I vaguely remember fiddling with my vTX antenna in the shop whilst moving things around on the copter. But whatever the reason, today when I was testing my heavy lifter, the damn thing just fell off. Fortunately I was only 20 feet or so off the ground, and maybe 30-40 feet away from my launch position. But the funny thing is that the GoPro that was hanging under the copter caught the whole thing. Not exactly a classic "brown trousers" moment...more like a "WTF" moment. I'm sure glad I'm flying a X8 coax, or else that big RHCP 1.3gHz antenna might have made a mess out of one of my props (or worse). As it is, it just fell harmlessly to the ground.
Check your transmitter is still working properly, Steve -- typically transmitters get very upset when there is no antenna presenting an electrical load to the final output stage. You may not notice a problem until you get the transmitter some distance away from the receiver. Andy.
I've also noticed that those small screw-thread SMA connectors can vibrate loose -- so I secure them with a piece of PVC tape (aka "bodge tape" by glider pilots) around them. Andy.