I What causes the unexpected annihilation point in the Magic-Tee configuration?

AI Thread Summary
The discussion revolves around the unexpected annihilation point in a Magic-Tee configuration, where the user encounters a phase shift anomaly at 3/8 of a wavelength instead of the expected 1/4. Key points include the possibility of impedance mismatch at the reflecting arm affecting the phase of the reflected wave, and the impact of optical phase shifts due to reflection at different media. The user also considers the effects of using a variable short circuit and the precision of their simulation setup, questioning whether discrepancies arise from the simulation parameters or physical setup. The conversation highlights the complexity of wave interactions in the configuration and the need for further experimentation to clarify the observed phenomena. Ultimately, the user expresses a desire to explore alternative simulation tools to resolve the issue.
Leopold89
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I have some problems understanding the magic-tee. There is a configuration for the E and H arm, where the signal output is blocked. As far as I understand you should be able to set one arm to 0 and the other to 1/4 of a wavelength, so the reflected wave's phase will be shifted by pi compared to the incoming and they will annihilate.
But now I found a setup where the annihilation happens at 3/8 of a wavelength. As far as I understand there are two possibilities:
1. is the impedance at the reflecting arm not matched, leading to a different phase of the reflecting wave?
2. maybe a phase shift from optics (reflection at a medium with n2>n1)?

Could you explain this really weird phenomenon to me, please?
 
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Is it possible to send a diagram so we are certain which ports we are discussing?
 
220px-MagicTee.jpg

Here is the image. I used port 1 as input, port 2 as output, blocked port 3 and 4. Then I set the height of port 4 to 0mm and of port 3 to 3/8 of the wavelength.
 
I suspect the disparity is caused by the wavelength in the waveguide.
 
If you mean this conversion from vacuum wave length to waveguide wavelength, then unfortunately no, I have considered this.
 
Is Port 2 terminated in a matched load?
 
tech99 said:
Is Port 2 terminated in a matched load?
Yes, the load of around 523Ohm (even though I would have expected around 533.9Ohm) is the same at port 1 and 2.

tech99 said:
If you terminate arm 4 with a non contacting variable short circuit (https://www.ainfoinc.com/waveguide-...sliding-short-plates-4-9-7-05-ghz-fdp58-udr58) then the zero setting is lambda/4 not zero. This may give the offset you are observing.
I do not understand why. If the wave in arm 3 gets phase shifted by pi and the wave that would go into arm 4 instead is unchanged, I would expect destructive interference. But if both are phase shifted by ##\pi##, then I would expect constructive interference.
Also the offset is ##\phi = 2\pi\frac{2\Delta x_i}{\lambda_w}##, right? So if I set arm 4 to 0, I get an offset of 0.
 
So, I now set both arms, 3 and 4, to ##\frac{\lambda_w}{4}\approx 43##mm length, but get the minima for dampening at ##\approx 2.4##GHz with -70dB and ##\approx 2.6##GHz with -50dB, referring to the frequency of the signal generator, while I would have expected 2.45GHz.
 
  • #10
That is only a 2% error. Maybe the offset of the movable short circuits is not so precise.
 
  • #11
Even air has sufficient relative permittivity to slow radio waves a little, as I found when I passed them through a bell jar for a demonstration. When I pumped the air out of the bell jar I found the jar was slightly defocusing the beam.
 
  • #12
Usually I would agree, but in this case I have a computer simulation, so everything is perfect. Perfect vacuum, precise control, perfect conductor and so on.
 
  • #13
Leopold89 said:
Usually I would agree, but in this case I have a computer simulation, so everything is perfect. Perfect vacuum, precise control, perfect conductor and so on.
And a perfect mesh generator. I am so jealous. I wish I had one of those.
 
  • #14
I am not sure if you have done the experiment or are using a simulation. If the latter, how can we discuss errors arising from it, in view of its being perfect?
 
  • #15
It is a simulation, CST to be precise.
tech99 said:
how can we discuss errors arising from it, in view of its being perfect?
Here is a misunderstanding. I was not suggesting that the setup/simulation was wrong, but that I may have missed something in my calculation of the arm position. Maybe the corners add a weird contribution. Maybe I was reading the wrong books, because I thought the S matrix would have entries dependent on something like ##e^{2\pi i\frac{2\Delta x}{\lambda_w}}##.

But if you say you block the signal at ##\frac{\lambda_w}{4}##, then I have to try another simulation software and perform an experiment.
 
  • #16
Could it be that ##\frac{\lambda_w}{4}## only works, if you have ##\cos\alpha=\frac{\lambda}{\lambda_w}=\frac{1}{\sqrt{2}}##? Because otherwise you have a different ##\lambda_w## in the arms than at the input?
 
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