Open Circuit Termination for Waveguides in High Frequency Labs

[cauchyriemann]

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?

 

[tech99]

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.

 

[tech99]

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.

 

[cauchyriemann]

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.

 

[Baluncore]

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.

 

[tech99]

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.

 

[Baluncore]

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?

 

[tech99]

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.