An increasingly common array of four vertical antennas arranged in a square that is electronically steered in four, switchable directions. Once used mostly as transmit antennas, particularly on lower frequencies where Yagis are not practical, these arrays are also becoming common as receive antennas. 4-square controllers, which perform the switching, are both home-brew and commercially available.
DX Engineering offers a variety of 4-square systems, controllers, and accessories from leading manufacturers, including its own branded Receive 4-Square Array System (vertical receive antennas, control console and 4-square receive controller, coaxial cable, tools and connectors), its Transmit Four-Square Hybrid Controllers for 160M, 80M, and 40M, and the COMTEK ACB-4 Hybrid Four-Square System, including phased array relay unit and control console for use with four quarter-wave ground mounted verticals.
