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Old 3rd Oct 2019, 5:33 am   #123
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Antenna recommendation for 40 and 80M bands

I've seen papers on the Q of solenoidal inductors and the effects of different sizes of screening cans. I haven't kept references.

For ATUs you want a case at least twice as long as the length of your roller-coaster, and you want space around it of about half its length, or 4 diameters in all directions. you can have capacitors in the clearance space, but envisage the fields and avoid plate metal cutting them. You're trying to avoid eddy currents in anything.

ATU wiring can be lossy, copper tape is useful here, or copper tubing. Skin effect and low impedance circuits say surface area beats cross-sectional area.

I have one roller-coaster where the winding is about 5" in diameter and stationary, the contact rotates and slides, following the coil. The coil is silver-plated flat bar formed into a solenoid shape.

Have a look at the 'Transfer' variable inductor in some classic Marconi transmitters and you'll get a feel for what is needed for a really good ATU.

In general, surplus parts and homebrew is the way to go.

Aluminium cases are better than steel. Eddy currents become reflective more than dissipative.

What you don't want to see is tightly packed stuff in small cases with thin spindly wiring.

David
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