Behavior of parallel plate mode in a slot-coupled patch antenna with a stripline feed

There are many attractive features in a slot-coupled patch antenna with a stripline feed (SCPA). For example, additional circuit integration under the bottom ground plane of the line can be easily achieved, providing the electrical isolation in between. The use of the stripline for the slot-coupled configuration, however, causes the power leakage to be parallel plate mode, resulting in a decrease of the radiation efficiency. In our previous work, it has been found that the leakage power to the parallel plate mode becomes a minimum at the resonant frequency of the radiating patch. In addition, we have found that frequency dependent characteristic of the leakage power is insensitive to the slot length. The background of the above phenomena involving the parallel plate mode is explained using a lumped element circuit representation of the SCPA. The approach is based on the moment method in which the spectral domain Green´s functions to account for the effect of the parallel plate mode.