Wideband Dual Circularly Polarized Helical Antenna With Reduced Mutual Coupling for MIMO Applications

A novel single helix antenna with wideband and dual circularly polarized (CP) characteristics is proposed in this communication. The left-handed CP (LHCP) and right-handed CP (RHCP) radiations are achieved by exciting the helical antenna from the top end and bottom end, respectively. A grounded copper plane is loaded to the top of the helix to support the microstrip feed line and adjust the radiation characteristics. The principles of achieving the dual CP in the broadside direction are analyzed. Besides, the methods of enhancing the isolation between the top and bottom ports (two orthogonal CPs) are investigated. The proposed dual CP helical antenna is further developed to a $2\times2$ multiple-input multiple-output (MIMO) antenna, rendering an eight-port MIMO antenna. A copper open cavity wall is adopted to surround each element to reduce the mutual couplings between the elements. The dual CP helical antenna element and the dual CP helical MIMO antenna are prototyped and verified experimentally. Measured results agree well with the simulations. The dual CP helical element shows an overlapped impedance/3dB-axial ratio (AR) bandwidth from 3.7–4.1 GHz (FBW = 11%) when LHCP and RHCP are excited individually. The isolation between the two orthogonal polarizations is over 20 dB. The measured gains are over 10 and 9 dBic over the band when the RHCP and the LHCP are excited, respectively. Besides, the isolation between elements of the MIMO antenna is better than 20 dB.