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Folded unipole antenna
The folded unipole antenna is a type of monopole mast radiator antenna used as a transmitting antenna mainly in the medium wave band for AM radio broadcasting stations. It consists of a vertical metal rod or mast mounted over and connected at its base to a grounding system consisting of buried wires. The mast is surrounded by a "skirt" of vertical wires electrically attached at or near the top of the mast. The skirt wires are connected by a metal ring near the mast base, and the feedline feeding power from the transmitter is connected between the ring and the ground.
It has seen much use for refurbishing medium wave AM broadcasting station towers in the United States and other countries. When an AM radio station shares a tower with other antennas such as FM broadcasting antennas, the folded unipole is often a good choice. Since the base of the tower connects to the ground system, unlike in an ordinary mast radiator tower in which the base is at high voltage, the 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.
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 (MW), frequency modulation (FM), and amplitude modulation (AM) broadcast antenna installations.
Since folded unipoles are most often used for refurbishing old broadcast antennas, the first subsection below describes a typical monopole antenna used as a starting point. The subsection that follows next describes how surrounding skirt wires are added to convert an ordinary broadcast tower into a folded unipole.
The picture at the right shows a small folded unipole antenna constructed from an existing triangular monopole tower; it has only three vertical wires comprising its "skirt".
A typical monopole transmitting antenna for an AM radio station is a series-fed mast radiator; a vertical steel lattice mast which is energized and radiates radio waves. One side of the feedline which feeds power from the transmitter to the antenna is connected to the mast, the other side to a ground (electricity) system consisting of buried wires radiating from a terminal next to the base of the mast. The mast is supported on a thick ceramic insulator which isolates it electrically from the ground. US FCC regulations require the ground system to have 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long; there is usually a ground-screen in the immediate vicinity of the tower. To minimize corrosion, all the ground system components are bonded together, usually by using brazing or coin silver solder.
The mast has diagonal guy cables attached to it, anchored to concrete anchors in the ground, to support it. The guy lines have strain insulators in them to isolate them electrically from the mast, to prevent the high voltage from reaching the ground. To prevent the conductive guy lines from disturbing the radiation pattern of the antenna, additional strain insulators are sometimes inserted in the lines to divide them into a series of short, electrically separate segments, to ensure all segments are too short to resonate at the operating frequency.
In the U.S., the Federal Communications Commission (FCC) requires 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.
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Folded unipole antenna
The folded unipole antenna is a type of monopole mast radiator antenna used as a transmitting antenna mainly in the medium wave band for AM radio broadcasting stations. It consists of a vertical metal rod or mast mounted over and connected at its base to a grounding system consisting of buried wires. The mast is surrounded by a "skirt" of vertical wires electrically attached at or near the top of the mast. The skirt wires are connected by a metal ring near the mast base, and the feedline feeding power from the transmitter is connected between the ring and the ground.
It has seen much use for refurbishing medium wave AM broadcasting station towers in the United States and other countries. When an AM radio station shares a tower with other antennas such as FM broadcasting antennas, the folded unipole is often a good choice. Since the base of the tower connects to the ground system, unlike in an ordinary mast radiator tower in which the base is at high voltage, the 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.
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 (MW), frequency modulation (FM), and amplitude modulation (AM) broadcast antenna installations.
Since folded unipoles are most often used for refurbishing old broadcast antennas, the first subsection below describes a typical monopole antenna used as a starting point. The subsection that follows next describes how surrounding skirt wires are added to convert an ordinary broadcast tower into a folded unipole.
The picture at the right shows a small folded unipole antenna constructed from an existing triangular monopole tower; it has only three vertical wires comprising its "skirt".
A typical monopole transmitting antenna for an AM radio station is a series-fed mast radiator; a vertical steel lattice mast which is energized and radiates radio waves. One side of the feedline which feeds power from the transmitter to the antenna is connected to the mast, the other side to a ground (electricity) system consisting of buried wires radiating from a terminal next to the base of the mast. The mast is supported on a thick ceramic insulator which isolates it electrically from the ground. US FCC regulations require the ground system to have 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long; there is usually a ground-screen in the immediate vicinity of the tower. To minimize corrosion, all the ground system components are bonded together, usually by using brazing or coin silver solder.
The mast has diagonal guy cables attached to it, anchored to concrete anchors in the ground, to support it. The guy lines have strain insulators in them to isolate them electrically from the mast, to prevent the high voltage from reaching the ground. To prevent the conductive guy lines from disturbing the radiation pattern of the antenna, additional strain insulators are sometimes inserted in the lines to divide them into a series of short, electrically separate segments, to ensure all segments are too short to resonate at the operating frequency.
In the U.S., the Federal Communications Commission (FCC) requires 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.