How to Build the World's Lightest Quadrifilar Helix Antenna

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!

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: http://jcoppens.com/ant/qfh/index.en.php 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!

Here are the antennas which didn’t work:

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SpeedBall Payload Details and Hinting at launch

Hello all,
White Star’s SpeedBall-1 flight and ground systems are are expected to be ready to fly within 2 days.

This flight is no longer a test flight, but a full trans-atlantic crossing attempt. That means we’ll likely be inserting it into the very next Jet Stream trans-atlantic crossing opportunity that occurs. We’re targeting <36h crossings, but the payload is designed to >72hr longevity in the air, so we may take a longer crossing if needed.

Launch will occur around 8PM Eastern Time from Space Port Indiana, in Columbus IN. SpacePort Indiana has graciously arranged for PraxAir to donate all helium for trans-atlantic attempts free of charge.
Launch Date will be announced 2-4 days in advance of the Jet Stream’s arrival. Please sign up for launch notification on our main home page to be sure you don’t miss it – http://www.whitestarballoon.org on the right side.

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How to Measure a Very Large Balloon

Hi everyone.  We’ve been working constantly at getting this Balloon off the ground.  Until now, though, we’ve had no way to directly measure how far “off the ground” we’d get. Fortunately, Brad and Dan spent their Friday night building just such a measurement device.  When you’re flying a constant altitude balloon like ours, the amount of helium it holds, combined with the weight of the robot pilot dangling off the balloon, directly determines the altitude that you will fly at. When your goal is to cross the Atlantic using the thin Jet Stream, you need to fly at a pretty precise altitude, or you’ll miss it entirely!

Unfortunately, our balloon manufacturer was unable to give us a precise volumetric measurement of the delivered envelopes. Unwilling to bet our sweat and tears on such a response means developing a method to measure the volume of air going into the balloon.

We came up with a few ideas that wouldn’t work:

  • Buying dry ice, letting it sublimate inside the balloon, and comparing the before and after weights. (It would take almost 110 lbs of dry ice to generate 1000 cu ft of gas! That’s expensive!)
  • Buying a canister of CO2, and measuring the pressure in the bottle before and after filling. (Compressed CO2 is as expensive as dry ice, and we don’t have accurate enough pressure gauges)
  • Filling the balloon with air, measuring the pressure inside the envelope, then adding a small quantity of dry ice, and measuring the pressure increase. (Requires a constant volume inside the balloon, which may rip its seams)
  • Making a “bicycle pump” mechanism, and adding measured quantities of air at a time (This would take FOREVER!)
  • Placing a temperature sensor and a heater in the air stream, to measure the heating effect in order to measure the air speed, which can tell you the volume that flowed through a tube over certain amount of time. (Would take too long to calibrate the temp sensor and heater.)
  • The last idea inspired us, though.  If you can measure the speed of the air you’re putting into the balloon, and measure the cross sectional area of the tube through which the air travels, you can measure the volume of air that’s moved through the tube over time!  It’s a simple equation:

    Air Speed (M/S) * Cross-Sectional Area (M^2) * Time (S) = Volume (M^3)

    How can you measure air speed?  Well anemometers work OK, but they’re big and bulky and would block our tube.  Airplanes measure their airspeed using a device called a “Pitot Tube.”  It looks like the image to the right.

    The Pitot Tube works by measuring the difference between the ambient air pressure and the ram-air pressure.  Air is forced into the opening of the pitot tube at high velocity, and the difference in pressure gives you the fluid velocity by the following relationship:

    Where (Pt – Ps) is the differential pressure, and Rho is the fluid density.

    That seems pretty simple, doesn’t it?  Well, we were able to pull it off with parts lying around the LVL1 hackerspace in one evening!

    We happened to have a long plastic conical nozzle, about 3 inches long, sold at auto parts stores as part of a miscellaneous kit of  vacuum hose adapters.  Looked pretty much like a pitot tube already!  It also happened to allow some tubing we had ( 1/6″ ID 1/8″OD) vinyl tubing to fit up right out the tip perfectly.

