Folded unipole antenna

Lore & Background

The folded unipole antenna was first devised for broadcast use by John H. Mullaney, an American radio broadcast pioneer and consulting engineer. It was designed to solve some difficult problems with existing medium wave, frequency modulation, and amplitude modulation broadcast antenna installations. The antenna is most often used for refurbishing old broadcast antennas, converting an ordinary broadcast tower into a folded unipole by adding surrounding skirt wires.

In a conventional monopole antenna, the mast is supported on a thick ceramic insulator isolating it from ground, and the feedline connects between the mast and the ground system. For a folded unipole, the base of the tower is connected directly to the ground system by shorting out the base insulator. Then a series of vertical wires—typically four to eight—are installed from an attachment at or near the top of the tower, surrounding it and kept a constant distance by insulated stand-off members. These skirt wires are joined to an electrically isolated conductor ring near the base, and the new antenna feed connects between the ground system and that ring.

The skirt wires add inductive reactance to the antenna mast, which helps neutralize a short mast's capacitive reactance. For a short monopole, the inductive reactance introduced by the skirt wires decreases as frequency decreases and the bare mast's capacitive reactance increases, so cancellation can only occur at a single frequency. With a longer antenna mast, at least a quarter-wave tall, the contrary reactances can be made to cancel each other at more than one frequency, widening the antenna's low-reactance bandwidth. If the greater part of the unbalanced radio current flows in the skirt wires instead of the mast, the outer ring of skirt wires effectively adds electrical width to the mast, improving bandwidth by causing the unbalanced currents to function like a cage antenna.

Reader's Guide

The folded unipole antenna's significance lies in its ability to solve practical problems in AM broadcasting, particularly when a tower is shared with other antennas such as FM broadcasting antennas. Since the base of the tower connects to the ground system, unlike in an ordinary mast radiator where the base is at high voltage, transmission lines to any antennas mounted on the tower, as well as aircraft lighting power lines, can be run up the side of the tower without requiring isolators. This makes it a good choice for refurbishing existing towers.

The antenna's design allows it to be constructed by modifying an existing monopole antenna, which has led to its widespread use for upgrading old broadcast installations. The skirt wires provide inductive reactance that can compensate for the capacitive reactance of electrically short masts, and with longer masts can widen the antenna's bandwidth. The ability to have the greater part of the unbalanced current flow in the skirt wires rather than the mast further improves performance by effectively widening the antenna electrically. However, not all possible unipole improvements can be achieved on every monopole, as the modifications depend on the existing tower structure.

Did You Know?

• When an AM radio station shares a tower with other antennas such as FM broadcasting antennas, the folded unipole is often a good choice.
• The skirt wires are kept a constant distance from the tower by insulated 'stand-off' structural members.
• US FCC regulations require the ground system to have 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long.