The simple wire dipole is a ubiquitous antenna, familiar to anyone that has ever attended a course on antennas or considered electromagnetic radiation. The design of these structures is deceptively simple - each arm should be just a little less than a quarter-wavelength long and driven in anti-phase to provide a linearly polarized radiator with 73 Ohm input impedance.
When the basic dipole is used in an array, however, the close proximity of the elements in the array causes interaction between elements, altering the currents (and thus impedances and radiation characteristics) of each element. This interaction, or mutual coupling, affects the way that the element should be designed, both in terms of the length of the element arms and the anticipated input impedance. The figure shows the comparative reflection coefficient of a dipole in an array environment when designed for single element usage (original) and when adjusted for use in an array (adjusted). The input impedance of the dipole in the array is designed so that it performs as though it were a single isolated element. ...Comparative single element reflection coefficient of a dipole designed for single-element operation and a dipole designed for inclusion in an array.