Cage aerial

Historic Radio Engineers Club station, Riverhead, New York, in 1922; a cage T-antenna 60 ft high by 90 ft long. The conductor is made of a "cage" of 6 wires held apart by wooden spreaders; this increased capacitance and decreased ohmic resistance. This antenna achieved transatlantic contacts on 1.5 MHz, at a power of 440 W.

A cage antenna (British 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, lowered conductor resistance, and a wider bandwidth 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 and mediumwave bands.

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, and only optionally connected at the top, if at all. 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.

In the entirely optional case that the cage is electrically connected near the top of a mast and that mast's base is shorted to the ground plane, then the electrical function of antenna changes to a "folded unipole". When configured as a folded unipole, the cage wires are instead called "skirt wires". A folded unipole might or might not obtain the wider bandwidth expected of a cage antenna, depending on how the unbalanced current splits between the mast and the skirt.

Example

At 1,000 kHz the wavelength is 300 metres (1000′). Therefore, the minimum 1/ 4  wave height of the cage antenna is a bit less than 75 metres (250′) for the cage, plus about 2 metres (6′) 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).