A Reflectionless Circularly Polarized High Gain Microstrip Filtering Antenna with Wideband Response

A high gain reflectionless circularly polarized (CP) filtering antenna is proposed. A quadrature hybrid coupler (QHC) is cascaded with two identical, mutually isolated and orthogonally linearly polarized (LP) third-order filtering antennas to produce a CP response. Out-of-band reflections from the CP filtering antenna are dissipated through a matched load connected to the QHC's isolated port. Each of the two LP filtering antennas has two stub resonators and a patch resonator, coupled together. The square-shaped patch resonator is dual-fed and acts as a common radiating resonator for the two LP filtering antennas by utilising its two orthogonal modes, TM ₁₀₀ and TM ₀₁₀ . The patch is backed with a defected air cavity to increase its gain and decrease its quality factor while keeping the feeds mutually isolated. The filtering antenna operates at 6.84 GHz with a 9.06 dBic peak gain, a wide 3 dB gain bandwidth of 15.79% and axial ratio below 3 dB in the gain passband. Moreover, a wideband reflectionless performance is obtained with | S ₁₁ | ≤ -10 dBbandwidth of 34% that extends on both sides of the gain passband. Suppression over 29 dB is achieved beyond 35% of band-edge frequencies of the gain passband. Additionally, the monotonically decreasing all-pole response provides low dispersion losses. The low profile (0.064λ ₀ ) structure can operate as a left-hand or a right-hand CP filtering antenna, interchangeably. The proposed reflectionless single-layered microstrip design assures easy, cheap manufacturing and compatibility with surface-mount devices.