Analysis of microstrip antenna structures using the 'add-on' technique

The so-called add-on method (IEEE AP-S Int. Symp. Digest 1987, pp. 276-279) is applied to microstrip antenna problems. While the treatment of scattering problems requires the synthesis of an incident field by a distribuiton of an equivalent current source over the plane containing the scatterer, only the current source at the driving point is needed for the antenna problem. The equivalent current source is modeled by a small square patch carrying one subdomain basis function with a unit amplitude. This function is also used for the unknown current on the body. The source is considered tangential to the plane, as the unknown current. The source is connected to the radiating structure via a model of a long transmission line, 70 to 99 patches long, as a part of the problem (this corresponds to a line 2.7 to 3.1 wavelengths long). Once the problem is solved, the profile of the current distribution along the line may be studied, and the standing-wave pattern revealed from which the amplitude and phase of the reflection coefficient are readily deduced.