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