After some not-very-scientific testing I've determined that there isn't enough heating in the antenna to affect the tuning to any noticeable degree. The next possible culprit at this stage seems to be the coax.
Now that I have had time to think about it the coax has been abused enough lately that it is entirely possible it was pinched ... it is cheap enough that the insulation is a foam dielectric and one pinch would move the center conductor unacceptably close to the little shielding it has. Prolonged PSK31 transmissions would heat the pinched area ... further compromising the insulation and increasing the reflected power ratio.
Something else that cropped up during testing was the different resonant points for the loop antenna when measured near the antenna (but not near enough to affect tuning) compared to the end of the coax ... 7.035Mhz at the antenna & 7.040Mhz at the rig ... doesn't seem right at all.
Time to get the wallet out and buy some better coax ... this time I'll run a length of through the wall to the loop antenna, out of the high traffic areas!
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