[Hallicrafters] Capacitor self resonant frequency

[James Liles]

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