Our balloon experiment is using a 920 MHz frequency-hopping radio link (we also have 2.4 GHz radios, but will probably stick to 920 MHz). The balloon will carry a dipole or vertical bazooka tuned for 920 MHz, but it will be rotating and squirrelly. It will also be 30 - 50 km away. So I decided a good Yagi on the ground will help ensure we maintain our fade margin.
I looked at a couple options from L-Com. The best 900 MHz Yagi they had offered 14 dBi in 109 cm, but was $120. The next best offered 13 dBi for $44, so I bought it. I didn't realize until it arrived that it was 145 cm long!
So, this will prove to be unwieldy, at best, especially from a moving car. We might decide to have a fixed ground tracking station, but that approach will need investigation.