Coupling signals to a stripline transmission line

[Ravaner]

Hello. I have a question about the place where signal enters or gets out from a line (strip or not) Sorry I don't know the exact word in english, may be tap or tapping ... For different lines 1/4 or 1/8 wavelength these points are sometimes @ a 1/4 from the cold point, sometimes elsewhere. I searched for a longtime (Smith diagram ... ) and found no explanation.

 

[scottdave]

Is it an antenna? Can you describe more about the setup? Please give some examples, rather than - sometimes 1/4 wavelength, and sometimes other.

 

[Baluncore]

I searched for a longtime (Smith diagram ... ) and found no explanation.

There is no easy answer to such a general question. A quick summary is;
A transmission line is a two port component, with a characteristic impedance and a time delay between the end ports.
A differential signal is coupled into a transmission line using two connections at one end port, one of those may be a ground plane.
Any impedance mismatches at the ends of the line create reflected waves. That can be very useful.
If you connect part way along a transmission line you get two shorter transmission lines with something where they join.

The wave that travels forwards along a transmission line is not affected by any backward traveling waves.
The energy traveling along a stripline travels in the dielectric between the two conductive surfaces.
The velocity is determined by the dielectric constant of the material between the conductors.

An impedance connected to one port of a line will appear different through the line from the other end port because there will be phase shifted reflections at impedance mismatches. The Smith Chart is one way used to calculate those impedance transformations. Some useful common effects are;
A λ/4 line, short circuited at one end will appear to be an open circuit at the other end. That makes a metal insulator at wavelength λ.
A λ/4 line, open circuited at one end will appear to be a grounded at the other end. That makes a floating ground at wavelength λ.
A λ/8 line, open or short at the other, will appear to be a reactive capacitance or inductance at λ.
Start here; http://mirror.thelifeofkenneth.com/lib/electronics_archive/RF-Circuit-Design.pdf

The Smith Chart can show it all. http://www.qsl.net/va3iul/Smith Chart/smith.html
https://ia800603.us.archive.org/32/items/TheoryAndProblemsOfTransmissionLines/Chipman-TransmissionLines_text.pdf
http://www.qsl.net/va3iul/Microstrip_Stripline_CPW_Design/Microstrip_Stripline_and_CPW_Design.pdf
http://www.qsl.net/va3iul/Files/Old_Radio_Frequency_Books.htm

 

[berkeman]

The Smith Chart can show it all. http://www.qsl.net/va3iul/Smith Chart/smith.html

Nice link! Bookmarked.

 

[scottdave]

There is no easy answer to such a general question. A quick summary is;
A transmission line is a two port component, with a characteristic impedance and a time delay between the end ports.
A differential signal is coupled into a transmission line using two connections at one end port, one of those may be a ground plane.
Any impedance mismatches at the ends of the line create reflected waves. That can be very useful...
. . . .
Some useful common effects are;
A λ/4 line, short circuited at one end will appear to be an open circuit at the other end. That makes a metal insulator at wavelength λ.
A λ/4 line, open circuited at one end will appear to be a grounded at the other end. That makes a floating ground at wavelength λ.
A λ/8 line, open or short at the other, will appear to be a reactive capacitance or inductance at λ.
Start here; http://mirror.thelifeofkenneth.com/lib/electronics_archive/RF-Circuit-Design.pdf

The Smith Chart can show it all. http://www.qsl.net/va3iul/Smith Chart/smith.html
https://ia800603.us.archive.org/32/items/TheoryAndProblemsOfTransmissionLines/Chipman-TransmissionLines_text.pdf
http://www.qsl.net/va3iul/Microstrip_Stripline_CPW_Design/Microstrip_Stripline_and_CPW_Design.pdf
http://www.qsl.net/va3iul/Files/Old_Radio_Frequency_Books.htm

Nice summary of what is going on.

 

[Ravaner]

Many, many thanks for this really exhaustive reply.

 

[sophiecentaur]

Hello. I have a question about the place where signal enters or gets out from a line

The answer really depends on the application. A signal can be fed directly to the end of a transmission line directly from an amplifier. There are many alternative physical ways of coupling (connecting) to the end of a line with the appropriate matching.
If you want to couple a signal in or out of a point, somewhere along transmission line, then a Directional Coupler can be used. These can consist of a length of line (1/4 wave long, often) that's placed parallel to the line. At each end of the coupled line, there is a load or a line to or from an amplifier. When the ports are perfectly matched, the outputs are a sample of either the forward or backward power on the main line. The coupling ratio depends on the spacing between the lines and can be anything between -3dB and -40or more dB, depending on whether you want a lot of power or you are just sampling / measuring. This wiki link has descriptions and some theory.
Using two 3dB directional couplers, you can combine the outputs of two power amplifiers without loss and that's frequently done when individual device powers are limited.