Open Circuit Termination for Waveguides in High Frequency Labs

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Discussion Overview

The discussion revolves around the use of open circuit terminators in waveguides and RF elements, specifically addressing why an open waveguide does not function as a true open circuit and the implications of radiation on performance. The scope includes theoretical and practical considerations in high-frequency applications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Meta-discussion

Main Points Raised

  • Some participants explain that an open waveguide radiates energy, which causes it to behave differently than an open circuit coaxial line, leading to resistance at the open end.
  • Others suggest that to mitigate radiation effects, a waveguide should be made a quarter of a guide wavelength longer and terminated with a short circuit.
  • It is noted that a transmission line or waveguide should ideally be terminated in its characteristic impedance to absorb forward energy and prevent reflections and standing waves.
  • Some participants argue that leaving the waveguide open results in partial matching due to radiation, which does not achieve 100% reflection as desired for certain applications like couplers.
  • A participant raises a concern about the clarity of the original question, suggesting that interpretations may vary and multiple answers could be valid.
  • Another participant acknowledges the need for clear communication but maintains that the original question was specific enough to be interpreted literally.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of open circuit terminations and the implications of radiation. There is no consensus on the best approach, and multiple interpretations of the original question are acknowledged.

Contextual Notes

Some limitations in the discussion include assumptions about the behavior of waveguides and the impact of environmental factors on performance, which are not fully explored.

Who May Find This Useful

This discussion may be of interest to those involved in RF engineering, waveguide design, and high-frequency laboratory work, particularly in understanding the nuances of circuit termination and energy propagation.

cauchyriemann
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Hi everyone,

I am trying to understand that why do we use open circuit terminator for wave guides or any RF element such as coupler and etc. I know if we just leave it open, it does not work as open circuit.
But why?
 
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cauchyriemann said:
Hi everyone,

I am trying to understand that why do we use open circuit terminator for wave guides or any RF element such as coupler and etc. I know if we just leave it open, it does not work as open circuit.
But why?
This is because an open waveguide radiates energy and so it does not behave as would an open circuit coaxial line. The energy lost by radiation makes the open end display resistance. The answer is to make the WG a quarter of a 'guide wavelength longer and use a short circuit instead.
 
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tech99 said:
This is because an open waveguide radiates energy and so it does not behave as would an open circuit coaxial line. The energy lost by radiation makes the open end display resistance. The answer is to make the WG a quarter of a 'guide wavelength longer and use a short circuit instead.
And the reason it radiates is that electrons are being caused to accelerate back and forth around the metal aperture, which does not happen with coax.
 
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tech99 said:
And the reason it radiates is that electrons are being caused to accelerate back and forth around the metal aperture, which does not happen with coax.

Thanks for your answer. I understood well.
 
A transmission line or waveguide should be terminated in it's characteristic impedance because that will absorb all forward energy and so will not cause reflections and standing waves on the line.

A short circuit or an completely open circuit will result in close to 100% reflection with very significant standing waves. The line will then appear to have an non-standard impedance since impedance is dependent on the wavelength and the electrical length of the line.
 
Baluncore said:
A transmission line or waveguide should be terminated in it's characteristic impedance because that will absorb all forward energy and so will not cause reflections and standing waves on the line.

A short circuit or an completely open circuit will result in close to 100% reflection with very significant standing waves. The line will then appear to have an non-standard impedance since impedance is dependent on the wavelength and the electrical length of the line.
Agree, but I think the OP wants to obtain perfect reflection in this case for his work on couplers etc. If the WG is left open to the air, it is partially matched due to the radiation and does not provide 100% reflection.
 
tech99.
Some of us have come to understand that an OP does not always express their question clearly and that mind reading may be the only way to identify what the OP really meant by the question. My interpretations of the question differed to yours. I believe it is better to allow multiple answers to satisfy the multiple interpretations, rather than censoring others.

I would be interested to know how you were appointed gatekeeper, and what makes you think your clairvoyance is any better than others?
 
Baluncore said:
tech99.
Some of us have come to understand that an OP does not always express their question clearly and that mind reading may be the only way to identify what the OP really meant by the question. My interpretations of the question differed to yours. I believe it is better to allow multiple answers to satisfy the multiple interpretations, rather than censoring others.

I would be interested to know how you were appointed gatekeeper, and what makes you think your clairvoyance is any better than others?
I agree with your suggestions about mind reading, but in this case the question seemed fairly specific and I just read it literally. I was not intending to use clairvoyance and I am sorry if I have caused offence.
 
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