REFLECTOR: Blue Mountain Mag Swing on Broadcast

[reflector at velocityxl.com]

Has anybody solved an issue with thier EFIS mag heading swing when they broadcast
 
I've got a XL and BMA Gen 3 EFIS/1.  I've worked pretty hard to get the heading accurate (including moving it from the nose/canard area to the strake where there's no metal aside from the wing bolts, which are about a foot away, where the com coax runs through to the winglet), but now I find that when taxing around the airport, if I broadcast the mag swings way to one side, and then returns when I unkey the mic. How irratating. Anyway, I ran shielded wire to the mag, which I think is unterminated on the mag end, and terminated on the EFIS end, but I'll have to check. Thoughts?
 
Also, the strobe wires run through the same area, but don't seem to affect mag heading.
 
Brett