Photo of the capacitors with copper straps used to minimize inductance.
Photo of capacitors with straps.
VHF test of Polypropylene capacitor Q Test of the Polypropylene dielectric at 205Mc
Polypropylene and Ceramic .01uf self resonant test comparison
Note: It is not possible for a .01uf Ceramic disc capacitor to be self resonant as high as 30Mc. Some believe that they can be resonant at >30Mc with 1/2" leads. This is a theoretical impossibility, simply hearsay.
Q comparison .001 Ceramic and Polypropylene The Q of a capacitor is indeed important. It is the opposite of the dissipation factor which is composed of ESR (The series resistance of the connections and Dielectric Absorption). The highest frequency that a capacitor can be used and linearity are directly related to Dielectric Absorption. Dielectric Absorption is the cause of the Hysteresis effect in capacitors. Class 2 and especially 3 Ceramic capacitors are known for linearity problems that are caused by the Hysteresis effect of Dielectric Absorption. See the Butterfly chart at the end of the Clifton Labs research paper.
Beware new ceramic capacitors Class 2 and 3 Ceramic capacitors normally experience varying capacity with voltage and it is different for AC or DC. The new multi layer capacitors display instability, non-linearity, low Q, excessive dielectric absorption, and abhorrent variations with voltage. The old class 2 and 3 Ceramic capacitors had the same maladies but were far in a way better than the newer caps. Click on the link for a demo.
Dielectric absorption measurement Dielectric absorption is a measure of how quickly the charges in the dielectric realign to their neutral position as the applied voltage is changed. It is responsible for the instability, non-linearity, and limits as to frequency limitations. After watching the video comparison, it is obvious that Ceramic class 3 capacitors should not be used in critical analog applications. If you are still not convinced, take a hard look at the Butterfly chart in the Clifton Labs research paper.
The notion that Polypropylene film or for that matter, any film capacitors are more inductive that the disc ceramic version is a long held but misguided notion. As you see from the above measurement of the series resonant frequency of the .001uf Polypropylene and disc Ceramic capacitors, the Polypropylene resonates at 76Mc and the disc Ceramic, 67Mc. Take a moment and experiment with your capacitor stock and you will see that it's quite simple to evaluate the qualities of these capacitors. You will find the Orange drop/dip and yellow tubular capacitors exceptionally suitable for analog applications where stability and linearity are important as well as coupling and bypass. The class 2 and 3 ceramic capacitors are suitable only for coupling and bypass applications.
Note: The Polypropylene film/foil dielectric is efficient to well into VHF and Polyethylene film/foil for lower frequencies.
Using a Q-Meter also reveals a Q ratio advantage for the Polypropylene capacitor of 130/100 over the Ceramic.
I encourage you to take a moment and think about the following.
The impedance of a capacitor at self resonance (Including the leads) is composed only of the resistance of the wire connecting it to the circuit, no capacitive or inductive reactance. As the frequency rises above self resonance, it becomes inductive and behaves like an inductor albeit no more so than a wire the same length. Is this undesirable? The inductance in these capacitors is equal to a straight length of wire equal to the combined length of the capacitor and leads. Keep the leads short and forget that a capacitor is part of the circuit.
Do the math. Fact one, a straight one inch length of #22 wire has approximately 22nh of inductance (.000000022nh). You can calculate it or accept the fact. The formula as best I can paint with this keyboard is.
L = 0.002l [2.3log(4l/d) -0.75] uh
where: L = inductance in uh
l = length of wire in cm
d = diameter of wire in cm
Now calculate the resonant point of any .001uf capacitor and one inch length of wire .000000022nh giving you a 1/2 inch connections. It will be self resonant at approximately 34Mc.
The formula: 1/(6.28X the square root of (LxC)) Keyboard doesn't do math symbols.
You will find that the capacitors actually present less inductance than a straight wire of the same length as the cap. The .001uf capacitors that I tested with the Grid dip meter were connected with a copper strap to reduce the inductance of the connection by approximately 25%. The Polypropylene cap was self resonant at 75Mc and the Ceramic cap 67Mc. Both the Polypropylene and Ceramic have less inductance (=< 6nh), than a 1/4 inch length of wire. View the video strip at the beginning of this note.
There is a Myth circulating that a .01uf disc Ceramic capacitor with 1/2" to 1" leads is self resonant at 30Mc. This is theoretically and physically impossible. Do the math with 1/2" leads. 1/2"+1/2" =1.0". One inch of #22 wire = 22nh of inductance. Using the formula above, you will find that using the 1/2" leads renders a resonant frequency of about 11Mc --- not 30Mc.
Now, lets use a copper strap to connect the leads directly --- no leads (See the Video clip). Check the resonant frequency and you will be very lucky to get 21Mc on any capacitor. Now do the math again using an inductance of 6nh ---- equal to a total of 1/4" of wire and you will get approximately 21MC. This is what we get with the copper strap and is included in a video clip. The research papers concur regarding a .01uf cap.
CONCLUSION: THE INDUCTANCE IN THESE CAPACITORS IS OF NO CONSEQUENCE to well into VHF. THE INDUCTANCE IS ESSENTIALLY IN THE CONNECTIONS --- KEEP THEM SHORT and forget the campfire hysteria. Polypropylene Orange drop capacitors are essentially equal to Ceramic class 1 caps. See the research papers and do the tests yourself.
Dielectric absorption is the limiting factor in capacitors. It is the fundamental cause of distortion, non-linearity, and instability. The level of dielectric absorption present in Ceramic class 2 and especially 3 capacitors render them inappropriate for Analog applications Polypropylene capacitors do not have this problem anymore so than Mica or class 1 Ceramic capacitors do.
Note: If you have an unstable stage in a radio, replace the Ceramic bypass capacitors with Polypropylene. Don't just say it won't work, try it. Also, there is a notion that if the stage is unstable, install a 100pf Mica cap in parallel with the Ceramic bypass. Careful what you wish for, now you have a parallel trap that you didn't expect, blocking the bypass of an unexpected frequency. Do the math before you do this.
I hope that you have found this interesting.
Kindest regards Jim K9AXN