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) amplitude modulation (AM)broadcast antenna installations, it has seen much use, both in the United States and other countries as well.
Typical setup
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.
Some claims have been made that ground system losses are reduced for the folded unipole antenna. However the experimental work of Ronald Rackley, et al., have shown this to be incorrect.[1] Such claims do not consider that for the same conditions, the total displacement current in the ground system is nearly the same for a folded unipole as for a conventional series-fed unipole. Therefore ground system losses for these two radiator configurations are very similar.
Using a folded unipole will increase the bandwidth of electrically short antennas because its feed-point comprises less reactance that needs to be tuned out.[citation needed] There are additional performance claims made by some when the resulting antenna allows most all of the reactance to be removed with a tuning-short, rather than electrical components in a tuning network.
One usually constructs the folded-unipole from an existing monopole antenna by shorting out the base insulator, thus connecting the tower directly to the ground system. Then a series of vertical wires, typically four to eight, are installed from an attachment near the top of the tower, kept a constant distance from the tower by structural members containing insulators, then attached together near the bottom of the tower. The resulting skirt connects to the tower top and remains insulated throughout its entire length. The wires of the skirt connect at the feed-point near the bottom.
Below is another folded unipole antenna constructed from an existing series fed monopole. This antenna has only three folds comprising the skirt.
A possible improvement over the basic folded-unipole antenna is the “self resonant” unipole antenna, described in U.S. Patent 6,133,890.
Abstract: A self-resonant vertically polarized folded unipole antenna for long wave (LW), medium wave (MW) broadcasting and for the 160-meter amateur radio band with a grounded tower connected to radially descending fold wires terminated near the base of the tower in an open polygonal ring, possibly a C-ring with a reactive load in series with this ring. This reactance cancels the reactive component of the antenna input impedance causing the input impedance to appear resistive at the feed point. This leads to outstanding linearity and bandwidth up to and possibly exceeding plus or minus 16 kHz, sometimes exceeding plus and minus 20 kHz. The antenna is particularly useful for AM broadcasting of high quality music with response capable of being better than that of FM.
Another possible improvement to the folded-unipole is described in U.S. Patent 4,658,266.
Bobby L. Cox II and James R. Moser of Kintronic Laboratories have performed a folded-unipole study of considerable depth.[2]
When a common tower is used for both AM and FM transmission, the folded-unipole is often a good choice. Since the base of the tower connects to the ground system, the transmission lines to any antennas mounted thereon run up the side of the tower with no isolation necessary. Another broadcast antenna that had a grounded base is the shunt-fed (or slant-wire) antenna. This antenna comprises a grounded tower with a feed-line wire attached at a point of approximate transmission-line match. A series capacitor tunes out the inductive reactance of this feed-line (this feed is also called a gamma match when the feed wire is parallel to the radiating element). When the folded-unipole antenna replaced the slant-wire fed antenna, a marked improvement of performance often occurred. This gave rise to the notion that folded-unipole antennas had power gains or other wonderful characteristics not supported by engineering theory.
When a folded-unipole replaces an antenna of defective or otherwise compromised design, there will be an improvement in performance. Series-fed monopole antenna sites require care, keeping weeds as short as possible, since weeds will dissipate radio frequency power, severely reducing antenna efficiency. Folded-unipole antenna sites are affected less by otherwise attenuating disturbances near the ground.
See also
- isotropic antenna
- omnidirectional antenna
- whip antenna
- driven element
- monopole antenna
- balun
- amateur radio
- shortwave listening
References
3. Raines, J. K., Folded Unipole Antennas: Theory and Applications, New York: McGraw-Hill, 2007.
4. Raines, J. K., "Simple Formulas for Folded Antennas", Microwave Journal, January, 2009 (electronic edition).http://www.mwjournal.com/Media/pdf/raines3-1.pdf
