Sure, mica is used as a dielectric up to c. 1GHz, however, my point
was and is that mica transmitting caps are typically rated to carry
less current above 5MHz than they are below 5MHz. Since this is the
opposite of what is the case with ceramic transmitting doorknob caps,
I thought it should be mentioned.
On Jul 12, 2006, at 6:00 AM, Tom W8JI wrote:
>> On Jul 12, 2006, at 2:13 AM, Peter Chadwick wrote:
>
>> Yes, but WHY? What's the mechanism that allows mica to be
>> a perfectly good insulator in other application, way way
>> up in frequency?
>> 73
>>> Peter G3RZP
>
>> It's called Dissipation Factor. For example, Nylon,
>> Delrin and Vinyl are excellent insulators at 50Hz, but at
>> 50MHz they are not.
>> R L MEASURES, AG6K. 805-386-3734
>
>
>
>
> Mica isn't bad at a "certain frequency", and the dissipation
> of one type of mica cannot be directly applied to the
> complex workings of a component. There are many mica caps
> that are quite good to 1GHz and beyond.
>
> Small mica capacitors are actually quite good at radio
> frequencies. For example, CDE's small case snubber mica's
> handle up to 9 amperes RMS continuously.
>
> The peak performance of a 100pF CDE silver mica is at 10MHz,
> where it has the lowest ESR (under 0.01 ohm ESR). Even at
> 300MHz, ESR is under 1/10th of an ohm!
>
> Silver micas and multilayer micas are commonly used in high
> current VHF and UHF high power applications. One example are
> the metal mica stacks used in VHF power amplifiers.
>
> There is great danger in making hard "rules of fact" based
> only on personal opinion of what indirect or unrelated data
> might indicate.
>
> See:
>
> http://www.cde.com/
>
> and look at Snubber Micas and other mica caps
>
> 73 Tom
>
>
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>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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