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RE: Helix antennas



Hi Zach,

Take a look at Kent Britains' WA5VJB "Cheap Yagi" designs: 

http://www.clarc.org/Articles/uhf.htm 

They are very easy to construct and are much less complicated to build
and physically support than a helical antenna at 2M and 70cm. Kent
designed them to be very forgiving of minor construction errors without
significant performance degradation. In other words if you are
reasonably careful when measuring, cutting, and drilling you can be
certain they will perform as advertised. That has been my experience for
the dozen or so that I have built and used.

In my experience the secret to success is to use mast mounted preamps.
Using Kent's "Cheap Yagi" designs and mast mounted preamps working all
of the currently operational analog and digital LEO's is a whole lotta
fun! I'm a big fan of being able to actually hear the satellite that I'm
working ;-) 

Note: for a great explanation of why preamps are important read David
Bowman's G0MRF preamp article on page 24 of the July/August 2005 AMSAT
Journal.

RE: Are circularly polarized antennas required for LEO's ?

As a test I ran a switchable polarity 9el x 9el commercial 70cm antenna
on the mast for a week or so. I found that the LEO birds signal
polarization does change during the pass and the fades can be 20dB or
more requiring one or more flips of the polarity switch(s) during the
pass. I went back to linear antennas for simplicity's sake. With medium
gain linear yagi's and mast mounted preamps there is more than enough
signal to noise ratio for armchair copy with the LEO's. 

73 de Steve, WI2W FN20

p.s. - thanks for the AO-51 contact on 5/9/06 !



-----Original Message-----
From: owner-AMSAT-BB@amsat.org [mailto:owner-AMSAT-BB@amsat.org] On
Behalf Of Zachary Schrempp
Sent: Wednesday, May 10, 2006 9:05 PM
To: amsat-bb@amsat.org
Subject: [amsat-bb] Helix antennas


Hello folks,

I am looking for advice on building helix antennas for satellite work.
Is anybody using helices for both V and U mode to yagi-comparable
success, and if so, how many turns are you using? I have been over the
basic overview in the Satellite Handbook and the helix looks like an
inexpensive, easy-to-build yagi alternative (if not capable of
switchable polarization). In real practice, has the lack of the
polarization switch from RHCP to LHCP really caused much difference?

I have been operating FM birds with handhelds for about 6 months and I
am getting to work on a base station that will be capable of working the
linear satellites as well. I'm trying to avoid the extra cost and
complexity of building circularly polarized yagis. Any suggestions would
be appreciated.

-- 
Zachary Schrempp, KE7EYQ
AMSAT #36023
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