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Microwave Oven Load Matching - Rear Shorting Wall

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4 years 1 month ago #31 by sricha1217
sricha1217 created the topic: Microwave Oven Load Matching - Rear Shorting Wall
I'll start with the question: How is the rear shorting wall of a waveguide determined and optimized? How does one determine where it should be placed (dist from antenna). Thanks immensely for any guidance.

The background: My company is building a microwave oven, so I need to match the cavity+load to the magnetron, which requires that I add a matching element (at least one) to the waveguide. If I create a slotted line along the load side of the waveguide from the antenna and then insert tuning elements further to the load side of the waveguide from the slotted line, I can match the waveguide to the cavity+load. That's all fine - in fact the Rieke diagram on the magnetron datasheet clearly states the safe phase and magnitude combinations that are allowable. However, that is all on the load side. As I understand it, changing the rear wall would have no effect on the impedance "seen" by the antenna at the plane of its axis looking toward the cavity+load. However, changing the rear wall distance would obviously changes the impedance "seen" by the antenna at the plane of its axis looking toward the rear wall and therefore profoundly affects the total impedance seen by the antenna. How is the rear shorting wall of a waveguide determined and optimized?

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3 years 10 months ago #109 by sricha1217
sricha1217 replied the topic: Microwave Oven Load Matching - Rear Shorting Wall
Although I haven't found an answer to my question, I have learned one very important item which is instrumental in understanding the behavior of the rear wall. Until recently, I believed that the rear wall was to be considered a shorted wall along the waveguide whose impedance was defined by its distance in terms of the GUIDED wavelength. Well, it is most definitely a shorted wall, but since its distance from the antenna is so short, the transmission pattern that ultimately develops in a long section of waveguide has not fully "developed". I'm sure there's a way to state that using proper terminology, but it is very similar to the "near field" vs. "far field" comparison with signals radiating in free space. In the far field, the wave impedance can be taken as approx. 377ohms, but in the near field, it is very different.

So the actual impedance presented to the antenna by the rear wall is NOT what one would calculate simply by considering a short circuit and rotating around a smith chart by the distance between the rear wall and antenna. However, that doesn't explain what the rear wall is SUPPOSED to present to the magnetron. A short circuit? An open circuit? A specific reactance known only to the magnetron manufacturer? Also, how is one to determine how to present the same reactance using a waveguide of dimensions other than the one in the datasheet?

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