[Amps] Tufnol rod

[peter.chadwick at Zarlink.Com]

Tufnol is, as Steve says, a proprietary brand of SRBF (Synthetic Resin
Bonded Fibre). However, it's also available in thin sheet form (1/32 and
1/64 inch thick) which I suspect is a somewhat different material, as in
this form, it is supposedly moldable under heat and pressure. It's also (in
this form) flexible, and is used on model steam locomotives for making leaf
springs, offering a bit more flexibility than spring steel for multiple
leaf applications, and I believe, having different hysterisis
characteristics.

The best electrical grade at one time certainly used Egyptian cotton - what
is special about Egyptian cotton as opposed to any other, I don't know. For
an RF choke former, at HF, I wouldn't worry about its characteristics
except for the hygroscopic nature. Even then, it's probably OK mounted off
the chassis on a nice glazed ceramic insulator. Varnishing might be another
alternative.

Has anyone had any experience with the machinable ceramics now available
e.g. from Farnell in the UK? They look ideal for a lot of amps
applications, although rather expensive.

Rich comments on the cold flow of Teflon/PTFE. It can be a problem, but may
be manageable if there's not too much stress. Fibreglass tube is the best
bet, although it too may be a bit hygroscopic.

73

Peter G3RZP