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10580Re: [loopantennas] Varicaps vs old fashioned variable capacitors

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  • T. Hills
    Apr 30, 2014
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      On 4/30/2014 5:54 PM, Richard (Rick) Karlquist wrote:

      On 4/30/2014 7:39 AM, n3ikq@... wrote:

      > Put another way, Does the mechanical complexity of a remotely tuned
      > variable capacitor provide any advantages over varicap diodes with all

      One other problem with varactors is testing. The loop design
      I published in the National Contest Journal cannot be tested
      with the MFJ-259 impedance meter (and probably similar units)
      because the drive level is too high. I only recently discovered
      this. I did all the development work originally with an
      HP 4815 vector impedance meter. I also found that if I
      used an Agilent E5071C VNA, I had to reduce the drive level
      below the default level of the instrument.

      Rick N6RK

      When I measured the batch I got I built a buffered VCO with the L.O. coil from an old MW radio using a socket for the varactors to test the frequency output at 12 discrete voltages using a 12 position switch and resistors.

      Fed the output to my scope and the output from the scope to a cheap frequency counter. A bit of Math (and a spreadsheet) gave me the cap. values at different voltages and monitoring the waveform on the scope showed if they were noisy or unstable. Neither the scope or the counter had to be NBS traceable or anything since I was comparing apples to apples so to speak and in circuit stray capacitance would be more than the measurement error anyway.

      Still have the test jig around here somewhere....

      Tim Hills
      Sioux Falls, SD
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