CDDA
Initialism of circularly disposed dipole array.
CDDA: antenna array for direction-finding without rotation
A circularly disposed dipole array is a fixed antenna arrangement in which multiple dipole elements are mounted at equal angular intervals around a circle, typically six, eight, or twelve elements evenly spaced. Unlike mechanically rotated directional antennas, a CDDA samples incoming signals from all azimuthal directions simultaneously, then uses phase and amplitude data from each dipole to calculate the bearing of a signal source through electronic processing.
Each dipole in the array is usually a thin resonant conductor, one half-wavelength long at the target frequency, fed through a receiver or transceiver channel. The outputs from all channels are fed to a direction-finding processor that compares the signal strength and phase relationships across the array. Since the dipoles are spatially distributed, a signal arriving from a particular bearing will reach some elements before others and at different amplitudes, creating a signature pattern that correlates to a specific azimuth.
Application and Performance Trade-offs
CDDAs are favored in communications intelligence, radio navigation systems, and emergency direction-finding where continuous, rapid bearing updates are needed without mechanical rotation. They are particularly effective at frequencies from about 30 MHz to several GHz, though the physical size and element spacing scale with wavelength. A CDDA operating at 150 MHz with eight elements might occupy a circle two meters in diameter, while a 2 GHz array could be much smaller.
The main limitation is accuracy: bearing resolution depends on array diameter relative to wavelength. Below a certain diameter, or at frequencies where the array is electrically small, ambiguities in azimuth may occur, and elevation angle cannot be determined without additional vertical elements. Multipath reflections and ground effects also degrade performance. Despite these constraints, the speed and economy of not requiring mechanical drive systems make CDDAs practical for mobile platforms, ship-mounted systems, and fixed monitoring stations.