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High Altitude Balloon Transmitter

SUMMARY: A small group of high school students taking an AP class for college credit launched a high-altitude weather balloon with a small payload. In addition to a video transmitter and GPS transmitter, they decided to include a simple transmitter built from scratch. This is the story of the project, with emphasis on the simple transmitter’s design, construction, implementation, and reception (which surprised me, being detected ~200 miles away and lasting the entire duration of the flight!) [sample.ogg]

6/16/2010 – TRACKING

I’m completely amazed at how well the transmitter/receiver worked! For only a few milliwatts, I was able to track that thing all the way from takeoff to landing in Gainesville, FL a few hundred miles away. Here is the data assembled in a special, annotated way!

CLICK HERE to view the signal tracked from Gainesville, FL

ANALYSIS: the text on the image describes most if it, but one of the most interesting features is the “multipathing” during the final moments of the descent, where the single carrier signal splits into two. I believe this is due to two Doppler shifts: (1) as the distance between the falling transmitter and the receiver is decreasing, producing a slight in increase in frequency, and (2) a signal reflected off of a layer of the atmosphere above the craft (the ionosphere?) before it gets to the receiver, the distance of which is increasing as the craft falls, producing a decrease in frequency. I’ll bet I can mathematically work backwards and determine how high the craft was, how fast it was falling, and/or how high the layer of the reflecting material is – but that’s more work than this dental student is prepared to do before his morning coffee!

HERE IS SOME AUDIO of some of the strongest signals I received. Pretty good for a few milliwatts a hundred miles away! [beeps.ogg]

6/16/2010 – THE FLIGHT

The launch:

This is the design team:

Walking the balloon to its launch destination at NASA with an awesome rocket (Saturn 1B – identified by Lee, KU4OS) in the background.

The team again, getting ready for launch. I’ve been informed that the reason their hands are up is to prevent the balloon from tilting over too much. I’d imagine that a brush with a grass blade could be bad news for the project!

Last minute checks – you can see the transmitter and battery holders for it taped to the Styrofoam.

The transmitter in its final position. Note the coil of yellow wire. That serves as a rudimentary “ground” for the antenna’s signal to push off of. I wasn’t very clear on my instructions on how to make it. I meant that it should be a huge coil wrapped around the entire payload (as large as it can be), which would have probably produced a better signal, but since I was able to capture the signal during the whole flight it turned out to be a non-issue.

The antenna can be seen dropping down as a yellow wire beneath the payload. (arrow)

Awesome photo.

Launch! Look how fast that balloon is rising!

It’s out of our hands now. When I got the text message that it launched, I held my breath. I was skeptical that the transmitter would even work!

One of the students listening to my transmitter with QRSS VD software (score!)

Video capture from an on-board camera was also attempted (900MHz), but from what I hear it didn’t function well for very long.

6/15/2010 – IMPROVED BUILD

Here you can see me (center arrow) showing the students how to receive the Morse code signal sent from the small transmitter (left arrow) using a laptop running QRSS VD (my software) analyzing audio from and an Icom706 mkII radio receiver attached to a dipole (right arrow).DSC_7082

I amped-up the output of the oscillator using an octal buffer chip (74HC240) with some decent results. I’m pleased! It’s not perfect (it’s noisy as heck) but it should be functional for a 2 hour flight.

Closeup of the transmitter showing the oscillator at 29.4912 MHz, the Atmel ATTiny44a AVR microcontroller (left chip), octal buffer 74HC240 (right chip), and some status lights which blink as the code is executed.01_closeup

This is my desk where I work from home. Note the styrofoam box in the background – that’s where my low-power transmitter lives (the one that’s spotted around the world). All I needed to build this device was a soldering iron. 02_workstation

Although I had a radio, it is not capable of receiving 29MHz so I was unable to test the transmitter from home. I had to take it to the university to assess its transmitting capabilities.03_room

At UF I used an oscilloscope to measure the waveform of the transmitter. 04_measure

I connected the leads to the output of the transmitter, shorted by a 39ohm resistor. By measuring the peak-to-peak voltage of the signal going into a resistor, we can measure its power.04_measure2

Here’s the test setup. The transmitter is on the blue pad on the right, and the waveform can be seen on the oscilloscope on the upper left.05_lab

Here’s a closer view.

