microwave absorption cell

[[email protected]]

Hello,

I am looking for a design for an absorption cell in the range 10 MHz to
10 GHz.
My intention is to measure the absorption of a crystal powder.

The cell should have coaxial connections, no waveguides.
The two-port cell will be put into an oscillationf field, RF is
injected into port 1 and a power meter on port 2 measures the
transmitted power. The magnetic field will be modulated to make
detection easier.

My first idea is to use a copper tube with 10 mm ID two semirigid coax
cables soldered into one end and the other with a plug.

Any better ideas?


Daniel

 

[[email protected]]

Hello,

I am looking for a design for an absorption cell in the range 10 MHz to
10 GHz.
My intention is to measure the absorption of a crystal powder.

The cell should have coaxial connections, no waveguides.
The two-port cell will be put into an oscillationf field, RF is
injected into port 1 and a power meter on port 2 measures the
transmitted power. The magnetic field will be modulated to make
detection easier.

My first idea is to use a copper tube with 10 mm ID two semirigid coax
cables soldered into one end and the other with a plug.

Any better ideas?

Not necessarily "better", but how about dimensioning your copper tube
and placing the connectors so as to make a re-entrant resonant cavity?
Fabricate it so you can put a RF-transparent container of the powder
under test inside, then remove it. Compare the transmission loss with
and without the powder (a "tare" measurement with the powder container
present, but empty, would be helpful).


Mark L. Fergerson

 

[Rene Tschaggelar]

Hello,

I am looking for a design for an absorption cell in the range 10 MHz to
10 GHz.
My intention is to measure the absorption of a crystal powder.

The cell should have coaxial connections, no waveguides.
The two-port cell will be put into an oscillationf field, RF is
injected into port 1 and a power meter on port 2 measures the
transmitted power. The magnetic field will be modulated to make
detection easier.

My first idea is to use a copper tube with 10 mm ID two semirigid coax
cables soldered into one end and the other with a plug.

Any better ideas?

Yes, indeed.
What do you intend to measure ? You can measure
nuclear magnetic resonance and you can measure
electron spin resonance. Both being standard
approaches of physical chemistry. The two differ
by 3 orders of magnitude in the same field.
Forget to measure 10MHz to 10GHz in the same
resonator.
What quantity of powder do you want to measure
at what field ? What is the linewidth and what
will be the absorption? Meaning what is the
concentration of measureable particles ?

Rene

 

[Don Lancaster]

Yes, indeed.
What do you intend to measure ? You can measure
nuclear magnetic resonance and you can measure
electron spin resonance. Both being standard
approaches of physical chemistry. The two differ
by 3 orders of magnitude in the same field.
Forget to measure 10MHz to 10GHz in the same
resonator.
What quantity of powder do you want to measure
at what field ? What is the linewidth and what
will be the absorption? Meaning what is the
concentration of measureable particles ?

Rene

Whatever it is you put between the connectors WILL BE a waveguide and
WILL HAVE many orders of resonant MODES.

The behavior of the powder will be wildly different depending upon its
spatial location.

Your first step is to understand what things like TM(1,3) mean.

The problem is enormously more complex than you think it is.



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[[email protected]]

What do you intend to measure ? You can measure
nuclear magnetic resonance and you can measure
electron spin resonance.

It is an ESR like measurement.
My Frequency of interest is between 4 and 5 GHz, fields will be up to
0.5 T, but I dont know the exact frequency yet.
I know about ESR setups, but I only found designs with waveguides. The
cell has to be quite small (1cm).
This is for someone who wants to measure his sample in my microwave
setup, so it should be a simple design, just for a fast try.


Thank you


Daniel

 

[[email protected]]

Hi Don,

I know the cell act as a cavity resonator, but well below its cutoff
frequency. The cell has to be small since it has to fit into a 11 mm
bore cryostate. Anyway a resonance would not be a problem thince the
absorption line will be modulated by an external field (lock-in), and
the absolute value is not important.
I am looking for a proven simple design, nothing for high precision. I
know a lot of these experiments are done with simple cells as the one I
described, I just don't want to design my own.
Another idea would be to use a rigid coax, drill a hole into the
shield, extract the dielectric and put the sample inside.

Thanks for your input
Daniel

 

[[email protected]]

Hello Mark,

that's a good idea,
I fould a similar design for a reentrant cavity in Rev. Sci. Instrum.
67 (12), December 1996, p 4294, this
seems to be a popular method.


Daniel

 

[vasile]

Hi Daniel,

Hello,

I am looking for a design for an absorption cell in the range 10 MHz to
10 GHz.
My intention is to measure the absorption of a crystal powder.

The cell should have coaxial connections, no waveguides.
The two-port cell will be put into an oscillationf field, RF is
injected into port 1 and a power meter on port 2 measures the
transmitted power.

