transmission planes

Hi Everybody!
I wish to make a special purpose bearer for radio signals. The bearer is to be a section of a plane within which the speed of light is to be reduced to c0/2 (c0 is speed of light in empty space). I anticipate that the bearer will be made of 2 sheets of metal, sandwiching some dielectric.
I have a question about bearer geometry also. The bearer I describe above is to be a slice from a circular pie, perhaps about pi/20 or so in terms of angle. (see drawing)
If I transmit a signal from the point O toward arc A, will the signal encounter a change in impedance on its way?

Thanks guys!