Microstrip line to slotline transition
For broadband application, Microstrip to slotline transition is generally used. It provides return loss of about -10dB with bandwidth on the order of 10:1.
In microstrip to slotline transition, slotline acts as a ground plane for microstrip line. As shown in the figure, microstrip is printed on one side and slotline on the other side. Both are orthogonal to each other.
To achieve multi-octave bandwidth of operation , radial stubs are used at the ends of slotline and microstrip line of radius λ/4. Characteristic impedance of microstrip line is 50 Ohm and that of slotline is 70 Ohm. To match the impedances multi section based impedance transformer is used.
This microstrip to slotline transition is widely employed in planar microwave integrated circuit designs. It is also used in tapered based slot antenna designs. As mentioned dotted microstrip radial stub acts as short circuit and slotline radial stub acts as open circuit at higher microwave frequencies. These radial stubs cancel reactance of each other and hence able to achieve broader bandwidth.