10580Re: [loopantennas] Varicaps vs old fashioned variable capacitors
- Apr 30, 2014
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.
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....
Sioux Falls, SD
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