s11 of transmission line

hi

i am thinking why we get ripples in the reflection coefficient S11 curve for a matched microstrip line.

i expect a horizontal line at S11<-10dB if line is properly matched but alwyas get a ripple at same frequency away from each other.

any explanation for this behaviour


thanks

ripples? as in what ur question is not as clear as it should be let me know.

i can surely help you.

regards
sajid mohammed.

hi there

well i think ripples means something which is moving up and down around some value.

suppose reference values is -5dB but S11 is varying betwween 0 dB and -10 dB with frequency this is called ripples

i hope this is clear explanation for you

thanks

are u talking about simulation.or when u are testing your stripline on the analyzer.most probably u are talking about analyzer because there aint no ripples on simulations.if it is the network analyzer then first thing is change the IF bandwidth.which depends on the frequency of operation.

regards
sajid mohammed.

ripples are usually due to bad calibration and/or IF bandwidth selection.

here the solution that limit the ripples sa much as possible
see the two photos

Hi Abdoeng

thanks for your answer to my post . well i understand these settings in simulation software helps to remove ripples if your structure is highly resonant.

but what i am talking about is the ripples in the S11 of matched transmission line.

actually i forgot where i read but theseripples have to something to do with the length of the transmission line .i mean if transmission line is multiple of wavelength longs you will get these nodes and antinodes in your s11 however i want to know the exact explanantion of this behaviour

you can test this behaviour urself.
just model any matched microstrip line and keep its length ( electrical) long and than observe s11 ,
you will understand what i am talking

thanks for your comments
shahid

A transmission line that is perfectly matched to the source and load will give no S11 ripple. For example a 50 ohm source connected to a 50 ohm transmission line, terminated by a 50 ohm load gives no S11 ripple vs frequency - in theory.

In practice, there is always some mismatch giving S11 ripple. Can you upload a plot of your simulations/measurements?