There are times when an increase in harmonic content can’t completely be explained by circuit or PC board design. If you’ve already done a good EMC design and are still getting radiated emission problems, then perhaps resonances in the product enclosure are, in effect, amplifying the internal harmonics. This internal amplification can cause a myriad of mysterious couplings internally to your product with resulting radiated emissions.
Any metal structure can become resonant if driven by a noise source. For example, I’ve seen the tines on a microprocessor heat sink resonate in the 2+ GHz region. More commonly, you’ll discover resonant modes created by the product enclosure. For example, for a rectangular enclosure, we have:
Where: epsilon = material permittivity, mu = material permeability and m, n, p are integers. Cavity resonance can only exist if the largest cavity dimension is greater, or equal, to one-half wavelength. Below this cutoff frequency, cavity resonance cannot exist. In this configuration (where a < b < c), the TE011 mode is dominant, because it occurs at the lowest frequency at which cavity resonance can exist.
The resonant frequency of the circular cavity is 1.225 GHz, very close to the calculated 1.274 GHz.
To read more about constructing a simple demonstration of resonance, click here…
design-4-emc.com 
Ken, one thing you could do with this experiment to illustrate a couple of concepts – resonance, tuning, coupling – is to insert a screw (post) into the cavity of your resonator. If you have it in the right place, you should see your peak move up and down as you twist the screw in and out.. Where to put it? Well, I would start with the pictures in either Ramo, Whinnery & Vab Duzer, or check out chapter 7 in a book by Willima Sinnema, titled Electronic Transmission Technology: Lines, Waves, and Antennas, 2nd ed. Highly recommended if you want to focus on the concepts and avoid most of the math.
Thanks Randy – I’ll give it a try. One thing that confuses me, though. While watching the peak and squeezing the lid tighter, effectively reducing the height of the container slightly, the peak always seems to move downward, rather than upward in frequency, as I believe should happen. Can you think of an explanation for this?
I think I have an answer for that. When I get back home on Monday, I’ll send you a short write up on what I think is going on. Also, does your site here have the capability to support a download? If so, I’ll contribute a spreadsheet I have which does calculations for rectangular and circular waveguides and resonators, if you are interested.
Thanks Randy, I’d love to see the spreadsheets. I’ll send you email directly with upload info.