There are a large number of licence exempt radio communication modules available (e.g. see link) - however there are only 3 types that may be used for air to ground communication (see link) - these are:

• Radio Location: 173.7 – 174 MHz and 458.9625 – 459.1000 MHz 10 mW ERP (Effective Radiated Power) 12.5 kHz or 25kHz bandwidth
• Data Transmission: 434.040– 434.790 MHz 10 mW ERP 25 kHz bandwidth and 433.05 - 434.79 MHz 1mW ERP 25kHz bandwidth
• Video: 2.400 – 2.4835 GHz 10 mW EIRP (Effective Isotropic Radiated Power) 20 MHz Bandwidth

Modules must conform to EN 300 220

And as of September 08 - 500mw on 869.40- 869.65 MHz < 10% duty cycle

Given the very low powers (10mW is about the power used by a dim LED)- its going to take some doing to use these in balloon to ground communication - but it can be done - over 400Km has been achieved.

Below is an example of a 10mW NBFM (Narrowband FM) transmitter module for use at 434.650MHz.

The Radiometrix NTX2 is the module of choice for UK HABing.

• http://stakeholders.ofcom.org.uk/spectrum/information/licence-exempt-radio-use/licence-exempt-devices/short-range-devices-information - Useful Ofcom Short Range Devices Information Sheet
• http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-policy-area/spectrum-management/research-guidelines-tech-info/interface-requirements/IR_2030.pdf - The full requirements document.
• http://www.rfsolutions.co.uk/acatalog/index.html