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RE: 1/4"3m ribbon helix reply to replies



Folks

For the past two or three years at the AMSAT-UK Colloquium we've had an
antenna range set up where, amongst other antennas, we test all manner of
helices.

The general line is:

o Helices in contact with any former (PVC or otherwise) perform badly.

o Helices with an off-centre boom not only have mediocre performance, but
also their pattern is way, way off centre (15 or 20 degrees).

The microwave oven test is not enough. As has already been mentioned, the
dielectric effect of the PVC is significant enough to de-tune the helix. I
documented this a while ago at
http://www.g6lvb.com/radcomclarifications.htm#Fragile

This being the case, if one can model the helix taking into account the
dielectrc properties of the former, then you can make some improvements.
Trouble is the last time I went into Home Depot I didn't see documented the
dielectric properties noted on any of the PVC pipe I saw. You'll have to
measure it emprirically I'm afraid.

On a slightly different subject, I was very pleasantly surprised how
favourably the TVRO offset fed dishes with helical feeds performed on the
antenna range with respect to gain. One could make favourable comparisons to
patch prime fed dishes such as the well-respected G3RUH patch/dish.
Unfortunately, we had no means of checking the G/T: the patch feed will
probably outperform a helix in this regard.

BTW my 1.1m umbrella dish turned in a gain of 24.5dB which was not far off
what I was expecting, as being a deep dish it is under-illuminated, and
there are understandably some aberrations from a true parabolic surface.

73 Howard G6LVB


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