[Amps] Mica cap substitute \ alternative

[Ian White GM3SEK]

Harold  Mandel wrote:
>Steatite, (porcelain); Corning Glass, PTFE....
>
>-----Original Message-----
>From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] 
>On
>Behalf Of Gary Smith
>Sent: Wednesday, July 12, 2006 5:18 PM
>To: R L Measures; g3rzp at 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