I agree that simple S11 impedance measurements tend to produce false
(low) resonant frequencies. And the higher the Q of the choke the bigger
the error.
But I can wind a high-Q air-cored choke, measure its resonant frequency
using a GDO (or another "non-attachment" technique I developed where
nothing gets closer than 9 inches to the choke) and then see an almost
identical result using an S21 measurement on the VNA. Attaching the VNA
will typically add 0.2pF or less equivalent parallel capacitance.
That S21 technique also produces results that match Fair-Rite's u' and
u'' data very well; but of course the Fair-Rite data doesn't tell us
anything about winding capacitance! The change in C that I'm getting
between different core materials swamps any likely measurement error.
If anyone has estimated C for a particular choke design I'd be happy to
replicate the choke and compare results.
Steve G3TXQ
On 01/11/2014 06:17, Jim Brown wrote:
On 10/31/2014 9:43 AM, Steve Hunt wrote:
I measured the choke resonant frequency using a 2-port VNA S21
measurement.
I have yet to find a swept antenna analyzer or VNA that will measure
chokes correctly above about 10 MHz, and I've tried that the the
DG8SAQ VNWA. I don't have access to the good (and very expensive) HP
Impedance Meters that Fair-Rite uses.
Other than that, the only data I trust is the scalar data (magnitude
only) using the method documented on my website.Every choke I've
measured with one of those analyzers came out MUCH lower in frequency,
but my data matches Fair-Rite's data with reasonable accuracy.
73, Jim K9YC
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