What is the physics behind the generation of orthogonal modes in antenna to ensure circular polarisation? Please suggest your answers.
Thank you.
It depends on the antenna nature. For example, a patch antenna can be excited on two orthogonal edges with a 90 degree phase shift to get CP. Two orthogonal dipoles excited with 90 degrees phase shift can also generate a CP. In other antennas, adding a slot in the antenna can generate a circular current on the antenna surface and yeild it CP. This is the case of a patch antenna with an L-shaped slot for exampe.
Circular polarisation can be achieved using one of the following ways:
1. Feed with 90 deg. phase shift, like hybrid coupler
2. Transmission line feed, power divider and keeping one of the line wavelength/4 ahead or behind.
Also, using various design patterns like:
1. A Cross-Slot of Unequal Arm length
2. Y-Shaped Slot of Unequal Arm Length
3. Designs with Slits
4. Designs with Spur Lines
5. Designs with Truncated Corners
6. Designs with Peripheral Cuts
7. Designs with a Tuning Stub
For more details you can refer "Compact and broadband Microstrip Antennas" by Kin-Lu Wong
You need to make best efforts to make reception orthogonal, then residual non-orthogonality components (which will no doubt exist) you'll need to take out will a calibration. In the calibration you'll launch and measure co and cross-polarisations.
Good luck, Neil
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