This paper presents differential-fed patch antennas with excellent cross-polarization. This paper provides a detailed graphic illustration of factors that lead to deteriorated H-plane cross-polarization by the conventional single-ended feeding probes. A novel differential rat-race feeding structure was constructed to allow easy impedance matching. An experimental antenna was realized on low-temperature co-fired ceramic (LTCC) at 8 GHz. An excellent cross-polarization of less than -22.5 dB was achieved. When the operation frequency is high, the parasitic inductance caused by feeding probes may degrade the performance of antennas. This paper further proposes the use of differential aperture-coupled structures at high frequencies. An aperture-coupled antenna, realized at 40 GHz with low cross-polarization <-15 dB has been achieved.
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