Microwave Experiments

Time: 3 Min 41 Sec

This recording is of an experiment with the microwave oven at the office. It's a small, low-power Sharp Carousel with a turntable. The recording begins with background noise. The signals in my area are fairly noisy since I have a wireless access point right outside my office. No big files appear to be sent or received during this time.

The sequence of events is as follows:

1. Background
2. About 40 seconds of my microwave operating with no load. Note that the signal leaking from the unit is quite strong and is largely at the upper end of the band. There are also two distinct modes showing, which is also typical.
3. More background noise.
4. About 40 seconds heating a coffee cup full of water. The cup is located near the outside of the turntable, so it rotates in and out of hot spots in the microwave mode pattern. You can observe a roughly periodic pattern to the radiation from the oven. When the cup is not in a hot spot, the radiation pattern looks like the unloaded oven pattern. When the cup is in the hot spot, the frequency drops a great deal and radiation occurs down to the bottom of the band. The repeating cycle corresponds to the rotation of the turntable so it looks like it finds one really good hot spot per cycle.
5. Background noise again.
6. About 40 seconds heating a coffee cup full of water now located as close to the center of the oven turntable as possible. Thus, except for the handle on the cup, the load remains constant as the turntable turns. Observe that the radiation frequency is lower than in the unloaded case and does not have the periodic structure of the radiation from when the cup is located near the outer edge of the turntable.

RPI Microwave Experiment

A comment on the frequency shift from Ken Connor:

One expects the frequency to shift in a system with damping if the oscillator sees the damping. In a cheap oven, there is nothing protecting the tube from the load, so we can expect to see this effect. Maybe the simplest way of understanding this is to look at the basic harmonic oscillator — an RLC circuit or damped spring mass system.

There are many, many online references to read over.

A couple examples:

• Wikipedia Article: Harmonic Oscillator
• The Dampened Harmonic Oscillator (PDF)

In each case the damped oscillator frequency is less than the undamped (natural) frequency.

Download Recordings:

1.0 Microwave Experiments
2.4 Microwave Experiments