[Hallicrafters] Capacitor self resonant frequency

[Peter Bertini]

Hi Jim

I'm a bit confused, as normal :)  What "shunt" are you
referring to?

The setup I used shows produces a pronounced
dip (inverted V) at the series reasonant frequency
of the capacitor. It can be seen in the Photobucket
photo referenced in my first post.

A few folks have raised some concern about the stub
length introduced by the BNC tee introducing reactances
that may have affected the accuracy of my readings.
I wonder if adding a few more BNC adapters after the
Tee would show a significant shift in the apparent
resonant frequency?  Better yet, I should go back
Iand do this again I will try using a section
of strip line so the cap leads can be directly soldered
on the 50 ohm line without involving a Tee fitting.

I didn't have much luck coupling into my Millen
GDO. More experimentation is needed there.
I suspect my AEA analyzer could be used to
verify the readings, but the LCD display has failed
and I need to replace it.

Pete

On Tue, Jan 4, 2011 at 4:01 PM, James Liles <james.liles at comcast.net> wrote:

> Good morning Pete, Richard and all!
> Great links Pete – didn’t know just how much I didn’t know about caps!  I
> tried your setup with the spectrum analyzer although I don’t have a tracking
> generator.  I used a 10db attenuator to a “T” to the spectrum analyzer.  At
> the “T”, I wire wrapped a lead on a .001 20% 1Kv  disc ceramic measuring
> 3/8” and slipped it over the pin and went straight to shield with the other
> lead; very short leads.  Set the spectrum analyzer to linear and max hold
> and swept in 10 meg segments.
>
> Results were a decline to 59Mc and then an incline.  Didn’t try switching
> out the “T” for the adapter in the procedure that you sent; the measurement
> will probably be more accurate with that configuration.
>
> Now, because others were able to extract indications with a GDO, I decided
> to revisit the effort.  I could not attain a definite dip without some form
> of a loop in the leads, and the leads and shape seem to be responsible for
> the majority of the inductance.
>
> I wound a new coil measuring 1/4” in diameter for the GDO to concentrate
> the field.   Then took the same .001 cap and clipped one lead leaving but a
> stub and wrapped the other lead directly around the stub leaving a 1/4” wire
> between them, with almost no space between the lead and cap.  Directed the
> dipper coil directly at the point where the lead and cap meet and was able
> to distinctly measure a dip at 81Mc.
>
> Now regarding the first post that I sent.  I thought it was a great idea
> for a low cost method to measure self resonance in capacitors but the GDO
> method is easier to use and appears accurate ---- to be determined!
>
> One unexpected result with the setup in my original post is this.  I said
> to connect the test cap to the circuit and leave it there and connect one
> side of a shunt, leaving the other free to connect and disconnect.  There’s
> a problem Houston!  With the shunt connected, the shunt and test cap become
> parallel resonant under some conditions and the whole of the circuit series
> resonant causing the circuit to appear to be very low Q.  The test cap must
> be removed when the shunt is applied.  Am I going to use this method in the
> future?  NO, the GDO!
>
> Kindest regards Jim K9AXN
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