High altitude ballooning is an interesting hobby where you can launch a balloon into near space (100,000 feet) with a payload attached. The payload could consist of an RF tracking device so that you can follow the balloon, a digital camera to take images or an array of sensors like temperature, pressure and radiation.
The clever folks over at the UK High Altitude Society have most things covered from how to build a simple, licence free, tracking radio through to sizing your balloon to ensure the correct amount of lift. They even have a Google maps mash up where you can track the balloons live.
The tracking depends on a voluntary distributed listener network. These listeners tune their receivers to the correct balloon frequency (usually 434.075 or 424.650 here in the UK) and feed the audio into a computer program called dl-fldigi. The program decodes the audio signal and uploads the received details to the tracking page.
I decided that I don’t want to build a balloon. There are already several good teams of builders out there who are pushing the boundary and creating ever more impressive payloads. So I thought I would work on the reception of the tracking data. In the UK the licence free tracking device that transmits information about the balloon such as altitude, position and battery status is very low powered (10mw output). This means that a good, high gain receiving antenna is needed to pick up the signal at a good enough strength to allow it to be decoded. High gain antennas need to be kept pointed at the balloon as it moves across the sky and so project HABAT (High Altitude Balloon Antenna Tracker) is born.