Pico Balloon Project

My good friend, AL7CR, and I have been involved in a Pico Balloon project to send flights into the atmosphere and hopefully to travel some distance from the launch point before it ends its trip.  Around the globe would be really nice!!

This type of project is not new and many have been sent up all over the world.  In this blog I will describe the components of the project and the progress of our experiment. 

Pico Ballons

The choice of balloons ranges from inexpensive Chinese party balloons that you can purchased at  AliExpress to the larger weather balloon type  and although there are different grades of helium, a 99.9%  pure grade is probably the best one to try. 

Our payload includes a transmitter that will send the location of the balloon using it's built-in GPS receiver.  Although the transmitter produces a very low power signal, there is a network of amateur radio operators that detects the transmissions and reports the location to a central internet based network called WSPR net.  This network will track our balloon wherever it is in the world.  

Radio Transmitter

We use the WSPR-TX Pico Transmitter built by ZACH-TEK, a Swedish company.  The device is just the right size at 10.5 grams and is solar cell powered.  That means that when in the dark the transmitter will be quiet.

Here is the description of the transmitter.

The WSPR-TX Pico is a solar powered WSPR transmitter that is custom-made for Balloon flights.
It has the the following 

  1. Is a low power autonomous solar powered shortwave WSPR transmitter for the 20m and 30m amateur bands that continuously transmits its position and altitude during daylight. 
  2. Has an onboard GPS module and antenna for calculations of position and altitude.
  3. Runs open-source software on an Arduino compatible micro controller.
  4. The transmitter is the well known Si5351 and Phase-Locked-Loop oscillator that uses a Lab-calibrated Temperature Compensated Crystal reference Oscillator (TCXO) for exact transmissions.
  5. Weight 10.5gram
  6. Transmits its altitude using the WSPR Powerfield 
  7. Configuration is done with a Windows program. No programming skills required.
  8. Pre-built, tested and calibrated. Solar cells not soldered to PCB to make it sturdier during shipping, see assembly doc for instructions.  


  • Tranmission Frequency:
    10 and 14MHz with on-board low pass filter.
  • RF Power output:
    Slightly more than 10mWatt.
  • Size:
  • Power usage:
    0.3W (5V 20mA at idle, 60mA at transmit) Powered by solar cells or by USB to Serial converter
  • Weight:
    10.5g with solar cells

The WSPR Pico is a custom designed WSPR transmitter for balloon flights and uses the power encoding capability of he WSPR  to send its current altitude to complement the position report that is transmitted as a Maidenhed grid.  

The altitude is converted to dBm by internally dividing the altitude in meter by 300.

The result is rounded to the nearest value in dBm in the table.

To take an example - lets say the transmitter is on 11500m of altitude it will report that its power is 37dBm or 5W. (11500 divided by 300 is 38.33dBm - nearest value is 37dBm

Pico product picuture


Our balloons are Chinese Party balloons that we order through AliExpress.  They are quite reasonable at $1.30 each.  We prestreach these balloons prior to launch in order to extend the material to its limits.  This process is done using a controlled compressed air system that slowly admits the air to the balloon.  The prestreach control uses a Seeedurino microprocessor, two aquarium water pumps, a paddle type flow meter,  a pressure relief blow off valve and a FreeScale MPX 5010 GP pressure transducer.  The pressure transducer range is 0 - 10 kPa and is calibrated using a water manometer.  The flow meter is calibrated using a variable area flow meter that has a built-in control valve.  The operator of the prestreacher can control the speed of the pumps and also can control the flow using the valve in the flow indicator.  The operator can therefore control the rate of inflation of the balloon.