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25 noiembrie 2013

ARHAB Special design antenna

A couple of years ago I was asked by Florin Mingireanu and Catalin Beledea to assemble a team for Stratospherium I experiment.
The goal was to have the balloon tracked via APRS both on air and ground after landing.
We had to use a Byonics tracker, the 10W version but we was limited somehow by the battery due to the limit in weight for the payload.
Instead of maximizing the transmitted power and using the maximum power, I was thinking to use less power but with a better antenna. After all, this is how we are doing on QSO's, no?

Ground Plane antenna diagram
Using a ground plane antenna which have the main lobes above the ground plate or counterpoises was out of question. Other disadvantage was the big null below the radiator. If the antenna was put downside, the second null (the one continuing the radiator) was to be exactly where the tracking team was, so it was the time to design something new.
I approach the new design having in mind that the receiver has to be below the payload so, the diagram has to be relevant in this.

I used MMANA-GAL for simulations and, after various (crazy) designs, the turnstile antenna capture my attention.
Classic Turnstile antenna
But the "classic" turnstile was, somehow, too heavy for our balloon so it has to be simplified.
Well, with not too much expectations, I put into MMANA a simple 2 x 2 dipole to see if my intuition will show something interesting. The ideea was to put two dipoles in paralell to lower the impedance close to 50 Ohm.

The simulation result was encouraging so I trimmed further more the design and made a few PCB to verify the theory. Of course, there was a lot of coat hangers but, fortunately, my wife is very understanding!
I measured the SWR with a Anritsu Sitemaster 331 and the results was well under 1:1.2!
The final result is below; the drawings are hand made but I hope that are clear to everyone who want to made this antenna.
In "real life", the antenna performed very well, radiating a lot below the balloon and very little at low negative angles. Therefore, distant reception was not so good but under the "belly" it was plenty of RF. We used only 3W from the tracker and all was OK.
The same antenna (not only the same design but the same PCB and wires) was used in Stratospherium II and Eclipser I in Australia in 2012.
We observed two more "features": the antena performed well when the payload was on the ground (we received the signals from the payload at about 4 km) and the antenna helped the payload to remain stable, countering the drag force from the wind on the parachute after landing.
As for the name of this antenna, I don't know! Leave a short comment if you have some ideeas. Or questions.
73 de YO3HJV, Adrian
The wires

The PCB

Assembly
MMANA-GAL simulation
SWR results with ANRITSU SITEMASTER 331

Before launching Stratospherium I

Before launching Stratospherium II





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