[amsat-bb] Re: Help wanted on CP antenna design

[Zach Leffke]

Hi Phil,
	I've flown a couple of balloons as an advisor to an undergraduate
lab here at Virginia Tech.  We use CP on all of our flights, though
generally we use the standard 2m APRS freq and not 70cm.  Our first two
flights we flew a crossed dipole suspended above the payload (to get it out
of the view of the down facing camera) and had excellent results.  Our
second flight flew the Byonics MicroTrak 300, with only 300 mW of power and
had no problems hitting digipeaters and iGates from 100,000+ feet.  The
crossed dipole was constructed out of aluminum arrow shafts (rigid, but
lightweight).  In an effort to drop even more weight, we changed to a design
that uses fairly rigid wire in a "turnstiled loop" configuration, more
commonly known as an eggbeater, again with excellent results from flight.
	In both cases we decided to forego installing the ground plane
mentioned in previous posts.  This was primarily a design decision made to
save weight at the loss of a few dB (energy radiated up does us no good with
receivers on the ground).  One design that I have wanted to try but haven't
yet had the opportunity is the crossed moxon.  Essentially, it is like
taking the reflector and driven elements of a pair of yagis and crossing
them at 90 degrees.  It is slightly different from the Yagis as the end of
the reflector is bent 90 degrees toward the driven and the driven ends are
bent 90 degrees towards the reflector (for impedance manipulation).  L.B.
Cebik, W4RNL, wrote a very useful article in the August 2001 edition of QST
describing the construction of the crossed moxon, complete with design
dimensions for a 435.6MHz crossed moxon (google can direct you to a copy of
his article).  I do not have information about the cross-pole rejection, but
some time spent with 4NEC2 antenna modelling software may give you a bit of
insight.  
	With the bends in the elements of the moxon it focuses the pattern a
bit more towards the horizon (over the crossed dipoles).  I'm not sure about
your location, but for us in SW Virginia our balloons get whipped right
along in the jet stream and we have trouble keeping up with them during the
chase (peak lateral speed in the Jetstream for our flights was 143 MPH).
The result of this is that at no time (other than the initial release when
the balloon is under about 10,000 ft) does the balloon get much higher than
maybe 20 degrees elevation.  Having energy focused more towards the horizon
and less towards zenith (both in the balloon antenna and the ground
antennas) is probably not a bad Idea in our case.

If you search callsigns KK4MOB and KK4PWM on aprs.fi you can see the path of
our flights and the insanely high speeds in the Jetstream (around 30k-40k
ft).

Hope the responses help inform your decision.
GOOD LUCK!

-Zach, KJ4QLP

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Zilvinas, LY2SS
Sent: Wednesday, July 17, 2013 6:33 AM
Cc: AMSAT BB
Subject: [amsat-bb] Re: Help wanted on CP antenna design


On 2013.07.17 10:11, Simone wrote:
> Hi Phil,
>
> QFH is effectively circularly polarized, it is often used for GPS 
> receivers, since when you point it skywards the main lobe points 
> trough the zenith, while nulls are pointing to the horizon (where you 
> do not have satellites normally).
>
> In my opinion QFH risks to be quite bulky for a balloon, since it will 
> occupy a cylinder 1 wavelength high (70cm in your case).
>

QFH for 70cm band is 264 mm height only (and 84 mm in diameter).

73!
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