Re: Cap dielectric

In my previous post, my mind was thinking polyethylene but my fingers 
were typing polypropylene when I was talking about the "ideal" 
capacitor dielectric.  Polypropylene is also good for coil contruction 
(low loss, high dielectric strength, etc...), but the "state-of-the-art" in 
home-built pulse capacitors seems to be rolled polyethylene caps (R. 
Hull, R. Quick).  The low density polyethylene sheet you can buy from 
your local plastic dealer has good dielectric strength, low RF losses, 
and is relatively inexpensive and easily obtainable.  Large mica caps 
are expensive and from R. Hull's experience (reported in the TCBA 
newsletter), they ran warm while his rolled polyethylene caps stayed 
cool (higher losses in the micas?).  Glass is also lossy, bulky, heavy, 
and breaks easily.  The best thing about glass is that it's so cheap and 
ubiquitous, a salt-water capacitor bank  can be made for practically 

Steven Roys (sroys-at-radiology.ab.umd.edu)