Rat-Race Coupler
Understanding Rat-Race Couplers
The rat-race coupler is one of the most useful planar microwave circuits. It provides both 0-degree (sum) and 180-degree (difference) power splitting/combining in a simple microstrip or stripline ring structure. It is widely used for balanced mixers, push-pull amplifiers, and antenna feeds.
Rat-Race Properties
- Ring circumference: 1.5 lambda (3/2 wavelength).
- Ring impedance: Z0 x sqrt(2) = 70.7 ohms (in 50-ohm system).
- Port spacing: Three ports at lambda/4 apart, fourth port at 3lambda/4 from the third.
- Outputs: Sum port (in-phase splitting), Difference port (180-degree splitting).
Applications
- Balanced mixer: RF to sum port, LO to difference port. Better LO-to-IF isolation.
- Push-pull amplifier: 180-degree split drives push-pull transistor pair.
- Antenna feed: Monopulse sum/difference beam forming.
Frequently Asked Questions
What is a rat-race coupler?
A rat-race coupler is a 1.5-wavelength ring circuit with four ports that provides both sum (in-phase) and difference (180-degree) power splitting. It is the planar equivalent of the waveguide magic tee. Used for balanced mixers and push-pull amplifiers.
What is the difference between rat-race and branch-line coupler?
A branch-line coupler provides 90-degree phase difference between outputs. A rat-race provides 180-degree difference (and has a sum port). Rat-race is used when 180-degree splitting is needed (balanced mixers, push-pull). Branch-line is for 90-degree hybrids.
What limits rat-race bandwidth?
The rat-race has about 20-30% bandwidth. At frequencies away from center, the port spacings are no longer exact multiples of lambda/4, degrading isolation and match. Wideband alternatives include the Marchand balun and wideband 180-degree hybrids.