Vertical Integration of High- $Q$ Filter With Circularly Polarized Patch Antenna With Enhanced Impedance-Axial Ratio Bandwidth

Directly coupling antennas to filters can remove the transition loss between the two and reduce the size of radio frequency front ends. Circularly polarized (CP) patch antennas with a single feed are very popular in many applications due to their compact size and ease of design. However, a probe-fed CP patch antenna typically exhibits only 1%-2% useful fractional bandwidth by considering both impedance matching and axialratio (AR) bandwidths. Thick substrates were used to realize single-feed CP antennas with wider bandwidth. However, thick substrates may cause issues, such as surface wave excitation, mutual coupling, and low efficiency. In this paper, we present a new approach to enhance the impedance-AR bandwidth of probefed CP patch antennas without using thick substrates. By integrating CP patch antennas with cavity filters and sequentially rotating the CP filter/patch antenna elements to form a 2 × 2 array, an impedance-AR bandwidth of 8.3% is achieved with the thickness of the antenna layer to be 0.0292λ
₀
. In addition, a third-order Chebyshev filtering function is embedded in this antenna array to enhance the out-of-band interference rejection.