(its legality is another thing)

The use of 802.11 wifi is probably the fist thing that people think about when designing a balloon payload for the first time. So what sort of distances could be achieved? Well it depends upon the qualities of the transmitter, receiver and antennas.

Assuming: A Transmitter power of about 15dBm (about 30mW) – typical for most wifi routers. Payload antenna gain is 0dBi (omni-directional) A Large ground station antenna of 24dBi (e.g. 3ft diameter dish) The slowest wi-fi speed (1Mbps) – which gives the greatest range

1Mbps receive sensitivity is between -91dbm (average) and -96dbm (very good) for 
a (borderline) 10% PER (Packet Error Rate) .

This gives a maximum path loss of between:

15 + 0 + 24 + 91 = 130dB

and

15 + 0 + 24 + 96  = 136dB

Which at 2.4GHz equates to a distance of between 32 and 63Km depending on receiver

Licence exempt FM TV modules running in the 2.4GHz band are readily available in the UK for CCTV application – what's more more this band is explicitly allowed to used used in airborne TV applications. So what sort of distances can be achieved?

Assuming: A Transmitter power of about 10dBm (about 10mW) – all that is allowed. Payload antenna gain is 0dBi (omni-directional) A Large ground station antenna of 24dBi (e.g. 3ft diameter dish) A receiver sensitivity of between -85dB (average) -91db (very good) for a (borderline) 10db video signal to noise ratio.

This gives a maximum path loss of between:

10 + 0 + 24 + 85 = 119db

and

10 + 0 + 24 + 91 = 125dB

Which at 2.4GHz equates to a distance of between 9 and 18Km depending on receiver

The use of bigger antenna dishes would result in longer distances – as a general rule – doubling the dish diameter will double the workable distance.