With the amplifier off, the output power is just that of the oscillator. Although the wave should look like a sine wave, it’s noisy, and simply does not. While this is unacceptable if our goal is a clean radio signal with maximum efficiency, this is good enough to be heard at our target frequency. The PPV (peak-to-peak voltage) as seen on the screen is about 100mV. Since I’m using a x10 probe, this value should be multiplied by 10 = 1V. 1V PPV into 39 ohms is about 3 milliwatts! ((1/(2*2^.5))^2/39*1000=3.2). For the math, see this post07_no_amp

With the amplifier, the output is much more powerful. At 600mV peak-to-peak with a 10x probe (actually 6V peak-to-peak, expected because that’s the voltage of the 4xAAA battery supply we’re using) into 39 ohms we get 115 millivolts! (6/(2*2^.5))^2/39*1000=115.38. 08_amp

Notes about power: First of all, the actual power output isn’t 115mW. The reason is that the math equations I used work only for pure sine waves. Since our transmitter has multiple waves in it, less than that power is going to produce our primary signal. It’s possible that only 50mW are going to our 29MHz signal, so the power output assessment is somewhat qualitative. Something significant however is the difference between the measured power with and without the amplifier. The 6x increase in peak-to-peak voltage results in a 36x (6^2) increase in power, which is very beneficial. I’m glad I added this amplifier! A 36 times increase in power will certainly help.

The final schematic is here:

6/14/2010 – THE BUILD

Last week I spoke with a student in the UF aerospace engineering department who told me he was working with a group of high school students to add a payload to a high-altitude balloon being launched at (and tracked by) NASA. We tossed around a few ideas about what to put on it, and we decided it was worth a try to add a transmitter. I’ll slowly add to this post as the project unfolds, but with only 2 days to prepare (wow!) I picked a simplistic design which should be extremely easy to understand by everyone. Here’s the schematic:


The code is as simple as it gets. It sends some Morse code (“go gators”), then a long tone (about 15 seconds) which I hope can be measured QRSS style. I commented virtually every line so it should be easy to understand how the program works.

#include <avr /io.h>
#include <util /delay.h>

char call[]={2,2,1,0,2,2,2,0,0,2,2,1,0,1,2,0,2,0,2,2,2,0,1,2,1,0,1,1,1,0,0};
// 0 for space, 1 for dit, 2 for dah

void sleep(){
  _delay_ms(100); // sleep for a while
  PORTA^=(1<<PA1); // "flip" the state of the TICK light

void ON(){
 PORTB=255; // turn on transmitter
 PORTA|=(1<<PA3); // turn on the ON light
 PORTA&=~(1<<PA2); // turn off the ON light

void OFF(){
 PORTB=0; // turn off transmitter
 PORTA|=(1<<PA2); // turn on the OFF light
 PORTA&=~(1<<PA3); // turn off the OFF light

void ID(){
        for (char i=0;i<sizeof(call);i++){
                if (call[i]==0){OFF();} // space
                if (call[i]==1){ON();} // dot
                if (call[i]==2){ON();sleep();sleep();} // dash
    sleep();OFF();sleep();sleep(); // between letters

void tone(){
 ON(); // turn on the transmitter
 for (char i=0;i<200;i++){ // do this a lot of times
 OFF();sleep();sleep();sleep(); // a little pause

int main(void) // PROGRAM STARTS HERE
    DDRB = 255; // set all of port B to output
 DDRA = 255; // set all of port A to output
 PORTA = 1; // turn on POWER light

 while (1){ // loop forever
  ID(); // send morse code ID
  tone(); // send a long beep

I’m now wondering if I should further amplify this signal’s output power. Perhaps a 74HC240 can handle 9V? … or maybe it would be better to use 4 AAA batteries in series to give me about 6V. [ponders] this is the schematic I’m thinking of building.


This story was featured on Hack-A-Day! Way to go everyone!

About the author

Scott W Harden

Scott Harden has had a lifelong passion for computer programming and electrical engineering, and recently has become interested in its relationship with biomolecular sciences. He has run a personal website since he was 15, which has changed names from HardenTechnologies.com, to KnightHacker.com, to ScottIsHot.com, to its current SWHarden.com. Scott has been in college for 10 years, with 3 more years to go. He has an AA in Biology (Valencia College), BS in Cell Biology (Union University), MS in Molecular Biology and Microbiology (University of Central Florida), and is currently in a combined DMD (doctor of dental medicine) / PhD (neuroscience) program through the collaboration of the College of Dentistry and College of Medicine (Interdisciplinary Program in Biomedical Science, IDP) at the University of Florida in Gainesville, Florida. In his spare time Scott builds small electrical devices (with an emphasis on radio frequency) and enjoys writing cross-platform open-source software.

