Cavity Model

Another model available at our disposal in order to analyze microstrip antennas is the cavity model. This model is different from transmission line model as in it is provides a better way to model the radiation patterns and is closer in the physical interpretation of the antenna characteristics. The normalized fields within the dielectric can be found more accurately by treating the region as a cavity bounded by electric conductors (above and below) and by magnetic walls along the perimeter of the patch. An attempt is made to present a physical interpretation into the formation of the fields within the cavity and radiation through its side walls.

Physical Insight
When the microstrip patch is energized, a charge distribution is established on the upper and lower surfaces of the patch, as well as on the surface of the ground plane. The charge distribution is controlled by two mechanisms; an attractive and a repulsive mechanism. The attractive mechanism is between the corresponding opposite charges on the bottom side of the patch and the ground plane, which tends to maintain the charge concentration on the bottom of the patch. The repulsive mechanism is between the like charges on the bottom surface of the patch, which tends to push the charges from the bottom around its edges to the top surface of the patch.

E-plane pattern

H-plane pattern

Slot Conductance
Single slot conductance

Where,

Mutual Conductance

Where, J0 is the Bessel function of the first kind and order zero.

Characteristic impedance of microstrip line feed
For W0/h ≤ 1

For W0/h > 1

Beamwidths
E-plane

H-plane