On Jun 20, 2:09*pm, spamhog wrote:
MY CURRENT CANDIDATE
Amidon cyl. core for HF PTOs R61-050-180
1.8" length x 1/2" diameter, material 61.
Excess length is necessary to keep linearity throughout the range.
I wish diameter was larger, closer to a roughly square length/diameter
ratio, but that's what's available.

SECOND BEST (probably WAAAY SECOND)
A cylindrical RFI core. I can count many many reasons not to use one,
but I wonder if I am grasping at straws.

I know you're going super gorilla on all this... but attacking this
from the low end up: the KD1JV brass-screw-tuned PTO is very easy to
realize and while not super duper stable, it's about as good as you
can do for a couple bucks. He has examples of covering 5.0 to 5.5 MHz
(75M SSB rig):

http://kd1jv.qrpradio.com/ssbrig/SSB.HTM

and covering 3.0 to 2.95 or so MHz (40M CW rig):

http://kd1jv.qrpradio.com/ARRLHBC/ARRL_MMR40.html

Unlike ferrite materials where putting the core in increases
inductance, putting a brass slug into a coil reduces its inductance.

KD1JV's designs are supremely reproducible. He does not use unobtanium
parts, that's for sure!

I have used these and while they are not awful temperature stable by
themselves, with some compensation maybe they could be better.

Tim N3QE.

Tim.