Roger Hayter wrote:
Brian Reay wrote:

On 22/03/15 16:26, wrote:
gareth wrote:
I believe that there is a test for the efficacy of materials at RF by
putting
a small quantity in a microwave oven, together with a mug of water to
see if they get hot (and are therefore unusable)

Anybody tried this and can report back?

Yes.

It will tell you whether or not the material aborbs energy at about 3 GHz.

If it does, there is a very good chance, but not guarantied, that it will
absorb energy at low frequencies.

A low frequency test with equipment generally available to a ham would
be something like a dip meter and see if the material has an effect
on a coil.

And conversly, no effect at low frequencies does not guarantee no effect
at high frequencies.


It is a rather crude test, erring on the useless.

The dielectric properties of materials can vary widely with frequency
and a assuming a test at microwave frequencies can be extrapolated to,
say, HF, is somewhat bold.

I have no empirical data, but my impression was that dielectrics
invariably show more loss as the frequency rises, at least up to the GHz
region. (I don't think it applies e.g. at light frequencies.) Do you
have any evidence of the existence of material that is lossy at HF but
not at GHz frequencies? Genuine question, I don't know if there is an
answer.

See this:

http://www.doitpoms.ac.uk/tlplib/die.../variation.php

The bottom line is that the dielectric constant of a material at a
particular frequency depends on the atomic structure of the material.



--
Jim Pennino