Thanks Steve (and Dave),
The possibility that the "Q" of older variable caps degrades over time because of environmental factors is interesting, and I appreciate your experimentation regarding this. That would certainly explain why the brand new variable caps from Mike's Electronic Parts always seem like hot performers out of the box, while the older ones installed years ago seem like duds by comparison. By coincidence I had replaced all of the variable caps in the larger Rockwork Cliff "DXpedition" FSL antennas prior to last year's trip, and did notice improved tuning performance.
<<< This is beginning to lend credence to Dave Aichelman's comments about these caps seem to deteriorate in selectivity/Q when used in FSLs that are exposed to cruddy conditions such as coastal salt air, and that cleaning the cap can restore it to "as-new" condition. >>>
I should thank Dave personally for this information, since during the past week I've had several requests from DXers who want me to replace the variable caps in their FSL's with the latest models, and now it seems that this type of work can be done by the DXer himself, without any effort on my part :-)
As for the "cruddy conditions" such as coastal salt air, Steve, I can certainly appreciate how that would accelerate the degradation of a variable cap. But in reality those kind of coastal locations frequently are ideal DXing venues for the compact FSL antennas, such as the wacky Rockwork Cliff in Oregon, where Tom R. has completely shattered the west coast record for South Pacific NDB's received during a DXpedition. Overseas salt water beaches are also prime FSL DXing venues, and they frequently combine extreme heat and humidity along with their salt air corrosion. I suppose it's a situation similar to pushing a race car constantly for top performance-- before you show up at a challenging race track, you need to "pay the piper" with new tires, new brakes and a perfectly tuned engine in order to be competitive. Before using FSL antennas at an exotic beach involving some serious travel $$, installing a brand new variable cap (or detailed cleaning of an old one) should be considered mandatory.