Maximum directivity of a series-fed microstrip array antenna for wireless application

An omnidirectional colinear microstrip array antenna is modeled as a ladder network. A formula is derived for the maximum directivity of a series-fed colinear antenna with an infinite number of ladder sections. It is shown that the maximum directivity is asymptotically limited and depends upon the product of the series impedance and shunt admittance of the equivalent ladder network. The result is verified by simulation and experiment.