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]

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.

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]

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

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 strange 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.