//www.curezone.org/forums/fm.asp?i=1470194
It seams that lower frequencies are able to get deeper in to the tissue (as deep as intestines).
Others claim that theoretically higher frequencies have this ability, best is to build your own with one with lower and one with higher frequency and try both for your self.
Mouser.com has fast shipping low prices and low shipping fee ($6). You probably can build both versions under $25 dollars including enclosures (if you already own soldering iron walmart sells 25W unit for about $12, you will also need wires and solder).
this link is to parazapper schematic with dual frequency selector in one device using second switch for the HZ selector:
http://www.paradevices.com/schematic.html
Here are more links to zapper schematics:
http://www.royalrife.com/zapper727.html
http://keelynet.com/biology/zapper.htm
This link points out some zapper features to look for:
http://www.paradevices.com/best-zapper.html
This one shows you how to build the original shoe box zapper:
http://www.zapperlab.com/
Additional info:
//www.curezone.org/forums/fm.asp?i=1104588
I think this should have your head scratching for a while.
Good luck,
Gill.