A cage antenna (

British British may refer to: Peoples, culture, and language * British people, nationals or natives of the United Kingdom, British Overseas Territories, and Crown Dependencies. ** Britishness, the British identity and common culture * British English, ...

cage aerial) is a radio antenna where a conventional design has been augmented by replacing a single long conductor with several parallel wires, connected at their ends, and held in position by ring spacers or support struts mounted on a central mast (if any). The "cage" is either mounted around a central mast (either conducting or non-conducting) or suspended from overhead wires.

Purpose

A cage can improve any antenna design by replacing a single wire in any section that carries large, unbalanced (radiating) current; the only issue is whether the improvement will be substantial enough to warrant the extra effort. The multiple parallel wires electrically simulate a single fat wire, roughly equivalent to a wire with a diameter half as wide as the distance separating the cage wires, giving the modified antenna higher

radiation resistance Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna. Radiation resistance is an ''effecti ...

, lowered conductor resistance, and a wider

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

without as much weight and wind resistance as the equivalent fat wire would entail. Cage aerials have been built in different variants for broadcast stations in the

longwave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...

and

mediumwave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayt ...

bands. One advantage of the cage aerial is that the supporting tower can be grounded. Grounding the tower allows it to easily be used for other radio services, such as a high mounting point for smaller

VHF Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF ...

UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...

antennas. A grounded tower also simplifies the installation of aircraft warning lamps, since their electrical feeds do not need to have special blocking filters to prevent the escape of RF through the power lines. For commercial transmitting towers the height needed for such an antenna is typically not a problem, since the mast height is selected for a TV or FM antenna mounted at its top, which need to be placed high for line-of-sight transmission at such high frequencies. When used as the main section of a vertical antenna, the cage is usually built to have a length one-quarter of the operating wavelength, and to surround the entire length of the central mast. If the tower is grounded, the cage wires must be electrically insulated from the tower for their entire length, only connected at the top, if at all. The cage electrical connection near the top of the mast is always optional, but usually that connection is made: When the cage wires are all connected to the grounded tower at or near its top, the type of antenna is in some places called a " folded unipole", which was extensively studied by John H. Mullaney in the mid-20th century. When the combination is a "unipole", the cage wires are usually called a "skirt". The cage wires are always fed at the ring connecting the cage's lower edge, with the other electrical side of the feedpoint always being the ground system, regardless of whether or not the tower itself is grounded. ;Example: At 1,000 kHz the wavelength is 300 m. Therefore, the minimum

wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (re ...

height of the cage antenna is a bit less than 75 m for the cage, plus about 2 metres to raise the lower end of the cage out of reach from the ground (depending on the details of the antenna feed, the lower end of the cage can carry a very high RF voltage). {{Antenna Types Radio frequency antenna types Antennas (radio)