A compact circularly-polarized patch antenna with wide axial-ratio beamwidth (ARBW) operating at dual bands compatible for GPS satellite navigation application is proposed in this paper. The antenna consists of two stacked patches, which account for GPS L1 band (1575 ± 2 MHz) and L2 band (1228 ± 2 MHz). To realize significant size miniaturization, four pillars are introduced and strongly coupled to the lower patch. To adjust the beamwidth response of the far-field components, the antenna is backed by a modified metallic cavity, which in turn widen the 3-dB axial ratio beamwidth (ARBW) at two operating bands effectively Finally, the proposed design with excellent performance is realized in a size of 0.368 λ0 (diameter) and 0.105 λ0 (height). Simulated and measured results show that the ARBWs at L1 and L2 bands are as wide as 173° and 133°, respectively. For practical application, its satisfactory GPS signal receiving capability is also verified by outdoor positioning test.

A singly fed circularly polarized (CP) antenna based on coupled radiator pair with stable CP performance is proposed for the accurate positioning of smart watches in this article. It is first demonstrated that two identical coupled monopoles sharing a finite ground can be equivalent as a second-order filter network consisting of composite J/K inverters. The coupled radiators will produce a pair of even and odd modes, whose resonant frequencies and bandwidths are determined by the coupling strength. After that, the characteristic mode analysis (CMA) is conducted to verify the circuit model, and it reveals that the even and odd modes are orthogonal to each other, which are candidates for CP radiation. In particular, the phase difference of the two modes is determined by the coupling coefficient, and it can be elaborately manipulated by properly changing the current distribution of characteristic modes through ground size, slot loading, and radiator length. At last, two antenna prototypes are designed, implemented, and measured to validate the working principle. Different from conventional CP antennas operating in degenerate modes, the proposed antennas are insensitive to perturbation segments and can maintain stable CP performance, which is highly demanded in smart watches under wearing conditions.

The fundamental working principle of front-to-back ratio (FTBR) enhancement of the quadruple inverted-F antenna (QIFA) is clearly revealed for the first time, and a unified design method is then proposed in this article. The characteristic mode analysis (CMA) demonstrates that the QIFA actually operates in two orthogonal modes, and analytical derivation validates that each mode acts as a pair of parallel magnetoelectric (ME) short dipoles for radiation. More specifically, the FTBR of the antenna is determined by the amplitude ratio and phase difference of the two pairs of equivalent ME dipoles. The equivalent model and analytical method can greatly facilitate the design. First, by leading or lagging phase of either characteristic mode through excitation, the beam direction can be flexibly prescribed as forward and backward. Second, the bending direction and bending degree of radiators, respectively, determine the phase and amplitude of its equivalent magnetic dipole, by which the FTBR can be improved. A practical design is carried out with the proposed method, and the antenna achieves an electrically small size of $0.068\lambda _{0} \times 0.068\lambda _{0} \times 0.052\lambda _{0}$ and an FTBR of 12.3 dB.

A novel hybrid water antenna with tunable frequency and beamwidth is proposed. An L-shaped metallic strip is adopted as the feeding structure of the antenna in order to effectively broaden the operating bandwidth. The L-shaped strip feeder and a rectangular water dielectric resonator constitute the driven element. Five identical rectangular water dielectric elements are mounted linearly with respect to the driven element, which act as the directors and contribute to narrow the beamwidth. By varying the height of the liquid water level in the driven element, the proposed antenna is able to tune to different operational frequencies. Furthermore, it is also able to adjust to different beamwidths and gains via varying the number of director elements. A prototype is fabricated by using 3-D printing technology, where the main parts of the antenna are printed with photopolymer resin, and then the ground plane and L-shaped strip feeder are realized by using adhesive copper tapes. Measurement results agree well the simulation ones. A tunable frequency ranging from 4.66 GHz to 5.65 GHz is obtained and a beam steering along a fixed direction with a gain variation less than 0.5 dB is realized.

A travelling-wave-fed slot spiral antenna with wide axial-ratio (AR) bandwidth and beamwidth for GNSS applications is proposed in this paper. The evolution procedure of the proposed antenna can be classified into three steps. Firstly, 12 identical spiral slots are arranged concentrically in a compact area, and AR roundness in low elevation plane is improved by virtue of good rotational symmetry of slot radiators. Secondly, a two-turn travelling-wave feed line is sequentially coupled to the slots, and wideband response of desired magnitude ratio and phase difference of $E_{\theta }$ and $E_{\varphi }$ is obtained by optimizing the number and growth rate of microstrip turns. Thirdly, by supporting the antenna with a modified metallic cavity, the CP radiation performance in low elevation plane is well improved, and thus a widened AR beamwidth is obtained. Finally, the proposed antenna with satisfactory performance is realized in a diameter of $0.56\lambda _{0}$ and height of $0.12\lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at center frequency 1.4 GHz. Measured bandwidth of $|S_{11}|$ fully covers the entire GNSS bands, and the 3-dB AR bandwidth ranges from 1.14 to 1.67 GHz (37.7%). Moreover, the 3-dB AR beamwidth exceeds 140° at 1.4 GHz.