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5447Re: [loopantennas] Re: Remote tuning

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  • Chris Trask
    Aug 1, 2008
      >
      > > So, it occurred to me that it would be good to come up with a cheap
      > > and easy way to turn a tuning capacitor <snip>
      >
      >
      > Paul,
      > That's basiclly a good idea and it's probably workable if the 180
      > degree mod can be done with a servo powerful enough to turn the cap.
      > The small tuning caps from cheap radios are usable in loops and turn
      > easily. But these days remote tuning is usually done with a varactor
      > diode. This replaces the cap and is tuned by a DC bias across it.
      > This variable voltage is usually sent up the signal feedline, isolated
      > by capacitors. Other posters on this forum have given links to examples
      > of this. The servo would, however, be a good way to rotate the loop.
      > You can find circuits on the web to operate the servo directly without
      > the R/C stuff.
      >

      I remotely tune my receiving loops with varactors. The tuning voltage is also the amplifier supply voltage, and the amplifier is designed to work with the varying supply voltage.

      I'm going to be working on a remotely tuned transmitting loop where the motor is controlled with the DC voltage going up the coax. Using a variable capacitor without a stop pin on the shaft relieves me of the trouble of having to add stop switches and diodes.





      Chris

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