Roger Hayter wrote:
wrote:

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.



Thanks. That is quite interesting. But it discusses mechanisms that
only come into play above about 10GHz, and doesn't deal with losses, or
energy dissipation, below this frequency. It implies that a good
dielectric at microwave oven frequency will also be good below that
frequency, but not necessarily the converse


It is certainly something to bear in mind when designing circuits for
above 10GHz. My rule of thumb is 'use PTFE', but I have no scientific
justification for that!


Any mechanism causing greater loss at lower frequencies would have to be
different from what is discussed in those references - not that I am
committing myself to whether such a mechanism exists.

If you really want to know more about the subject, Google dielectric
constant versus frequency.

Most of the good stuff is in PDF's.

There are materials whose constants take a dive in the 10's of MHz range.


--
Jim Pennino