Measuring Dielectric Constant

[mprathap]

Hello All,

I followed the procedure as described in **broken link removed** to measure the dielectric constant of FR4.

I made a open microstrip line of width 3 mm and length 4.5mm. The characteristic impedance of such a line according to the analytical formula turns out to be around 48.8 ohms if the dielectric constant is 4.4.

However when I measure the input admittance and calculate the characteristic impedance, it turns out to be around 12 ohms. Therefore my calculation does not yield a value near 4.4.

According to the measurement, the dielectric constant comes to around 16.2 which is clearly wrong.

Any tips on what I might be doing wrong.

Thanks.

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[biff44]

It is a very simplistic papaer, using closed form equations to approximate microstrip impedances. No attention to microstrip disconinuitites was made, such as end effect, connector effect, radiation, dispersion, ........

And, could be a mistake in the equation derivations too.

Better transmission line type dielectric measurements utilize a substrate that you measure, etch off some length of line, measure again, and the deduce the dielectric contant from the change in measured data (the discontinuities cancel out in such a case).

 

[albbg]

I don't think is a problem related to the method. In fact the impedance value you expect from your microstrip is 48.8 ohm instead of the measured value of 12 ohm: there is some problem in the measurement or in the microstrip geometry.
You stated that you built an open microstrip: how did you measure its impedance ?
Why you didn't terminate the other side on 50 ohm load ?
Furthermore, in your post you didn't specify the dielectric height of your substrate.
However, if you don't need a very precise value you can simply consider your microstrip as a parallel plate capacitor (L and W should be large enough in order the inductance to be negligible). By means of a network analyzer measure its capacitance; frequency should be kept low (around 1 to 10 MHz) to avoid errors due to parasitics effects such as fringe or inductive. From the simple formula:
C=8.84*εr*W*L/h
where C is capacitance in pF, W width of microstrip in m, L lenght of microstrip in m and h height of substrate in m.

You can recover the value of εr as:

εr=C*h/(8.84*W*L)