Re: NEC-LIST: Coil Q

Re Q of "Air-core" Inductors:

Using an HP LCR meter and Q adaptor, at 1 mHz, about 10 years ago we
compared a 10 uHy 100 A helical inductor (1 1/8 inch copper tubing) and a
hand-made 50 Amp air core toroid (1/2 inch aluminum bar stock, bolted with
1/4-20 hardware) in clear area at least 10x the inductor dimensions from any
conducting objects (hung by bungi cords from rafters in our shop). The
helical inductor measured around 650, the toroid nearly 1000.

Putting them adjacent to a 4 x 4 ft. aluminum plate (1 diameter distant,
axis orthogonal for the toroid, parallel for the helical inductor) reduced
the toroid by about 20%, and the axial inductor by at least 2/1. Putting
them in a standard Collins Radio metal transmitting equipment cabinet
without a back door which was about 6 feet tall by 4 feet wide and 3 feet
deep reduced the helical inductor to 150-200, and the toroid down to around
5 or 600.

Continental Electronics, who manufacture quite high power broadcast
transmitters and other high power RF generating devices, have used a high
current air-core toroid inductor design for many years in combiners and
filters of various sorts, at currents of 10's if not hundreds of amps. I
don't think anyone there has every written a paper about the design, but
there are pictures of them as used in the equipment in some of their older
sales brochures.

Ben Dawson

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