Microstrip to SIW transition

Hello guys,

I designed a Microstrip to SIW transition at CST. It has been working perfectly as long as I use waveguide port for exciting the microstrip (s11 < -20 db). but when I use a Coaxial cable for exciting everythings goes wrong. S11 is almost 0 when I use the Coaxial cable.

Does anybody have any idea why it happens ?
my dielectric is a little thick 1.524 mm. But I can not change it since it is going to connect to some other circuits.

Thanks

hi,
is your coaxial cable is 50ohm? only if your coaxial cable is 50ohm, you will get good response as before (S11 <-20dB). make sure you design 50ohm cable.

I used one of the manufactured connector dimensions for simulation, so probably the dimensions are right.

if your cable is right/50ohm, you should get good S11. impedance missmatch of the cable causing your antenna to go bad!