Microstrip ring resonator

Thread Starter

tlemsalim

Joined Feb 15, 2021
6
Hi,

Please does anyone knows how to design a microstrip ring resonator from scratch in order to measure permittivity of certain materials, at microwave frequencies? If yes, would you mind explaining how do you choose the microstrip ring resonator dimensions ? I.e. mean radius of the resonator, width and length of feed lines, coupling gaps lengths, ...?

Thank you.
 

Delta Prime

Joined Nov 15, 2019
1,147
Hello there :)
Welcome to AAC.
I have designed microstrip ring resonators and have run into many problems in the gigahertz range.
measuring permittivity with a microstrip structure, you will be measuring the effective dielectric constant, no ifs ands or buts. If the technique involves coax, waveguide or stripline, and you are careful not to introduce appreciable air gaps or glue layers, you just might be able to directly measure the "real" dielectric constant!
First this is (usually) low frequency (MHz, not GHz) measurement (permittivity will not act like a lumped-element capacitor at X-band!). The second problem is that the accuracy of the measurement is affected by the ability of the research laboratories and manufacturers to maintain the sample thickness; a 10% thickness tolerance equates to a potential 10% permittivity error.;)
 

Thread Starter

tlemsalim

Joined Feb 15, 2021
6
Thank you very much for your answer @Delta prime :) It is well appreciated.

Without considering the manufacturing issues that can be faced (assuming that the experience is taking place on a software only), how can the dimensions of the device be determined/estimated/calculated? Can I calculate the ideal ring mean radius, feed line height and width, and/or other parameters to measure permittivity of materials at high frequencies?

Moreover, I understand your point when you say that this method of measurement is usually suited for low frequencies, but in my experiments I want to stick with high frequencies and see by myself the results and try to understand why exactly the results are not as accurate as for low frequencies.

From research papers, I found many suggestions of dimensions. However, I couldn't find any paper or book that explain how to find these dimensions (see attached files).
 

Attachments

Thread Starter

tlemsalim

Joined Feb 15, 2021
6
In the below paper (see end of text), some design formula are presented in order to find the device dimensions. However, it seems to me that some variables in the equations are unknown, and yet the author somehow shows results from the equations. For example, in the ring mean radius equation (see attached files), although it says that the effective permittivity as well as the feed line width were found using a software (I still don't understand how though), it says nothing about the mode number "n". All I know about the mode number "n" is that it is a positive integer, but I don't know which "n" should I take.

In order to overcome this issue, I tried to find the value of "n" by substituting the values presented in the paper and re-arranging the ring radius formula in terms of "n". However, I got n=1.215 which doesn't make sense since "n" should be an integer. If I don't find a solution in a few hours, I might just continue my research assuming that n=1. In the meantime, I will try learning how to find the line width and effective permittivity on a software.

Link to the paper: https://www.researchgate.net/public...ator_based_sensing_technique_for_meat_quality
 
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