Focal cloud

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Antennas
Common types Dipole Fractal Loop Monopole Satellite dish Television Whip
Components Balun Block upconverter Coaxial cable Counterpoise (ground system) Feed Feed line Low-noise block downconverter Passive radiator Receiver Rotator Stub Transmitter Tuner Twin-lead
Systems Antenna farm Amateur radio Cellular network Hotspot Municipal wireless network Radio Radio masts and towers Wi-Fi Wireless
Safety and regulation Wireless device radiation and health Wireless electronic devices and health International Telecommunication Union (Radio Regulations) World Radiocommunication Conference
Radiation sources / regions Boresight Focal cloud Ground plane Main lobe Near and far field Side lobe Vertical plane
Characteristics Array gain Directivity Efficiency Electrical length Equivalent radius Factor Friis transmission equation Gain Height Radiation pattern Radiation resistance Radio propagation Radio spectrum Signal-to-noise ratio Spurious emission
Techniques Beam steering Beam tilt Beamforming Small cell Bell Laboratories Layered Space-Time (BLAST) Massive Multiple-input multiple-output (MIMO) Reconfiguration Spread spectrum Wideband Space Division Multiple Access (WSDMA)
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A focal cloud is the collection of focal points of an imperfect lens or parabolic reflector whether optical, electrostatic or electromagnetic. This includes parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion in photography.

In a perfect lens or parabolic reflector, rays parallel to the device's axis striking the lens or reflector all pass through a single point, the focal point. In an imperfectly constructed lens or reflector, rays passing through different parts of the element do not converge to a single point but have different focal points. The set of these focal points forms a region called the focal cloud. The diameter of the focal cloud determines the maximum resolution of the optical system. Lens-reflector artifacts, geometry and other imperfections determine the actual diameter of the focal cloud.