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Radiation Pattern
An offset antenna pattern is determined by measuring the energy radiated by the antenna through various angles over a constant distance. The radiation pattern consists of three regions, known as main beam, or main lobe, sidelobes and backlobe. The main lobe represents the direction of maximum radiation; the sidelobes represent smaller beams of radiation, away from the main beam; and the backlobe represents radiation in the opposite direction to the main beam.
Feed Horn
The feed horn of an offset antenna is deliberately offset to prevent the feed horn itself from casting a shadow over the antenna dish and creating a ̶0;dead spot.̶1; Such a dead spot can severely degrade the signal coming from the reflector to the antenna, a phenomenon known as ̶0;aperture blocking.̶1; Offsetting the feed horn does raise the level of sidelobes, however.
Sidelobes
Sidelobes are important characteristics of an offset antenna pattern. A conventional offset pattern is characterized by asymmetric sidelobes, relative to the main beam, which reduces the sensitivity of the antenna in a particular direction and its ability to amplify signals. If an offset antenna is used to transmit signals, sidelobes may allow transmissions to be intercepted by people other than the intended receiver. If an offset antenna is used to receive signals, sidelobes increase the likelihood of it encountering interference or causing interference with other receivers in the vicinity.
Reducing Sidelobes
The issues caused by sidelobes in an offset antenna pattern can be reduced, at a cost, by designing the dish and feed horn of the antenna in a particular way. This may involve mounting the feed horn upwards in relation to the axis of the dish and removing the lower half of the dish to create a shallow dish with a longer focal length. Another method involves attaching metal strips, known as ̶0;scatters,̶1; to the surface of the dish, which literally scatters the radiation from the antenna to the reflector.