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New for May 2011! And under construction... The patch coupler is a form of quadrature coupler, related to the branchline. It looks sort like a branchline with the center filled in. We came across this topic at the request of one of our readers.
Her is one reference to the patch coupler, there are surely many more. In this article an elliptical coupler was proposed.
A Quadrature-Hybrid Design Using a Four-Port Elliptic Patch, Kin-Lung Chan et al, IEEE MTT, Vol. 45, No. 3, March 1997.
We didn't want to follow the math for now, so we used trial and error to come up with a patch coupler that works. We started with a GaAs substrate in an electromagnetic simulator, drew a rectangle (just like an ellipse, it is asymmetric), then played around with the dimensions until quadrature coupling occurred. This is no way to design a circuit in real life, we didn't have a target frequency in mind, just got lucky at around 120 GHz...
The port numbers are hard to read, P1 is upper left, then P2, P4 and P3 when you move clockwise. The rectangle measures 15 mils high x 13.5 wide, while the connections are 3.0 x 7.5 mils (and are slightly higher than fifty ohms impedance at 120 GHz). Overall dimension is 28.5 x 15 mils. The use of mils rather than microns was out of laziness, these were default units in the project.
Here is the coupled and direct ports excited from P1. There is approximately 0.25 dB loss due to the metal conductivity.
Here's the return loss and isolation:
The relative phases of the two 3dB ports are approximately in quadrature:
We'd appreciate another example, especially if it included measured data!