Hi Jim,
I just plugged a quick model into MultiNec and got results similar to
yours in terms of feedpoint impedance and pattern. The 2:1 VSWR
bandwidth (12.5 ohm reference) is about 175 KHz with antenna
centered at 14.1 MHz which isn't bad for a compact antenna. His
claim that the vertically polarized radiation off the ends is reduced
doesn't seem to hold water, however. If you plot the V-pol field
for the CobbWebb and a reference thin dipole at the same height
(30 feet in this case), you get a similar V-pol component off the
"ends" of the antennas. The electric field off the tips in the near
field may indeed be lower, however (I didn't look at that yet).
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
> Well... given that it's 1/4 wavelength on a side, the closest you could
get
> another one is 1/2 lambda away, a lot farther than the spacing of a
typical
> parasitic element in a Yagi. If you spaced it, say, 1/4 lambda apart,
(3/4
> lambda center to center), then the mutual coupling would be quite low.
> There are some other stacking arrangements though.. you could have them
> partially overlap, for instance (like dipoles in echelon).
Actually it's 1/8 lambda on a side (you were probably thinking
about a full wavelength quad - easy mistake since that's what
it looks like sans the gap). Also, it just occured to me that you
could mirror image the parasitic element so that the current
carrying portions (feed side for the driven elements) were
adjacent to each other (gapped sides opposite each other).
Since most of the current is on the feed side, the phase center
will probably be closer to that edge. With the above configuration
(current carrying sides placed close together), you should be
able to get close effective parasitic spacing. I'll take a crack at
modeling that. Might be interesting - 2 element CobbWebb
Yagi.
73 de Mike, W4EF........................................
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