improve coupler directivity
I always wonder why the directivity was poor.And how to increase it ?
Pls give some other details about the coupler. In general the directivity depends upon the different velocity of the odd and even mode in the structure. So equalizing these 2 velocities the directivity should improve.
Techniques to do this are:
1) Add lumped capacitors at the end of the lines
2)Dielectric overlays
3)Wiggy lines
Details are on RF asnd microwave coupled line circuits Mongia-Bahl-Bhartia already posted here
regards
ooops sorry of course what i said was referred to microstrip coupler.
whats the coupling?
In Stripline even and odd mode velocity are the same.
I know the related theoy but practically the result is poor,thogh the VSWR is good enough(about 28dB)
Is this a real coupler, or one simulated on computer? In real world, a lot of times you lose directivity in the load on the 4th port. If the load is not a good match, you get reflections that mess up the directivity.
In theoretical world, you might need more sections to get the bandwidth that you are trying for.
i think for stripline coupler you get loose coupling. you can refer to the CRLH transmission line type coupler. It can achive tight coupling.
to improve it try to do:
1) put capacitors at the begin and the end of the coupler
2) use overlay coupler
3) use shorter line then quter wave, and make it teied (more near one to other)