Folded unipole antenna

The folded unipole antenna was first devised for broadcast use by John H. Mullaney, an American radio broadcast pioneer, and consulting engineer. A variation of a monopole antenna, it consists of a vertical metal rod or mast mounted over a conductive surface called a ground plane. The mast is surrounded by a "skirt" of vertical wires electrically attached to the top of the mast. The wires are connected by a metal ring at the bottom and the feed line is connected between the bottom of the wires and ground. Designed to solve some difficult problems with existing medium wave (MW), frequency modulation (FM), and amplitude modulation (AM) broadcast antenna installations, it has seen much use, both in the United States and other countries as well.

A typical AM broadcast antenna is a series-fed monopole antenna above a ground system. The ground system normally comprises 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long and a ground-screen in the immediate vicinity of the tower. All the ground system components are bonded together, usually by brazing or using coin silver solder to minimize corrosion. These antennas have insulated bases. If required, insulated guy wires are used. Radio frequency power is fed across the base insulator between the ground system and the tower itself. In the US, the Federal Communications Commission (FCC) required that the transmitter power measurements for a single series-fed tower calculated at this feed point as the current squared multiplied by the resistive part of the feed-point impedance.

Electrically short monopole antennas have low resistance and high reactance. Longer antennas may have impedances that are more advantageous unless the electrical height exceeds about 5/8 of a wavelength. In any case, an electrical network at the base of the tower matches the antenna to its transmission line. If the tower is very short, it will have its capacitive reactance tuned out by this matching network. This network and tower combination often results in a narrow bandwidth, severely limiting the audio frequency fidelity of the radio station. Electrically short antenna systems have relatively small apertures and high losses. The cause of these losses is related to the relatively low radiation resistance of an electrically short radiator with respect to the r-f resistance of the ground system and matching networks, all of which are in series with the antenna current.

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Wikipedia