A focal cloud is the collection of focal point

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

s of any imperfect lens,
whether optical

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

, electrostatic

Electrostatic lens

An electrostatic lens is a device that assists in the transport of charged particles. For instance, it can guide electrons emitted from a sample to an electron analyzer, analogous to the way an optical lens assists in the transport of light in an optical instrument. The recent development of...

 or electromagnetic. This includes Parabolic antenna

Parabolic antenna

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...

s and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion

Circle of confusion

In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source...

 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

Feed horn

In satellite dish and antenna design, a feedhorn is a small horn antenna used to convey radio waves between the transmitter and/or receiver and the reflector, particularly in parabolic antennas...

, 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

Feed horn

In satellite dish and antenna design, a feedhorn is a small horn antenna used to convey radio waves between the transmitter and/or receiver and the reflector, particularly in parabolic antennas...

.

References:

Satellite LNB's & polarizers