    We hot glued the tubing into the nozzle. and mounted it on a thin sheet of aluminum with a cable tie and hot glue.  This would allow us to position the pitot tube in the center of the airflow in the filling tube, with minimal disruption in the airflow itself.  Disrupting the airflow too much is BAD for pitot tube accuracy.  Streamline it, and the things around and behind it.  Make sure it points directly into the airstream.

    For measuring balloon volume, we don’t need helium, plain old air is fine.  So, we called upon our in-house air source: the high-power vacuum cleaner blower head from a shop vac.  It would make quick work of inflating the giant balloon.

    We embedded the aluminum strip, with pitot tube, parallel to the airflow in the hard plastic extension tube of our shop vac.  A simple slot was cut on each side using a dremel cutting wheel, and secured with hot glue.  (Use safety glasses!)

    The pitot gives us ram air pressure, which is only half of the answer – the other half comes from the static source.  Where do you measure that from?  Well, we think that it should be measured in the balloon, a ways away from the fast-rushing airstream coming in.  To do that, we the open end of another tube of the same type about 4 feet into the balloon, away from the direction we’d be aiming the blowing the air in.

    Now, with two tubes to give us air pressure from two different places, we just needed a differential pressure sensor to measure difference at the static location and the the pitot tube in the airstream.  Fortunately for us, a few months ago we had designed and built a sensor module, complete with a differential pressure sensor that perfectly covered the pressure range we needed to measure today.  We just pressed the static tube and the ram air tube onto the two ports of the sensor, and were ready to measure the pressure!

    We connected the sensor board up to an Arduino microcontroller, and wrote a quick program to read the pressure in kPa every half-second.  We connected the Arduino to a PC running a serial port data logger to save the numbers as a text file.

    The procedure to get volume would be: record the pressure every half second, inflate the balloon to full, paste the recorded pressure list data into a spreadsheet to convert pressure to airspeed, and then each airspeed * time to get volume of air each time period, and integrate that M^3 volume over time.  For our balloon, we ended up with 28.04 M^3, which is just about 990 cubic feet.  That’s a reasonable answer.  We think it’s probably right.

    One thing we know for sure, is it was easy, cheap and fast.  It should work with helium right on the launch pad, which *might* really help amateur ballooning launches, where it’s really hard to tell how much helium you’ve put in the balloon.  We’ll publish some more details on how to precisely replicate this in the future.

    Pictures follow:

    Progress is furious!

    Hello all, we’re working our little tails off to meet our December 18 launch date. It’s going to be a photo finish. We haven’t been updating this much due to the frantic pace we’re working day and night on all this. Quick update:

    All electronics designed and PCBs ordered.

    Mission control team members coming together.

    Balloon firmware is in very early stages of development.

    Public flight tracking websites are ready, but backend database software is just starting.

    Major parts such as satellite modem are all here.

    Balloons are built, but not to the specification we required. Still trying to figure out what to do about this. They’re supposedly now built to float several thousand feet higher than we asked.

    Documentation is being created for safety rules, procedures, and test details. Pics have been posted to the Flickr stream of the latest whiteboard diagrams and such!

    White Star Balloon in the News

    We wanted to thank everyone who has helped us feature the project so extensively in the media and at their local hackerspaces.  Below are links to the coverage.  Please check them out if you haven’t already.

    We also wanted to thank the Facebook and Twitter community for helping us promote our the Apples in stereo benefit concert and project!  Much good karma comes your way!

    If we missed anyone, please notify us at whitestarballoon (at) lvl1.org so that you are added and acknowledged!

    Blogosphere Community

    37 Flood

    Consuming Louisville

    Loueyville

    Make

    Make Hackerspace Weekend Events

    Hackerspaces

    BloomingLabs

    Hive13

    Newspapers

    Courier-Journal

    Courier-Journal Features

    Metromix Louisville

    NPR

    WFPL

    Podcasts

    Evadot Podcast