As we know, patch antenna is combination of two magnetic dipole. If we place a perfect electrical conductor near a dipole, antenna performance will suffer significantly. Then how a patch antenna will radiate? is it only radiate by fringing field??
"If we place a perfect electrical conductor near a dipole, antenna performance will suffer significantly."
Debraj Kar You're thinking of an electric dipole here. However, as you mentioned, we typically model a patch antenna as a pair of magnetic dipoles. While image theory isn't usually considered in the case of the patch antenna, if you were to do so the dipoles would be in-phase with their mirror images and hence not cancel destructively as electric dipoles would. See Balanis' Antenna Theory text for a more thorough description.
Debraj Kar The radiation principles of a patch antenna is better explained by the Generalized Cavity model of rectangular patches, and the currents you are mentioning are surface currents. See Stutzman's "Antenna Theory and Design, page 486" to better understand the propagation regimes of microstrip antenna.
Regards,
Stuart Barth Then my question will be why patch antenna is considered as a array of two magnetic dipole. why is it not electric dipole? Which criteria makes it different? can u please mention the Balanis page number. Thanks
When ground plane is near to radiator then some current is induced in the ground plane since it is a conductor. Then the induced currents in the ground plane produce small fields, which are also radiated.
Debraj Kar If one was to think of the patch as an electric dipole (based on the half-wavelength resonance of currents along the patch), one would end up with the situation you described in your original posting: the electric dipole moment in close proximity to a good conductor essentially cancels itself out with its image. However, there is still a magnetic dipole moment induced by the electric fields of the radiating slots (where M = -n x E), which do not cancel each other out.
See Balanis (3rd Ed.) page 832, and page 187 for dipole sources and their images near ideal conductors.
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