I was at about 1400ft. The transmitter comes with a small whip aerial so arguably does pretty well already all things considered. I'd like to replace it with something better, but access to the turtledeck of the aircraft isn't easy.
I've just google-mapped the distance I was trying to transmit - about 15 miles and more than I realised - so in retrospect I think it was unfair of me to expect it to work.
My radio does 8.33 already so I hope I don't need to change it - I forget the exact model.