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how to increase directivity for coupler?

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amicloud

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improve coupler directivity

I design a coupler with narrow bandwidth with coupled stripline,but found the directivity is poor(about 10dB),though the VSWR is good.
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?
 

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.
 

amicloud said:
I design a coupler with narrow bandwidth with coupled stripline,but found the directivity is poor(about 10dB),though the VSWR is good.
I always wonder why the directivity was poor.And how to increase it ?
Click to expand...

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)
 

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    amicloud

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