Mode-Based Information Channels in Closely Coupled Dipole Pairs

Information channels supported by two orthogonal radiation modes in a pair of closely spaced dipole antennas are investigated. The radiation impedances of the two modes are closely related to the coupling behavior of the 2-element array as a function of the antenna spacing. Through a multiport impedance matching network, the common and difference modes of the coupled antenna system can be separated into different driving ports. Conjugate match can thus be independently performed for each port, resulting in a fully matched 2-element array. Such a 2-element array with closely spaced antennas can potentially offer a significant array gain or an ideal 2 times 2 multiple input multiple output channel capacity. Full-wave simulations of a pair of half-wave dipoles with 0.1 lambda spacing have demonstrated the feasibility of the approach for narrowband applications. By combining the common mode and difference mode, approximately 3 times of the gain of a single half-wave dipole can be achieved. Experimental validation with a test bed consisting of two lambda/4 monopoles and a rat-race coupler has been carried out. The concept may be extended onto closely coupled antenna systems with different numbers of elements.