A couple of weeks ago, I connected up my ex-Puckeridge giant
Litz coil (see my web site for a picture). I used a variation of the
stripping/soldering method described by Jim, M0BMU. He
recommended wrapping copper foil around the wires and then
applying a blowlamp. I used kitchen aluminium foil which worked
OK. The only failure I had was when I stripped too little of the PTFE
insulation off and the melting PTFE coated the wires with a black
goo that was impossible to remove.
The resultant coils - they are all different; mine has four parallel-
wound windings - each had less than one ohm DC resistance. I
connected up one winding, and the antenna (plus the coil at the top
of the mast) resonated at 150kHz. Connecting another in series
reduced the frequency to 88kHz. This is not the sort of coil you can
tap so it was back to the old dustbin to supplement a single Litz
coil. I haven't yet tried three windings to get to 73kHz. The built-in
variometer changes the frequency by only a few kHz.
My antenna current has increased by 20 per cent and there is
obviously a higher Q as I have had to prevent the pa from oscillating
on key up.
When we were first experimenting with LF there was a lot of
discussion about Litz wound inductors. The conclusion then was
that any advantage would be small compared with earth and other
losses. I think that was true, but it does show that once you have
reduced the other losses as far as possible there is some
advantage in going for a properly wound low-loss coil.
Next projects - a shed for the big loading coil (though perhaps one
of those plastic luggage boxes that sit on top of cars would do) and
a Litz-wound mast-head inductor.
Mike, G3XDV (IO91VT)
http://www.dennison.demon.co.uk/activity.htm
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