Control of surface waves in planar printed antennas

When printed antennas are fabricated monolithically on semiconductor substrate materials, the substrate thickness tends to be electrically thick, which can increase the impedance bandwidth of the antenna. However, it has the negative effect of increasing the power coupled into the surface wave. Thus, there is a need to control the surface wave behavior, especially before they reach the edge of the ground plane. In this paper, we present two schemes to control surface waves. The first scheme is to convert them into leaky waves so they radiate in a controlled fashion before they reach an edge. An example of this scheme is leaky wave antennas. Another example is the use of chokes around the edge of the ground plane. The second scheme, which is briefly discussed here, is to absorb the surface waves using a lossy material. A lossy material that is suitable to absorb surface waves is the resistive card. It has been used in the past to minimize edge diffraction from metal edges which can reduce the cross-section of aircraft, missiles, ships, etc. It is noted that thin resistive cards can be efficiently modeled by the resistive boundary condition.