I have an rf generator with auto tune circuit driving motorized plates. A loop antenna provides feedback. The existing is approx 6" diameter. What length should the circumference be for 13.56mhz?

 

Hello,

Have a look at these ham radio pages:
http://www.qsl.net/g3pto/loop.html
http://www.standpipe.com/w2bri/
http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
http://9m2pju.blogspot.nl/2011/04/9m2sxs-40m-20m-magnetic-loop-antenna.html

The 20 meter designs are the the closest to what you are looking for as the 20 meter hamradio band is from 14 to 14.35 Mhz

Bertus