Hello,

In our institute we have three ESR resonators, two of them are commercial (and sadly, information about the used materials is not given publicly), but they look like their made of Cu or brass. The third one is made using brass. In each case the inner surface is coated with thin layer of gold - which is very good solution. Using aluminum can be challenge if you want to do such coating. 

I personally think that there is no such thing as 'best material'. Few things are to be considered when making a resonator - e.g. thermal expansion coefficient (if you want to use it with sample cooling or heating as in my case) should be as small as possible for keeping resonant frequency constant, but it is minor, when it will be used in fixed temperature.

Anyway, I'm not an expert in this topic, just want to share what I know from my personal experience.

Regards,

Konrad

Dear Konrad Gruszka,

Thanks for you answer. I am using this cavity at room temperature so no worries about thermal effects. At the moment I am considering building irises for the commercial waveguide that I have out of brass.  Maybe  I could coat them with gold too, I will give you feedback when done.

Best Regards.

Moaz

I'll be very glad if you do.

Good luck!

Konrad

Brass is better choice in terms of cost. However, inner surface MUST be very smooth. Gold or silver coating offers good result. In place of coating, even gold flush can be used, if process is available. Flush is as good as coating in terms of performance, but less costly.

If high-Q resonance is preferable, I consider the form of cavity resonator is more important than the material.  TE011 mode in cylindrical resonator is the most widely used in our field (microwave conductivity measurement at low temperatures).  When the surface current flows across the joint (boundary part) between the parts of the resonator, large Ohmic loss will be caused.  The smoothness of the surface is also very important as is pointed out by Dr. Amipara Manilal D.

Dear Colleagues,

I want to thank you for all of your valuable feedback.  Today I have tested a rectangular cavity that I have  built out of the commercial waveguide and two irises made out of brass. Right now, I have an improvement of the quality factor close to 5.5 times when compared with irises made out of Aluminum.  The next variable that I am trying to explore is the dimension of the irises used, any ides?

Thanks again and my best wishes for you all.

Moaz

Good for your improvement in the quality factor !

The coupling Q ($Q_c$) is defined as the ratio of finite leakage energy per period ($\times  2\pi$ to be exact) to the stored energy.  The coupling to network circuit is indispensable for the characterization of the resonator, but it means the microwave leakage from the viewpoint of the isolated resonator.  Therefore, the weaker the coupling is, the higher $Q_c$ becomes.  You have two irises i.e. two ports and operate the resonator in transmission mode, so strongly under-coupling (with $\beta \simless 0.01$) is possible.

Your first question is mainly concerned with the unloaded Q ($Q_u$) of the ideally isolated resonator.  The inverse quality factors contribute in the additive manner to the net loaded quality factor ($Q_l$) i.e. $1/Q_l = 1/Q_u + 1/Q_c$.   

 Dear Ryotaro,

Regarding the irises dimension, my understanding from what you are saying is that making the irises small will increase Q. However, I have seen some publications ( e.g. from Agilent Tech. for measuring the dielectric constant) using the dimension of the iris to be b/2.2 where b is the narrow dimension of the waveguide. Any ideas why b/2.2 is preferred. Is there any harm for the dielectric measurements if I  use small irises since high Q is desired. My understanding as long as you can detect a change in resonance frequency and have a high Q-factor there is no importance of the irises dimension.

Best Regards,

Moaz

Dear Moaz,

Sorry for late reply.

I think I don't have any problems in weak coupling as long as I can detect the resonance peak in transmission coefficient (S21).  I don't know why some publications recommend the fixed value of the iris dimension.  Actually, when I tune the coupling, I prepare several thin (0.2-0.3 mm thickness) coupling plates with two small coupling holes (0.2-0.3 mm radius) on each plate, and adjust the coupling by enlarging the holes using a needle-like file.  If I have enlarged them too much,  which I can know by S21 using network analyzer, I discard that coupling plate and restart in another plate.  This method is based on the following paper which was one of the textbooks in my belonged-to laboratory.

Int. J. Infrared Millimeter-waves vol. 14, no. 12, pp. 2459-2487, 1993.

If you are interested in the relative change of resonant frequency and Q-factor, in cavity perturbation technique for example, weak coupling is further preferable because the interaction between electromagnetic fields in the isolated resonator and  those in microwave circuits (waveguides) causes the change of the coupling value itself when the sample condition is changed.

Best regards, 

Ryotaro

Dear Ryotaro,

First of all I want to  thank you for much for sparing the time to answer my question and sharing your experience in this research area. I will consider your notes when performing my experiment. I will post my results at some point in the near future and l will be looking for your feedback. 

Best Regards,

Moaz