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Re: Arrow Mod



I see a few problems with making a pair of circularly polarized Arrows on
the same beam.  The first is that you would need something like an
octagon-shaped cross section since the Arrow elements need a flat surface to
screw into so you couldn't use the boom that comes with the Arrow which has
a square cross section.

Adding twice as many elements will make setup take quite a bit longer, which
is a big consideration in portable operations.   It would also nearly double
its weight which magnifies a problem when you're trying to cantilever it
from a camera tripod.  I had to build a balancer for my Arrow antenna when I
put it on the tripod mount.  Otherwise, I had to tighten the friction locks
every time I moved it which was a pain when tracking a LEO satellite.

I don't think a CP antenna would buy you anything on the FM birds like AO27
or UO14 which don't have circularly polarized signals.  It might be useful
on the FO birds, but I've worked them from a camera tripod-mounted Arrow and
it's a handful trying to adjust for Doppler on an inverting transponder and
track the bird manually at the same time.  Also, you would have to worry
about switching polarity (which means you need to add some coax relays) at
some point in the pass so you don't lose the signal completely.

I believe that the linearly polarized antenna suffers only a 3dB penalty
over a circularly polarized antenna when receiving circularly polarized
signals.  I found that when working the FO birds that the polarity of the
Arrow antenna made little difference in the downlink signal strength.  It
makes all the difference in the world when working the FM birds, hence the
need to 'twist' the antenna to get the best downlink signal.   Most camera
mounts allow you to twist the antenna to match the polarity of the linearly
polarized signals.

The final problem is tuning such a antenna.  The interactions between
elements that are perpendicular are minimal, but are likely to be
substantial at 45 degrees of separation.  I assume that this is why everyone
goes through the trouble of separating a pair of yagis on a fiberglass boom
to implement circular polarization.

73,
Lee Devlin, K0LEE
Greeley, CO
http://members.home.com/k0lee


> I was sitting here this morning wondering if it would be possible to
> convert the Arrow design into 2 CP yagis on the one boom.  Could the
> additional elements be offset by 45 degrees?  Thus making it a 6 Element
CP
> Antenna on 2M with elements at 0/90/180/270 Degrees and 14 Elements on
70cm
> at 45/135/225/315 Degrees?
>
> I'm simply thinking about camera/tripod use.  This would provide a heavy
> (But not unwieldy) Mode J Antenna that would just about be the ideal for
> portable applications.  Would this be enough seperation, or would it cause
> too many problems?
>
> Dave, N8KXA


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