Topband: Unun Failure

[K3SV@aol.com]

I installed a Bytemark/Amidon Unun at the base of an inverted L, 72 feet 
vertical, 60 feet horizontal. The Unun was matching  36 ohms of the antenna 
to the 50 ohm feedline.  At 100 watts everything was fine, a good match at 
1830 with a fair amount of bandwidth, when I went to 500 watts output, the 
match went out-to-lunch.  Any ideas, there is no visable damage to the Unun, 
did the core get saturated?  I went back to 100 watts and the match was still 
out-to-lunch.

Thanks, 73 Bill