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March 23rd 15, 05:08 PM
posted to rec.radio.amateur.homebrew
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Microwave test?
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. 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. -- Roger Hayter |
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