Tnx Roy for the factual info below.

Frank GM0CSZ / KN6WH
========================================
You shouldn't lump "ceramic cap" into a single group and assume it has one
set of characteristics. There are many types of ceramic capacitors, with
widely varying characteristics. The four most common groups a

1. Hi-K. These are made with a class of ceramic having a very high
dielectric constant. This results in physically small capacitors which are
excellent for many RF applications such as bypassing. Their small physical
size results in a high self resonant frequency for a given capacitance.
The tradeoff is instability in nearly every other respect. They have a
very high temperature coefficient, are sensitive to humidity and physical
stress, and the capacitance varies with frequency and applied voltage. A
common type is Z5U, which has a specified maximum capacitance change of
+22 - 56% over the temperature range of +10 to +85 C.

2. "Temperature stable". These are made with ceramic which has
considerably lower dielectric constant than used for the hi-k types.
Consequently, they're larger for the same capacitance. Their temperature
coefficient is much lower, and they're much less sensitive to other
factors. They're good for many general purpose uses such as low- to
moderate-Q filters and resonant circuits, coupling, and so forth. Q can be
quite good. A common type is X7R, which has a specified maximum
capacitance change of +/- 15% over the temperature range of -55 to +125 C.

3. Temperature compensating. These are much larger yet, being made from
ceramics with a low and controlled temperature coefficient. They're
typically very stable with respect to voltage, frequency, mechanical
stress, and humidity. A common type is P2G (N150), which has a temperature
coefficient of -150 +/-30 ppm/C.

4. NPO. (Common type is C0G.) These are made from a sandwich of two
temperature compensating types with equal and opposite coefficients. They
have excellent Q and are very stable. They're good for use in resonant
circuits including oscillator tanks, high-Q filters, and other demanding
applications. Q can be very good. A common type is C0G, which has a
temperature coefficient of 0 +/-30 ppm/C.

NPO capacitors are substantially larger than "temperture stable" types,
and much larger than hi-k types. You won't typically find them in large
values.

Roy Lewallen, W7EL

"Bill" bravely wrote to "All" (25 Oct 05 21:08:34)
--- on the heady topic of " High Q caps for 60 Khz loop antenna?"

Bi From: Bill
Bi Xref: core-easynews rec.radio.amateur.homebrew:88513

Bi wrote:

Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

Bi Where can you buy .2uf NP0 discs?

Bi -Bill

How about 10 of 0.02uF?

A*s*i*m*o*v

.... "If it ain't broke, you're not tryin!" - Red Green

Yep, a bunch of NPO in parallel is a good way to go. Polystyrene is
nice too. BTW you can get some very impressive amounts of capacitance
out of multilayer ceramic capacitors.

73
NEO