A novel dual-frequency concept for series-fed microstrip patch array antennas is proposed. An identical near field distribution following an identical far field characteristic was achieved at two different frequencies. The conventional series fed array comes with a uniform aperture distribution for patches of same width. By exploiting the dispersion of this periodic structure the same cosine aperture distribution at two different frequencies is obtained. Due to the cosine aperture distribution a low side lobe level of 23 dB is intrinsically achieved for the two operating frequencies, 23 GHz and 25 GHz. This concept is particularly suited for small frequency ratios (f+/f− < 1.3).