Analysis of microstrip-slotline ring 3 dB directional couplers

The paper deals with the analysis of a new class of microstrip slotline ring 3 dB directional couplers which possess topological symmetry with respect to two of the input ports. Slotline or slotline microstrip T-junctions are used to obtain the frequency independent 180¿¿ phase shift. This provides for a high frequency independent isolation between the input ports. In the analysis effects of microstrip T-junctions and impedance transformation in the microstrip slotline transitions have been taken into account. The numerical results obtained predict that couplers operating over 30¿¿50% bandwidth are feasible. Some additional design flexibility resulting from the coupler impedance transformation properties has also been demonstrated.