Experimental study of the characteristics of top-loaded microstrip monopoles

The characteristics of microstrip monopoles fed at the center with antiphase currents are investigated experimentally. First the impedance is measured as a function of frequency. It is found that the radiation resistance at the resonance near L (the monopole length) equal to approximately 0.5 lambda is very low while the radiation resistance at the resonance near L equal to approximately 1 lambda is very high. For a given physical length of the monopole, the resonant frequency is dependent on the strip width and the substrate thickness. The effect of substrate thickness and strip width on the input impedance and bandwidth has also been studied. Finally the E-plane radiation pattern is found to contain two maxima, occurring at approximately + or - 40 deg from broadside.