Question of the Month Answers, September 2006

Edson Wander from Electrolux has submitted this month's question:

Dear Harlan,
I am looking for information about measuring microwave oven cavity reflexion (SWR). I am having difficulty finding the correct probe to simulate the magnetron with the network analyzer. Could you provide any direction?

From: Tharaka Perera, MillenniumIT

Dear Edson,
I guess it would be easier if you could prepare a Through, Reflect, Match (TRM) calibration kit for this procedure. By the way, you can even use a normal network analyzer probe and mathematically remove the errors in the SWR measurements. First, you have to connect probes straight then through and make the measurements, then connect each probe to open load and make measurements and then connect the probes to a matched load and make the measurements. Once you have these sets of measurements in place, you can eliminate the errors by the use of simultaneous equations.

From: Suranga Perera, MillenniumESP

Dear Edson,
I think the best option would be to make use of the existing network analyzer and remove the errors mathematically. TRL/TRM calibration schemes can be used for this purpose. I have seen many white papers on this topic on HP web sites.

From: Bill Gordan, SSD

Dear Edson,
First, place a square block of acrylic plastic in the cavity. Then use a thermal camera to measure the radiation pattern and use take two cameras to create a 3D picture of what is happening.

From: Huy Tran, Moog Aircraft

Dear Edson,
If a good probe is what you want for the SWR measurement, I believe you are going about it the wrong way. The thing to do is get a magnetron cavity and use the network analyzer to measure S11 and convert it to VSWR. This way you do not need a probe measurement.

From: Robert Kim, Newgen Telecom

Dear Edson,
As I know, a microwave oven is using a magnetron as a signal source operating at mostly 2.45 GHz (there is also an exception of using 915 MHz for other industrial use) and a magnetron has ANT part for signal radiation to the oven chamber, the small extruded part out of magnetron assembly. And to my knowledge, there is a waveguide between the oven chamber and the magnetron. I think you better insert a probe where the magnetron is attached to simulate what you want. The probe should be the same dimension as the ANT part of the magnetron and the opening should be properly shielded like a magnetron does in a real product. Connect the probe to the network analyzer through coax and measure S11 to monitor the reflection under various conditions, such as no load (food) or turntable moving condition, etc. Since the oven is thought to be a kind of passive device, the level of test signal is not of our concern if it can properly monitor the reflection. One thing you should make sure of, however, is that the ANT part might not be 50 ohm when connected to the coax, so you need to check the ANT first comparing its return in free space to that after installed to the oven whether it shows sufficient S11 as an ANT. That is what I'm suggesting for your measurement.

From: Aprem Joseph, ISRO

Dear Edson,
You can use a directional coupler to measure the return loss and compute the VSWR from the equation.