pattern was always the desired
"flat pancake". On the other hand, I wasn't trying for any up or down
slope to the pattern, and I can see that things might change in that
case. With the propagation velocities I was using, between 0.66 and
about 0.9, and the element diameters I was using, I suppose the
elements were always shorter than resonance, and the self and mutual
impedances were not changing in any dramatic fashion.

Or, perhaps my model was all screwed up! ;-)

Cheers,
Tom