Circular Polarization
It is also possible to fabricate patch antennas that radiate circularly-polarized waves. One approach is to excite a single square patch using two feeds, with one feed delayed by 90° with respect to the other. This drives each transverse mode and with equal amplitudes and 90 degrees out of phase. Each mode radiates separately and combine to produce circular polarization. This feed condition is often achieved using a 90 degree hybrid coupler. When the antenna is fed in this manner, the vertical current flow is maximized as the horizontal current flow becomes zero, so the radiated electric field will be vertical; one quarter-cycle later, the situation will have reversed and the field will be horizontal. The radiated field will thus rotate in time, producing a circularly-polarized wave.
An alternative is to use a single feed but introduce some sort of asymmetric slot or other feature on the patch, causing the current distribution to be displaced. A square patch which has been perturbed slightly to produce a rectangular microstrip antenna can be driven along a diagonal and produce circular polarization. The aspect ratio of this rectangle is chosen so each orthogonal mode ( and modes) are both non-resonant. At the driving point of the antenna one mode is +45 degrees and the other -45 degrees to produce the required 90 degree phase shift for circular polarization.
Note that, while circular patches can be used for these techniques, a circular patch does not inherently radiate circularly-polarized waves. A circular patch with a single feed point will create linearly-polarized radiation. If the circular patch antenna is perturbed into an ellipse and fed properly it can produce circularly polarized electromagnetic waves.
Read more about this topic: Patch Antenna
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