Having got the flight computer radio working with my homebrew antenna it was time for a couple of tests: test that the radio could actually transmit over a reasonably long distance and see how long the flight computer would keep running for.
Here's GAGA-1 ready for its trip (I made a little cover for the antenna to protect it on the journey):
10mW radio transmitter. As others have shown this works fine with line-of-sight and a good antenna. To run the test I got a helping hand from my parents who dropped me off at one end of Abberton reservoir with my Yaesu FT-790R and a Yagi antenna while they drove off to Abberton church with GAGA-1 sitting on the backseat transmitting a short RTTY message. The total distance between the two points was almost exactly 5.5km.
As they drove away I left the radio receiver on the ground with a little whip antenna on it. This was able to pick them up almost the entire way (at some points the signal fading, and while at the church there was a lot of noise).
Switching to the Yagi made the signal come in loud and clear, so loud and clear in fact that they never bothered to get out of the car. Over 5.5km from inside a car parked inside a copse the transmitter was working fine at the other end of this shot:
Here's a picture of the flight computer left and recovery computer on the right before mounting inside the box.