Waveguide standing wave pattern

JuanCarlos

Hi everybody, that's my question, I have been measuring the standing wave pattern within a waveguide (X band) using a slotted line, I put a short circuit termination and the theory said that we expect a rectified sine, but I don't get that, my result is in the picture



I can't explain my results and I have tried with two different microwave sources (10.5 GHz).

I appreciate your help.

sophiecentaur

That looks more like the phase variation along a line than the amplitude variation. Is that possible?

sophiecentaur

How does that pattern change when you change the load on the end (good match / fair match / short circuit) ?

JuanCarlos

I have measured the standing wave pattern in the air using the same source and the results looks better, of course that's not a short circuit but the wave is not inclined.



I've been thinking if it is about the propagation mode.

JuanCarlos

The picture is a short circuit, and when the load is changed, the swr and the phase changes, but the wave is still inclined.



That's an arbitrary load pattern.

sophiecentaur

You need someone with more experience with waveguide, I think. My actual work was all on UHF and coax feeder.
If I were chasing stange things like that, I think I'd put a 3dB attenuator (resistive) in the source to improve that match and put a good terminating load at the end. If I couldn't get a flattish line then, I would be scratching my head. I could suggest that harmonics in the signal source could be setting up another standing wave which adds to the one you want? (Not relevant if you are using a 'proper' receiver but could be, if you are just using a simple detector.) Solution: filter the output of the signal source.