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Brian Reay[_5_] · March 23rd 15, 01:42 PM posted to rec.radio.amateur.homebrew

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.

As a parallel, if you measured the parameters of a ferrite core at, say,
10MHz, you would not expect it to perform the same a, say, 10GHz.

For checking materials at HF, you would be better investigating using
the techniques used in diathermy machines, which work (or perhaps
worked, I'm not sure there are still used) at around 28MHz. These were
used in the 1930s/40s (and perhaps since) for medical reasons (and
possibly industrial ones), they were also adapted to 'jam' German
navigation beams by transmitting a stray signal during WW2. I expect
there are details on the internet, I can't say I am familiar with the
beasts, although I vaguely recall they used coils to generate the test
area- something like Helmholtz Coils.

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