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dielectric constant measurement

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masadi

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can i measure effective dielectric constant of microstrip or FR4 if its relative dielectic constant is unknown? how can i do this?

i'd ever read in some books that it can be measured using half wavelength line resonator, but relative dielectric constant has to be known.

thanks before.
 

how to measure dielectric constant

I don't know if it can help you but it exists RF method measurement on the effective dielectric permittivity with S parameter measurement.

Theses methods are based on Free Space measurement method.
You "just" need a line and a VNA (and calibration kit).

you compute directly the effective permittivity with the S parameter measurement.
 
how do you measure dielectric constant

free space method won't help you find the effective dielectric constant of a microstrip. The reason is that the free space method measures the reflection/propagation of plane wave (actually a focused beam), yet the effective dielectric constant of microstrip describes the propagation of the quasi-tem wave it supports.

The "half wavelength line resonator" should work, i.e. it shouldn't require the relative dielectric constant. Please tell us the reason to the contrary cited in your reference.
 

dielectric constant from ring resonators

Sorry my post has not been understood. I have told it is based on free space method, not you must use un free space method.

You are right with de free space method, you compute the reflexion and transmission. But you could deduce th effective parameter of the propagation media.

More information are avaible in the attached file.

Hope it helps you.
 

dielectricconstant measurement

Is there any lab document to guide how to measurement dielectric constant
step by step ? Thanks.
 

VSWR said:
Use the microstrip ring resonator methode:

www.ee.lut.fi/fi/lab/soveele/tutkimus/Determ.pdf
h**p://www.mri.psu.edu/centers/cds/microwave.htm
Click to expand...

I would suggest the CPW instead of resonator. Using resonator, you can only extract few points (resonance freq). I think the paper by Hinojosa clearly explains the steps to extract Er and Eeff.
 

Cut a small square piece of the material and measure it's parallel plate capacitance. Knowing the dimensions and capacitance, you can now calculate the dielectric constant.
 

could u show me any formulas to calculate dielectric constant from the dimension and capacitance ?

thanx
 

Hi,

formula for calculating the capacitance of a paralel plate capacitor (it does not include the fringing field):

C=ε0 x εr x A / d

where A is the area of the plate (w x l) and d is the distance between them

or practically

C[pF]=8.85 x e-3 x εr x w[mm] x l[mm] / d[mm]

(εr is the substrate dielectrical constant)

rgds,
Al
 

How do you measure capacitance ?
How to measure is correct ?
Thanks.
 

Hi,

you can use so-called substrate resonator method to determine the dielectric constant of a microwave laminate. If I uderstood correctly this would be your goal. It is based on measuring transmission resonances of a (square) piece of metalized substrate excited by inserting one corner into coaxial connector (e.g. APC7 or N) and measuring transmission using VNA on oposite diagonal corner. Lateral edges could be metalized too or left opened. Advantage of this method is that you can really use this particular piece of substrate to make circuit. Disadvantage is that the method doesn't work for substrates having moderate or high loss as Q factor of the resonances is low.

In case of FR4 you have to limit yourself to range of several GHz. This will require relatively big piece of substrate.

Method description and furher references can be found in the book by Gupta et. all. : Microstrip lines and slotlines.

Good luck!

flyhigh
 

hi,..

what will we get from that measurement method? ans what's the relationship to dielectric constant? resonance frequency or others?

thanks
 

For the parallel plate method (AL0117 and frannk), one should know, that it gives only a low freq. dielectric constant (at the freq. of the capacitor measurement, in the kHz or MHz range), and it do not give any info for the GHz range dielectric constant !
g579
 

Hi g579,

You are right, the calculated dielectric constant from the measured capacitance at low frequency (several hundreds of MHz) will be different then the one measured at several GHz with the ring resonator method. Everyone has to decide how accurate he need the result, and what measurement instruments he has available( a VNA is quite an expensive equipment)

I made some nice chip capacitors from 0.8 mm FR4 in 1986 when I built my first satellite receiver, and I use a vobbler and a Q meter (I still have it ) to determine the values. At a first approximation it works for me.

rgds,
Al
 

Hi
Any document wirtes this ?
Thanks.
 

Do you guys have any application note explaining how to measure permittivity of materials using coaxial transmission line (air line) method. As far as I know, it will be enough to have a coaxial airline and network analyzer to figure out the dielectric constant. Any application note will be very helpful.
Thanks in advance
 

Does any body know that where I can buy an 75ohm airline?
 

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