Hi Jim and LF Group
Jim Said
My 136k Loading coil has 80 odd turns of Decca litz wire on a sectional
manhole former. L is about 4mH, and Rseries about 5ohms, making Q around
700.
My 136kHz coil is a rewound Decca coil with two layers of coil and the
original variometer, which was also rewound. This gives a coil with an
inductance variation of 1.7mH to 2.2mH.
For 73k, another sectional manhole is stacked on top, wound with about
120 turns of 19/0.25 Teflon insulated stranded wire, which gives a total L
of about 15mH, and a Q of around 300. The coils are wound in sections,
with
the total turns divided fairly evenly between the 14 slots on the former .
The required number of turns wound in to each slot before moving to the
next, with the aim of minimising inter-winding C and maximising breakdown
voltage, rather like the old-fashioned RF chokes.
I am using a sectional manhole former for the the 73kHz coil wound exactly
as Jim describes but using the yellow 2mm Litz wire. It uses around 98 turns
(7turns per slot) to give an inductance of around 6.8mH (forgot to make a
note of that L measurment and the figure is from memory). When the two coils
were stacked the inductance was 10mH and 9.2 when the variometer was set to
minimum.
So G3LDO's Qs of less
than 200 suggest either poor inductor performance or Q measurement errors.
Using Jim's coil construction as a standard of comparison I would have
expected a coil of similar construction but using less turns per section,
and using Litz wire, to have a Q something in excess of 300. My
measurements, using the method that I have already described was still less
than 200 no matter how much care I took reducing the coupling to the
measuring instruments. I feel that this method of measuring Q is OK for
loaded Q as in a filter and that other methods described by Jim and Ha Jo
DJ1ZB are more appropriate for hi Q measurements.
My main reason for winding big loading coils was to stop the
things melting!
Well thats it. I had a coil catch fire when the windings slipped and an arc
occured through the plastic insulation.
Thanks to everyone who offered suggestions on this subject. I will but a
synopsis of this discussion on the Web page when I get it together.
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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