----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Wednesday, July 12, 2006 5:57 PM
Subject: Re: [Amps] Mica cap substitute \ alternative
> 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.
>
A solicitor would have framed the question "What is a bad insulator?" Hi.
>
>
> --
> 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
Take a look at Volume 74 No 5 May 1998 page 10.
The photo at the top.
> http://www.ifwtech.co.uk/g3sek
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
Mike k1ern
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|