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?

(I shall cease cross-posting my homebrew posts also to uk.radio.amateur
because that only brings forth the Mongolian Hordes of children behind
the fence of the kindergarten school who wish to become the centre of
attention and make fools of themselves by shouting off personal abuse.

It is a sad reflection on the state of amateur radio in Brit, but there we
go)

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.


(I shall cease cross-posting my homebrew posts also to uk.radio.amateur
because that only brings forth the Mongolian Hordes of children behind
the fence of the kindergarten school who wish to become the centre of
attention and make fools of themselves by shouting off personal abuse.

It is a sad reflection on the state of amateur radio in Brit, but there we
go)

Ending every post with a paragraph like this one is like waving a cape in
front of a bull and wondering why the bull charged.


--
Jim Pennino

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.

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

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

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.



--
Roger Hayter

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

"Roger Hayter" wrote in message
...

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!

My enquiry relates to PolyLactic Acid (PLA) which is what my 3D printer
extrudes, because I wish to use it to produce some insulator blocks for
the HRO-like Catacomb band switching in my "Vapourware" Retro RX.

gareth wrote:
"Roger Hayter" wrote in message
...

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!

My enquiry relates to PolyLactic Acid (PLA) which is what my 3D printer
extrudes, because I wish to use it to produce some insulator blocks for
the HRO-like Catacomb band switching in my "Vapourware" Retro RX.

It it highly temperature dependant and dependant on the base material
it was made from.

For the HF range, typical values range from about 2 to about 16.

Google polylactic acid dielectric constant.



--
Jim Pennino