On Sun, 12 Oct 2014 14:49:42 -0700, Jim Thompson
wrote:

...snip...!!!!!
That's pretty much what I expected. But I also have a 5mm "above the
PCB" maximum... do drum/pot cores come that small a diameter?

...Jim Thompson

Stop it! you're going to optimize a weak design following this path. If
you want to avoid FCC scrutiny stay below 10KHz. Don't even use ferrites,
no real need. You aren't transferring energy, you're transferring
'information', where almost NO power still will tell you stuff.

ARRRGGG!

I've got a system in mind that will cost less than $1 each in volumes of
100, and if you have some left over uController power, like inside TI's
MSP430, the rest is FREE! jeeesh!

ferrites?! barbaric! plus weight, mounting, wrapping, vibration etc etc
..... stay away from antique concepts, PLEASE!