Usually such measurements have incident power, reflected power and
transmitted power.
Your goal is to have zero reflected power, meaning a perfect energy
transfer into the cavity.
You can't have a 10MHz to 10GHz cavity even for a coaxial one.
On the cavity you'll find multi resonant modes (ie say at 5Ghz,
5.5Ghz, etc)
for every resonant mode the amount of transmitted energy will be
different.
Step one is to calibrate your cavity and see what you've got on the
transmitted side without any powder.
A Vector Network Analyzer (but also a good detection diode and an
analogic microvoltmeter ) could help you alot.


The magnetic field will be modulated to make

 

[Rene Tschaggelar]

It is an ESR like measurement.
My Frequency of interest is between 4 and 5 GHz, fields will be up to
0.5 T, but I dont know the exact frequency yet.
I know about ESR setups, but I only found designs with waveguides. The
cell has to be quite small (1cm).
This is for someone who wants to measure his sample in my microwave
setup, so it should be a simple design, just for a fast try.

The ESR measurements are all done in resonant cavities
with a Q in the several 1000 for CW mode. This also
with lock-in, modulated fields and such. What makes
you believe to get sufficient signal without a resonant
cavity ?

Rene

 

[[email protected]]

Hello Rene,

with a Q in the several 1000 for CW mode. This also
with lock-in, modulated fields and such. What makes
you believe to get sufficient signal without a resonant
cavity ?

The sample is a molecule which contains some magnetic atoms. This
should/could be a ferromagnet, and
the resonance of the ferromagnet would be visible, that's what I'll be
looking at.


Daniel

 

[[email protected]]

Hello Rene,

The sample is a molecule which contains some magnetic atoms. This
should/could be a ferromagnet, and
the resonance of the ferromagnet would be visible, that's what I'll be
looking at.

Well, if you have some idea where to look for resonance, that'll
narrow down the design options considerably. Frinst at first you
specified the range from 10 MHz to 10 GHz which seems a bit
unneccesarily broad.

Now, can you give us a few more details that might help us with your
design, like exactly what sort of resonance is this other person
looking for? What is the material? Does your existing equipment
provide for a static magnetic field, or just high-power RF, and in
what range(s)? FTM what's the range of RF available? I'm just saying
that the desired resonance may not _be_ measurable with your
equipment, or it may be mistaken for something else, or some other
effect may be mistaken for _it_.

If all the other person wants to do is go/no-go verify
ferromagnetism in the material there're much simpler ways to do it.


Mark L. Fergerson

 

[Rene Tschaggelar]

Hello Rene,


The sample is a molecule which contains some magnetic
atoms. This
should/could be a ferromagnet, and
the resonance of the ferromagnet would be visible,
that's what I'll be
looking at.

Interesting. Before invesing considerable amounts
into specialty gear I'd suggest to visit the next
ESR/NMR Lab and do some measurements there. I'm
aware there are not too many, but still. If you
tell them something about interesting materials,
and possible publications, they might do a
measurement for you.

Where are you located ?

Rene

 

[[email protected]]

Hi Mark,


What is the material?
a strange molecule/cluster
Does your existing equipment

provide for a static magnetic field,

the or just high-power RF

both, a strong static field up to 0.5 T and an RF Field from 10 MHz to
20 GHz.
I will modulate the static field and look with a lock-in at the RF
power.
The sample space is cylindrical with 11 mm diameter and 50 mm length,
the experiment will be done at 2 K.

that the desired resonance may not _be_ measurable with your
equipment, or it may be mistaken for something else, or some other
effect may be mistaken for _it_.

My real area of work is the influence of microwave radiation on
magnetic nanostructures, i.e. small rings and wires with
200 nm width. These measurements are very broadband, from 10 MHz to 20
GHz.
Unfortunately it is impossible fabricate these structures with a
controlled impedance over this range.
(the geometric shape is fixed by the physical properties needed, there
need to be other contacts to the sample which partially shorten the
RF...)
I have a very bad VSWR, which I need to compensate, because this
signal is stronger than the effect measured...
So you are right, resolving the desired signal will be difficult.

If all the other person wants to do is go/no-go verify
ferromagnetism in the material there're much simpler ways to do it.

Yes, I know. This thing is a molecule whose owner wants to measure
some energy levels and transitions between
them.

Thank you


Daniel

 

[[email protected]]

Hello Rene,

I am located in Konstanz, quite near to Switzerland.

Before invesing considerable amounts
into specialty gear I'd suggest to visit the next
ESR/NMR Lab and do some measurements there.

of couse that is the best option, though not as technically
interesting...
I already got some interesting ideas by not going over to the NMR lab.



Daniel

 

[Rene Tschaggelar]

Hello Rene,

I am located in Konstanz, quite near to Switzerland.



of couse that is the best option, though not as technically
interesting...
I already got some interesting ideas by not going over to the NMR lab.

Oh, Konstanz. They'd have an ESR lab there.
So if you're having a resonator with your
material in it, at least they'd have the
sweepers and magnets. They'd also have the
helium equipment. And whatever they don't
have, we have it in Zuerich.

Rene