[Amps] crossmodulation in PA ? Thanks for all good advice

[Peter Chadwick]

I'm inclined to the 'rusty bolt effect' theory. 
At one time, Plessey Microwave Materials sold an expensive 'anti intermodulation grease' which was used by a number of navies - a   sea water proof conductive grease for stays, chain etc. This was because rusty bolt effect caused trouble at HF on ships, with powers of 500 watts or so to 30 foot vertical whips, receivers which were pretty good for overload, and which tuned preselector filters.

Finding the culprit is more difficult, and Tom's suggestion of moving the tx power up and down and seeing how the unwanted products move could porve interesting - you may find that there's a threshold below which they suddenly disappear. That would suggest there's a dielectric brealing down at acritrical power level, which may make things easier to find.
If it is rusty bolt effect on the tower/antenna/connector etc, bandpass filters won't do anything for you, of course.

73
Peter G3RZP