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2kmtr How to Build the World’s Lightest Quadrifilar Helix Antenna

Project White Star, those intrepid souls trying to fly a weather balloon across the Atlantic, built an excellent helical antenna to be featured as part of their next payload. (The one pictured above is not their final antenna, but one of the experimental ones.)

After the scrub on the launch pad for flight attempt A, we went back to the books, to try and figure out what we could do to improve our odds the next time around. One of our biggest setbacks was the inability make an antenna suitable for our use: We needed an antenna tuned for 149 MHz, not needing a ground plane, weighing as little as possible.

After 4 tries, and some expensive test equipment, the end result was a Quadrifilar Helix antenna weighing only 80 grams!

For the metrically challenged, that’s the same weight as five empty soda cans!

Our ground test antenna was a 5/8ths wave whip antenna, which works well, but unfortunately needs a ground plane. Tests with both a quarter-wave dipole and a J-pole antenna were lackluster. Documentation from our satellite service provider implied that a quadrifilar helix antenna would provide the best coverage at all. While these antennas are pretty, their design and construction was voodoo magic at first.

Thanks to some design documentation here: and some help from the balloon community, we had some baselines for creating such an antenna. We still went through *quite* a few revisions. We went through 3 revisions that didn’t work, and one which works pretty darn well!

John Baichtal

My interests include writing, electronics, RPGs, scifi, hackers & hackerspaces, 3D printing, building sets & toys. @johnbaichtal

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