A Novel Low-Profile Circularly Polarized Diversity Patch Antenna With Extremely Small Spacing, Reduced Size, and Low Mutual Coupling

A novel low-profile circularly polarized (CP) diversity patch antenna with extremely small edge-to-edge distance, reduced size, and low mutual coupling is proposed under dual modes. Initially, the single microstrip patch antenna (MPA) is theoretically investigated by using characteristic mode analysis. It demonstrates that a large number of radiative modes, i.e., CM1 to CM7, are excited for the traditional MPA. By selecting its quarter patch configuration with shorting pins, only a few modes, i.e., CM2 and CM7, could be resonated for the quarter patch antenna (QPA). Meanwhile, their nonbroadside radiated fields could be transformed into the broadside radiation. Particularly, the polarizations of CM2 and CM7 are maintained orthogonally with each other. Next, a linear slot is etched on the patch to reduce the magnitude ratio between
$\vert E_{\mathrm {\theta }}\vert $
and
$\vert E_{\varphi }\vert $
components. After that, the resonant frequency of CM7 is moved closely to that of CM2 for nearby 90° phase difference. With these arrangements, the compact CP performance is acquired by combining its CM2 and CM7. Furthermore, a pair of QPAs with CP diversity is deeply studied. The results prove that the antenna has successfully acquired both of left-handed CP (LHCP) and right-handed CP (RHCP) performances at dual ports. Moreover, the low mutual coupling between them is generated under extremely small spacing. In final, the proposed antenna is fabricated and tested. Both of LHCP and RHCP patterns are formed of around 4.785 GHz. Besides, the proposed antenna occupies the small overall volume
$0.023\lambda _{\mathrm {g}}^{3}$
(
$\lambda _{\mathrm {g}}$
is the guided wavelength), that is only half of the traditional patch (
$0.045\lambda _{\mathrm {g}}^{3}$
), and maintains the isolation as high as about 24 dB under extremely small distance (0.3 mm).