low-loss (low RS) and have good leakage characteristics
(moderate RP). Ceramic capacitors are very rugged and
pack a lot of capacitance into a small package. Ceramics
are non-polarized and have a wide range of tolerances.
MICA AND GLASS
You will occasionally find silvered-mica and glass
capacitors in RF and transmitting equipment due to
their extremely low loss (low RS and high RP) and low LS. A
variation on the ceramic stack, mica and glass dielectric
layers are used instead of ceramic. They cannot be
sintered like ceramic, so this limits the capacitance that
can be obtained. Both types typically have a 5% tolerance.
About the Author
H. Ward Silver is an engineer, writer, and teacher
with over 30 years of practical experience in medical
electronics, instrumentation design, and broadcasting. He
is the author of Ham Radio for Dummies by Wiley Press
and numerous articles for QST Magazine. Ward’s ham
radio call sign is NØAX.
■ CAPACITOR FUNDAMENTALS
If you take apart an old radio, behind the tuning knob
you’ll see an air-variable capacitor with closely-spaced
metal plates. While these are typically small in value
(1,000 pF or less), they are adjustable. Air variables are
stable and low-loss, working very well at RF. An adjustable
variation of the mica capacitor in which the stack is
squeezed by a screw is called a compression trimmer.
Ceramic and plastic variables are also available with
values of up to several hundred pF. NV
For Further Reading
By now, you’re aware that there’s a lot more to a capacitor than its capacitance. To learn more, browse the
Capacitor Term Glossary at www.capacitorindustries.
com/ glossary.htm The information at http://xtronics.com/
kits/ ccode.htm will help you decipher capacitor value markings. A whole web page full of links about capacitors can be
found at www.hallbar.com/capacitors.html If you are really
interested and can find a copy, The Capacitor Handbook by
Kaiser (CJ Publishing, 2851 W. 127th St., Olathe, KS 66061)
is an authoritative, but easy-to-read reference.