A quick google of "fpv long range video transmitter" will find lots of info
on the topic. A number have received video from 10-15 miles out and check
out
http://fpvlab.com/forums/showthread.php?508-New-long-range-distance-record-%
26%238206%3B86-93km for an example of video received from 87 km away.

Lower frequencies offer better range than higher ones and 2.4ghz has more
than its share of wi-fi, blue tooth and similar signals so 1.2 ghz is what
many find the best to use. A HAB payload will almost always end up spinning
somewhat so to avoid antenna polarization issues it would be best to use
circular polarized Tx & Rx antennas. On aircraft a omni-directional antenna
must be used but the big advantage with HAB payloads where the Tx antenna
will always be pointing in the same direction (down) is that a higher gain
directional TX antenna can be used. The higher the gain the narrower the
signal beam is of course so just how much gain you decide to use depends on
how far away, horizontally, you want to be able to receive the signal. Our
group likes to stay fairly close to where the balloon is while tracking so
when we try this I figure a 90deg beam spread will be plenty. One could try
and use a much higher gain with even less beam spread, but one must also
allow for the fact that the payload may swing about some leaving the antenna
not always facing straight down. Anyway, do some research and have fun
trying which, IMO, is what it's all about.

Barry