Harold Mandel wrote:
>Steatite, (porcelain); Corning Glass, PTFE....
>
>-----Original Message-----
>From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
>On
>Behalf Of Gary Smith
>Sent: Wednesday, July 12, 2006 5:18 PM
>To: R L Measures; g3rzp@g3rzp.wanadoo.co.uk
>Cc: Amps Amps
>Subject: Re: [Amps] Mica cap substitute \ alternative
>
>At the risk of straying OT, what is a good insulator at 50 MHz or >?
That is too simple a question, because the answer depends on the
application as well as the frequency.
In locations where the RF+DC voltage gradient or the RF current density
are high, you need a better insulating material than you would for less
demanding applications. For example, the anode coupling capacitor at the
hot end of an RF choke is a very different application from the RF
bypass capacitor at the cold end. The first one has to handle a high DC
voltage and also a large RF current; the second one has the same DC
voltage but a much lower RF current.
The answer also depends on the operating temperature. Many traditional
porcelain materials are not particularly low-loss, but it generally
isn't a problem if they get hot. However, temperature *is* a problem if
those materials are used in capacitors that form part of a tuned tank
circuit, because the capacitance may vary as the material heats up.
Also, compare PTFE with other plastics such as polystyrene and
polyethylene. They all have very low RF losses, but only PTFE can
operate at high temperatures without melting.
That is a very sketchy outline of some of the factors involved, but I
hope it shows that simply asking "What is a good insulator?" won't
provide all the answers you need.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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