The Eico model 710 measures down to 400kc The Millen model 90651 (and variants) goes down to the kc range The ARRL hanbook has simple GDM construction articles and coil data down into the 100kc range. Try a simple pickup loop. Also, I have driven a TC with a sig gen to resonate a simple absorption meter. I checked in ARRL handbook, 40th ed, 1963, p.526. Other editions will have the GDM plans. zeke