Focal cloud

Focal cloud of a reflector (red) compared to an ideal focal point (blue)

Part of a series on
Antennas
Common types Dipole Fractal Loop 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 Mobile phone 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 Bell Laboratories Layered Space-Time (BLAST) Multiple-input multiple-output (MIMO) Reconfiguration Spread spectrum Wideband Space Division Multiple Access (WSDMA)

A focal cloud is the collection of focal points of any imperfect lens, 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 this focal cloud illustration, a perfect reflector would focus all beams into the blue spot. Due to imperfections of lens construction, the actual signal beams follow the red path to several different focal points, which form the focal cloud. Lens-reflector artifacts, geometry and other imperfections determine the actual radius of the focal cloud.

Satellite dish effects

A focal cloud can render a satellite dish rather inefficient in relation to its size in specific frequencies. If the imperfections are formed in a way that the phase shifts of reflected waves, at a large portion of the dish, are near 180 degrees between them and the feed horn, this results in self-cancellation of the specific frequency.

For countering the effect, there are several techniques, either in construction of the reflectors or lenses, or in the way signal beams are concentrated. In satellite antennas, scalar rings are used to push the spread signal into the feed horn.

References