radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a tr ...

electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, a null is a direction in an antenna's

radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “A ...

where the antenna radiates almost no radio waves, so the far field

signal strength In telecommunications, particularly in radio frequency engineering, signal strength refers to the transmitter power output as received by a reference antenna at a distance from the transmitting antenna. High-powered transmissions, such as those u ...

is a local minimum. Nulls occur because different parts of an antenna radiate radio waves of different phase. In directions at which the antenna radiates equal amplitude radio waves of opposite phase, the radio waves cancel, resulting in little or no radio power being radiated in that direction. In other directions the radio waves from different parts of the antenna are in phase and reinforce, resulting in a maximum signal strength in the radiation pattern, called a lobe. In transmitting antennas designed to provide broad coverage nulls can be a problem, preventing reception in a given area. Null fill in the

vertical plane In astronomy, geography, and related sciences and contexts, a '' direction'' or ''plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. Conversely, a direction or plane is said to be hor ...

is used to prevent this.

Nulling antenna

Nulls can also be used to advantage. In a radio receiver the receiver's antenna can be adjusted so the direction of the interference source is located in a null of the antenna, to minimize reception of interference. Nulls have also been used intentionally to prevent an antenna from broadcasting to a certain area. For example, CIII-DT-22, a repeater of

Toronto Toronto ( ; or ) is the capital city of the Canadian province of Ontario. With a recorded population of 2,794,356 in 2021, it is the most populous city in Canada and the fourth most populous city in North America. The city is the anch ...

-based

Global Global means of or referring to a globe and may also refer to: Entertainment * ''Global'' (Paul van Dyk album), 2003 * ''Global'' (Bunji Garlin album), 2007 * ''Global'' (Humanoid album), 1989 * ''Global'' (Todd Rundgren album), 2015 * Bruno ...

TV station

CIII-DT CIII-DT (channel 41) is a television station in Toronto, Ontario, Canada, serving as the flagship station of the Global Television Network. Owned and operated by network parent Corus Entertainment, CIII-DT maintains studios at 81 Barber Gree ...

Wheatley Wheatley may refer to: Places * Wheatley (crater), on Venus * Wheatley, Ontario, Canada * Wheatley, Hampshire, England * Wheatley, Oxfordshire, England ** Wheatley railway station * Wheatley, South Yorkshire, England * Wheatley, now Ben Rhyddi ...

has a null towards

Windsor Windsor may refer to: Places Australia * Windsor, New South Wales ** Municipality of Windsor, a former local government area * Windsor, Queensland, a suburb of Brisbane, Queensland **Shire of Windsor, a former local government authority around Wi ...

to protect broadcast rights of American stations in the bordering

Detroit Detroit ( , ; , ) is the largest city in the U.S. state of Michigan. It is also the largest U.S. city on the United States–Canada border, and the seat of government of Wayne County. The City of Detroit had a population of 639,111 at t ...

area.

Radio direction finding Direction finding (DF), or radio direction finding (RDF), isin accordance with International Telecommunication Union (ITU)defined as radio location that uses the reception of radio waves to determine the direction in which a radio stati ...

(RDF) receivers use special antennas with very narrow, sharp nulls to find the location of transmitters. The antenna is rotated until the received signal is minimum; at that point the antenna's null is pointed along the bearing line to the transmitter.

Nulling antenna

See also