Rap 'n Tap
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From: John Davidson
Date: 8/21/00
Time: 4:57:18 PM
Remote Name: 206.16.140.254
Charlie, I do not know of any information web sites on this, there may be. I can give you a few tidbits from my own experience, I made a bunch of them.
First, they are sure neat! You just turn the knob and smoothly, presto the L changes, like magic. But, there is a down side, a deep dark side to these neat things.
For one thing, they are low Q, they have serious loss problems. Closed, they have high proximity losses, open they have high resistive losses with their inductance cancelled out.
There are homes in a radio for low Q coils, the best use for them I thought was as an antenna tuner. After all, connected to that radiating antenna, who cares much about the finer aspects of losses! But as hard as I tried to tightly couple the inner & outer coils, (using ball configurations, close spacing and others) I could never get one to have enough L range (high to low) to tune an antenna across the whole 2 octave BCB (achieved about 3/4). So I was always faced with adding a high-low band switch, yuk. That was just too much like a switch variable inductor for my arrogant attitude. I never could find a real good home for them in a set. Maybe you can break out this box.
To work with them, it really helps to be able to measure inductance. You can do that by using them in a loose coupler and tuning them with a known silver mica capacitor to a station of known freq. and dragging out a calculator. If you can measure capacitance, you can do it with 2 stations, 2 capacitances, same L setting and back out the self capacitance, (which varies with L settings).
Hope that is the kind of stuff that will help you along. _J_