Re: Spaced Coil measurements.
> Original Poster: Ed Phillips <evp-at-pacbell-dot-net>
> Hi Malcolm, All,
> Using your measurements I get 18.25 pF from Medhurst's formula. I
> plotted your data in Excel as 1/(f^2) VS C according to Howe. A
> linear trend line fit to the data crosses the axis at about -50 pF.
> 50 pF seems a bit high. You gave Ns = 38 turns for the whole coil?"
> My calculations agree almost exactly with Malcom's. I have always
> found very good agreement (typically better than 5%) betweem the
> measured and calculated SRF's. Do you have extra long leads, and do you
> really know the capacitance of your test capacitor?
Indeed! The first thing I did was to run for cover of the LCR
analyser to check the caps which is why the E12 numbers have been
given an extra significant digit. I suspected they were pretty close
once I saw the initial inductance figures coming out of the mix. The
leads were only just long enough for me to obtain the readings and
the top lead goes straight back down the coil to the cap at the
bottom. The relatively large shunt capacitances appear to swamp the
small self capacitance of the lead which is what I was after. More
checking with even greater spacing this weekend.