Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Cavity Modes

Cavity Modes

Cavity Modes

I have been searching my microwave textbooks for information on rectilinear cavity modes and have found very little practical information. For standard rectangular waveguide we have simple rules like the height is half the width making the next pair of modes 2:1 above the dominant mode. A cubic cavity seems like a very poor idea as the number of degenerate modes is large.

What I am after is the idiot’s guide to cavity modes. It seems that TE101 is the lowest mode.

My ultimate goal is to make a cavity oscillator using a MMIC. Initially I thought I could "play" with the idea by doing this at some low frequency (a few GHz) so the probe/launcher dimensional tolerances are not such an issue.

If I use a 25mm wide waveguide, 12mm high, then the cutoff frequency is f= c/lambda = 3E8/(2*25E-3) = 6GHz. So aiming for 7GHz, for example, how long should the cavity be?

It seems that for TE cavity modes, 1/lambda= sqrt(  (m/2a)^2 + (n/2b)^2 + (p/2d)^2 )

and it is best to make one of m, n or p zero.

If the cavity is 25mm x 12mm x 25mm, I get 6GHz * sqrt(2)= 8.48GHz, which is reasonable.

I don’t have an equation for the TM cavity modes.

So the real question is what ratio of sides should I use to get the highest ratio between the lowest mode and the next higher modes?

I would think that this was a standard textbook result, just not in any of my textbooks!   

RE: Cavity Modes

You need to find a graph called a "mode chart".  You might find one somewhere on the web.  Older microwave engineering books like Ramo, Whinnery, and Van Duzer have them.

RE: Cavity Modes

Alternatively, it would be pretty easy to make your own mode chart on Matlab or Mathcad.

RE: Cavity Modes

Ramo, Whinnery and Van Duzer (third ed) only has a small section (10.4) on this subject, without a mode chart. I did find a chart in Very High Frequency Techniques (c.1947) which shows it nicely so I recreated the chart in Mathcad.

The cavity I had in an earlier amplifier design was 29mm x 20mm and I easily got this to oscillate at 7.15GHz, 14.29GHz, 21.43GHz, 28.58GHz. Unfortunately the lowest cavity mode is 9.11GHz.

The only thing I can think is that the ceramic tile covering the base of the cavity is slowing down the mode in some difficult to calculate way.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close