The antenna was swinging on the mast, so anything up to about 30° out.
About 10 minutes after the start, the voltage at the rig started dropping to about 10V after a couple of seconds of transmit. This causes the rig to either FM or to just cut out completely. The voltmeter on by ignition-sense disconnect box wasn't showing a problem. I didn't manage to properly diagnose the problem but feeding the system from the cigar lighter socket seemed to fix the...