[Milsurplus] Fwd: FW: When is a Cap Not a Cap? Redux

[Alan Tasker]

Subject: Re: [Milsurplus] When is a Cap Not a Cap? Redux Hi Dave, I wish 
I had seen that discussion. However, as an electrical design engineer, I 
have had to learn about the ins and outs of components. Unfortunately, 
there are no "perfect" ones. The user has to learn "what they don't tell 
you" on your own, or hopefully, under the wing of an experienced 
engineer when you are fresh out of college. In general, a capacitor's 
usefulness in any application depends on two things, the characteristics 
of the dielectric material that sits between the two plates of the cap, 
and the construction technique of the cap. Both of these contribute to 
the the two parasitic elements, the ESR (equivalent series resistance) 
and ESL (equivalent series inductance). NON-ELECTROLYTIC I have been in 
the "chip-and-wire" business since 1977, so I have only limited 
knowledge of the "wound" type of capacitors, like Polyester, 
Polypropylene, etc. But one would not use these types at RF anyway. 
These are best used in audio or other low frequency applications where 
the "wound" equates to inductance, which is not a problem at audio 
frequencies. However, I understand that some of the dielectric materials 
used in these caps change very little over temperature. One would use 
these for timing applications. For RF use, the choices revolve around 
the dielectrics silver-mica, ceramic, or porcelain. Silver-mica caps can 
be found in a lot of the commercial land mobile radios (which some of us 
convert over to ham repeater frequencies). I have the impression that 
microwave frequency use favors porcelain. Ceramics can be used at RF, 
but you have to be careful. One of the properties of a dielectric is its 
"K" factor. The actual equation for capacitance boils down to C=FxKxA/d. 
F is a constant for all capacitors. K is a constant that depends on the 
actual material being used. A is the area, and d is the gap between 
plates. K is 1 for air. K is higher for mica, but not that high. For 
ceramics, K depends on the actual ceramic mix being used. With ceramics, 
the higher you go in K value, the more the capacitance changes with 
temperature, and the more it absorbes energy at RF. So called "Hi-K" 
caps are not your friend at RF. That is what you experienced with the 
ceramic cap you bought for RF use. For RF, it is best to stick with 
mica, or porcelain, or NPO ceramic caps. NPO is a ceramic that changes 
little over temperature, but it also has low losses at RF frequencies up 
to perhaps 500 MHz. Alan ----