Permanent link to this article: http://www.SWHarden.com/blog/2010-07-14-high-altitude-balloon-transmitter/


  1. hmm

    Why not just have an On LED that is on whenever the transmitter is on?

  2. Scott

    This way we can ensure the micro-controller is actually executing code rather than simply being supplied power (this is also why I added the clock LED). Additionally, since the output is a combination of short Morse code and long tones, it’s helpful to know when it’s transmitting and when it’s not. Ultimately the LEDs are useless, but I had them on hand so I used them because they looked cool =o)

    Whenever you get the option to make something blinky, go for it!

  3. Dante

    The students were delighted and everyone was impressed with the clarity of the transmitter……well done fine sir!

  4. Lee

    Nicely done. The large horizontal rocket in the pictures is the Saturn 1B.


  5. Lee

    One additional thought. You should include your callsign in the transmitted CW so you don’t run afoul of the FCC. As you noticed you easily covered the State with the signal and under the right band conditions it could be heard anywhere in the world.


  6. Scott

    I agree! Not only will I add a callsign on the next attempt, but also a legal lowpass filter to eliminate those pesky odd harmonics ;-)

  7. Borat

    You have a make a very nice time.

  8. TrueBLUEGuy

    Congrats! You did a nice hack out there, you could just use a CIP-8E/D 8-bit encoder decoder pair to send more than morse code, which basically is OOK(On-Off Keying).

  9. Gary

    Great little project, very minimal weight. A thought occus to me on the noise and RFI issues, should you be gating the power on the oscillator to provide the output? From what I remember of these things they have a significant settle time after power on.

    Would it not be far better to gate the oscillator output signal with a single HF transistor and keep the oscillator on all the time, letting it settle and possibly reducing noise.

    Then once you extract the GPS line of sight velocity from the Doppler you automatically get the temperature delta from the remaining frequency shift (these oscillators do shift with temp in a predictable way), provided you do a calibration run on the ground first.

  10. Scott

    Yes and no. I found settling time was less than half a second, and that it didn’t affect Morse code copyability at all. In the spirit of designing the simplest-case transmitter, I still feel my design was optimal for the goal of the project. In future launches, I will certainly consider a much-improved transmitter.

    P.S. rather than gating the transmitter output, I’d just enable/disable the amplifier with the “output enable” pins ;-)

  11. Petar

    can you give me the oscilator product number. I search for oscilator here in Bulgaria, but i found only with 2 legs.
    Btw, i`ll use and modify your schematic – thanks for posting! :)

  12. Anonymous

    Just buffer the output with a couple of MOSFET/BJT transistors, and it’s easy to go over 5W without any RF-specific circuit complexity.

    Nice idea circuit! (even though it does not fit the international radio regulations…)

  13. Anonymous


  14. Anonymous

    The transmitter is easy enough to build, but what would I be looking at for a receiver? As long as I can get an audio signal, I should be able to interpret it, but how good does the receiver have to be?

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  17. ed kemp LZ2ILR

    try taking half of the buffer straight and the other half inverted then feed the 2 streams into either side of a dipole to double your output voltage . feed it into a mini magloop tuned from aluminium wire and that should be good enough as an antenna and output bandpass filter. if using dipole, keep one wire directly to the balloon and the other dangling from the payload. now do you think that you can lose the can oscilator and use 2 inverters from the buffer chip with a crystal direct? or use the crystal to clock the cpu itself to keep timing acurate. add morse routine to read from temp and at sensors (very light) or gps chip (bah more cash!) i made a similar thing years and years ago but we didnt have cheap pic’s then. i used a binary counter wired direct to a paralel acess static ram chip to play the cw. there were 8 lines available and the first one was the identifier and the other 7 lines sent numbers 0, 10,20,-….70 on loop and were simply switched in depending on temperature. very simple circuit, not elegant at all. but you could read the code on an am radio.

  18. zoran

    Good idea, but I saw this too late. So we use Pixie2 only for transmit. 1/4 wave antena , keyer is microcontroler single chip. Keying call sign 9A5B and other information. Work well on ground.
    See more at http://www.rasadnik-strk.com/9a9y click PiRHAB-2 and scroll down to English text.
    9A3HP Zoran

  19. Muhammad Owais

    Can this octal buffer be used with fm transmitter? to amplify the power?

  20. Kirk

    Spot on with this write-up, I really think
    this web site needs far more attention. I’ll probably be back again to read through more, thanks for the information!

    website (Kirk)

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