[Hallicrafters] Capacitor self resonant frequency

[Carl]

Id spend the time figuring why you cant use the Millen. Is it the SS 
version? Most SS GDO's are too weak to work well if at all. The Model 59 I 
use has a healthy 955 acorn triode.

Been too busy with antennas to do much more on the cap resonances but IMO 
the GDO is the tried, true, and long accepted way of testing for resonance 
even at almost zero lead lengths.

Carl
KM1H



----- Original Message ----- 
From: "Peter Bertini" <radioconnection at gmail.com>
To: "James Liles" <james.liles at comcast.net>
Cc: <hallicrafters at mailman.qth.net>
Sent: Tuesday, January 04, 2011 5:22 PM
Subject: Re: [Hallicrafters] Capacitor self resonant frequency


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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