[amsat-bb] EZnec experts? (cubesat Circ Antenna)

Robert Bruninga bruninga at usna.edu
Fri Dec 6 08:58:20 PST 2013

EZnec kibitzers welcome:

See this model image:  http://aprs.org/psat/VHF-circ-tuned-ask.gif

We are placing two ¼ wave VHF whip antennas on a single feed at the corner
of a 1.5U cubesat.

We want them +/-45 degrees out of phase so that we get cross polarization
when paralleled to a single feed point for simplicity.

Modeling each one individually, we got the desired short one to -45 degrees
with a coordinate of 23.04” and the long one to +45 degrees with a
coordinate of 25.7”.  Great…

But when we connect them both to the same feed point, we get a combined
real R term about half of the others as expected, but do not get the
desired J0 term but get something near -45 degrees and a resonance several
MHz higher.

To fix that, we lengthened each exactly the same amount to get the
resonance down to 145.8 MHz and J=0 term.   Good antenna….

The QUESTION IS:  Are they still 90 degrees out of phase and will they work
as a cross polarized antenna?  And how can we verify that?

The feed point is the center of a 0.1” wire between the cubesat wire frame
and the combined end point of the antennas which are also 0.1” above the
top of the cubesat.

Maybe I should have included both wires in the original model, but only
connected one at a time to a source and grounded the other one  while
trying to find their individual +/-45 degree lengths.  This way, their
passive action as a possible counterpoinse to the whole frame may be
accounted for.

Also, to get within the 500 data points for EZnec, we had to make all of
the wire model 1” segments be a single segment per wire.  Is fidelity
maintained with only 1 segment per 1” wire?


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