In order to test the effects of Electromagnetic radiation (Radio waves) on electronics we need to conduct the test within a "chamber" that does not "leak" radio waves. If the container were to leak the test itself could interfere with the operation of cell phones, ipods, radios etc. The best such test chamber is one that is constructed of metal walls, floors, and ceilings. Radio waves bounce off metal, so the radio waves in such box continue to bounce around the box until they are absorbed by something (which could be the walls of the box). At certain frequencies these bounces all add up to a rather large signal. This is equivalent to the sound you get by blowing air across a soda bottle. The air resonates creating a pitch. In the case of our metal box the are MANY such resonances. However when the box is NOT resonating the strength of the electromagnetic field can be quite weak. Testing in this environment is difficult since the strength of the electromagnetic field keeps changing! One solution is to line the walls, ceiling and floor with a material that absorbs radio waves. This is called an Anechoic chamber, and they are VERY expensive.
Rather than avoid the resonances we can try and make use of them. Consider putting identical antennas in an anechoic chamber and a metal box of the same size. We then drive both antennas with the same amount of power and measure the strength of the radio waves in the tow chambers. The Anechoic chamber should have no resonances. Therefore the strength of the radio saves in the Anechoic chamber should not depend on frequency (which is why they were created). The simpler metal box will have frequencies with a much Stronger radio wave when compared to the anechoic chamber. These are the resonances of the box. When someone plays the flue they do not blow in the note. The flute sets up a resonance which is driven by the breath. However between the resonances the radio waves are much weaker than the anechoic chamber. The basic problem with the resonances is that there isn't enough of them! If you go high enough in frequencies the resonances get very close together. Below is a plot of the low frequency resonances of our large chamber. You can see how at low frequencies (below about 75 MHz) the chamber doesn't respond well to the radio wave stimulus.
Between 75 and 225 MHz you see more and more resonances but they are father apart. It is only after around 300 MHz that the resonances get very close together. This is data for a mostly empty box.
How do we get more resonances? Change the shape! This is what happens when a flute or clarinet player does when the open and close the "valve's" on the instrument. We want to keep all the radio waves in the box, so we don't want to add any holes. We can achieve much of the same thing by changing the shape INSIDE the chamber. The easiest way to accomplish this is to install a big rotating paddle. As the paddle rotates the inside shape changes as the resonances change their position slightly. Now we can run the tests inside the chamber with the paddle in many different positions. Each position will have a slightly different resonance. If we average all the results together....
In the above plot we have show the response of the chamber over a limited frequency range as we change the stirrer location. This was a simple stirrer of three flat aluminum placed mounted on a rotating pole in the chamber. The plates were arranged with 120 degrees between them (as well as different vertical positions on the pole). They were also placed at several different angles to the horizontal. All of this is trying to "break symmetry". Starting with the black trace (angle zero). The red trace represent a rotation of 2.5 degrees from Black. The Blue trace represents a rotation of 10 degrees from the black. Finally the Violet trace represents a rotation of 15 degrees from the black trace. We can see that with even this simple stirrer rotating the paddles changes the location and strength of the resonances.
The movie on the front of this website is a two paddle system we have been using (March 2008). The plot below gives us some understanding of the effectiveness of the paddles.
The blue line represents (roughly) the relative magnitude of the total magnetic field with both stirrers stopped. The red line represents the same data but averaged over the positions of the two stirrers. You can see that the peaks have been reduced. We are using a log periodic antenna in side the chamber to launch the waves. The receiving antenna is three loops of wire attached to the end of the cable. The loops are oriented 90 degrees from each other. (The Bandwidth here is 30 kHz and the points are separated by about 1 bandwidth.)
What we have accomplished is taken a "resonant rich" system and averaged out those resonances by moving them around!