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I did a little web checking on air variables and I am surprised that the manufacturers don’t specify the Q or DF. Search “Q of air variable capacitors”. Mostly they just claim the Q is high. One guy has a site that describes his results from measurements. I believe a properly working, good quality air variable should have a Q of at least 1000 (DF=1/Q=0.001), and likely significantly higher. Heck, even a ceramic NP0/C0G capacitor has a DF=.001, so you would think any sort of air variable should be better.
I use a Johanson 8050 .6pf-6pf air variable piston trimmer (http://www.johansonmfg.com/pdf/Air-Capacitor.pdf) in the forthcoming AMT5000. Johanson specifies the Q to be >10,000 (DF<.0001). I think that's about as good as it gets. BTW, not that it has anything to do with air variables, one of the best application guide and technical analysis I have seen is found on pages 4-9 of this document from Kemet: http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfilesname/F3101.pdf/$file/F3101.pdf