Rap 'n Tap
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From: John Davdison
Date: 8/23/00
Time: 11:20:52 AM
Remote Name: 206.16.140.254
Hi Gordon,
I don't really understand where you want to go. To tune the whole band with a slight variety of antennas you need the antenna inductance coil variable from about 50 to about 325 uH. One common old time architecture is to have the antenna coil (maybe #22 wire or so) variable with taps on what's called a "units-tens switch" for this range. Then mount a variocoupler rotor coil (maybe #18 or bigger space wound) of the proper L inside the tapped coil so that it rotates.
For the inside coil needs an L value to tune the whole band with your capacitor. If you use 365 pF, that takes about 250 uH.
Then you tune the antenna and the rotor separately, and adjust coupling by rotating the rotor. There will be lots of pulling and draging of resonances, so you have to retune both when you change one. It looks real neat, and even works pretty well, not outrageous.
_J_