In article , Roy Lewallen
writes:

Yes, powdered iron (I believe it's "iron dust" in the Queen's English)
is what you want. The major manufacturer of suitable cores is
Micrometals. Their products are sold by Amidon and others. Give their
web site a look for general info, and a google search should yield some
retail sources. For HF work, type 6 generally gives the highest Q, with
type 2 in second place.

Quite so, Roy!

Micrometals website is - http://www.micrometals.com

The website has a nice, free, downloadable program for calculating
the number of turns for any of their toroidal cores for any of their
powder mixes for many sizes of magnet wire.

One of their publications is the venerable "Q Book" which graphs
the Q of many different inductances wound on their forms over
frequency from LF to UHF. If Paul can talk them out of a copy
he will find it indispensable for estimating toroidal inductors...
provided there is a supplier on his side of the pond.

Amidon is a reseller (Micrometals' minimum order is at least 100
of any type, or was). Most of the small quantities found in ham
and electronics stores here have some typical Q curves enclosed
in the plastic packet, curves originally from Micrometals.

Len Anderson
retired (from regular hours) electronic engineer person