Folded unipole antenna

The folded unipole antenna is a type of monopole antenna; it consists of a vertical metal rod or mast mounted over and connected at its base to a conductive surface called a ground plane. 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 feed line is connected between the ring and the ground.

It has seen much use for refurbishing medium wave (AM broadcast) station towers in the United States and other countries. When an AM station (mediumwave, long antennas) shares a tower with FM transmitters ( VHF, short antennas), 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 on the tower can run up the side of the tower without requiring isolation, even though the tower itself carries mediumwave current.

Invention

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.

Typical installation

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".

Conventional monopole antennas

A typical AM broadcast antenna is a series-fed monopole antenna mounted above a ground system, but usually with no direct connection to 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.

Quarter-wave monopole antennas ordinarily have insulated bases, so the ground system and antenna mast are electrically separate, and the base of the mast and an adjacent ground plane connection point constitute the two electrical contacts for the feedline. If extra stabilization is required, any guy wires used are insulated from both the tower and the ground system; long guy wires are sometimes broken into a series of short, electrically separate segments, linked by insulators, to ensure all segments are too short to resonate at the operating frequency.

Radio frequency power is fed into the quarter-wave monopole system across the base insulator between a feed contact to the tower itself and another feed contact to the ground system. 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.

::$\ P = I^2\ R\$

Electrically short monopole antennas have low resistance and high capacitive (negative) reactance. Longer antennas may have send out signals out in directions that are increasingly more advantageous up to the point that the electrical height exceeds about  wavelengths tall. Reactance is zero only for towers slightly shorter than  wavelength, but the reactance will in any case rise or fall depending on humidity, dust, or ice collecting on the tower or its feedline.

Regardless of its height, the antenna feed system has an impedance matching system housed in a small shed at the tower's base (called a " tuning hut" or "coupling hut" or "helix hut"). The matching network is adjusted to join the antenna's impedance to the characteristic impedance of its feed line coming from the transmitter. If the tower is too short or too tall for the frequency, the antenna's capacitive or inductive reactance will be counteracted by an opposite reactance in the matching network, as well as raising or lowering the apparent resistance of the antenna to match the feedline. The combined limitations of the matching network, ground wires, and tower can cause the system to have a narrow bandwidth; in extreme cases the effects of narrow bandwidth can be severe enough to detract from the audio fidelity of the radio broadcast.

Electrically short antennas have low radiation resistance, which makes normal loss in other parts of the system relatively more costly in terms of lost broadcast power. The losses in the ground system, matching network(s), feedline wires, and structure of the tower all are in series with the antenna feed current, and each wastes a share of the broadcast power heating the soil or metal in the tower.

Folded unipole antennas

Heuristically, the unipole's outer skirt wires can be thought of as attached segments of several tall, narrow, loop antennas, with the central mast completing the final side of each loop. Equivalently, each skirt wire makes a parallel wire stub, with the mast being the other parallel "wire"; the closed end at the top of the stub, where the skirt connects to the mast, makes a transmission line stub inductor. Either way of looking at it, the effect of the skirt wires is to add inductive reactance to the antenna mast, which helps neutralize a short mast's capacitive reactance.

For the normal case of a short monopole, the inductive reactance introduced by the skirt wires increases as the frequency decreases and the bare mast's reactance becomes more capacitive. (With increasing frequency both the inductive reactance and capacitive reactance drop.) When carefully configured, the two contrary reactances can be made to cancel each other, at least in part, and to rise and fall by approximately the same amount. Approximate balance between the opposing reactances adds up to reduce the total reactance of the whole antenna at the decreased (and increased) frequencies, thus widening the antenna's low-reactance bandwidth.

If the greater part of the unbalanced radio current can be made to flow in the skirt wires, instead of in the mast, the outer ring of skirt wires will also effectively add electrical width to the mast, which also will improve bandwidth by turning the unipole into a " cage antenna".

Usually folded-unipoles are constructed by modifying an existing monopole antenna, and not all possible unipole improvements can be achieved on every monopole.
* First one connects the base of the tower 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; these wires surround the tower and are called a "skirt".
* The skirt wires are kept a constant distance from the tower by insulated "stand-off" structural members, and joined to an electrically isolated conductor ring that surrounds the base of the tower, also mounted on insulated stand-offs.
* The new antenna feed connects between the common point of the ground system and the ring at the bottom of the skirt wires.

The resulting skirt enveloping the mast connects only at the tower top, or some midpoint near the top, and to the isolated conducting ring that surrounds the tower base; the skirt wires remain insulated from the mast at every other point along its entire length.

Performance comparisons

When a well-made folded-unipole replaces a decrepit antenna, or one with a poor original design, there will of course be an improvement in performance; the sudden improvement may be cause for mistakenly inferred superiority in the design.

Experiments show that folded-unipole performance is the same as other monopole designs: Direct comparisons between folded unipoles and more conventional vertical antennas of the same height, all well-made, show essentially no difference in radiation pattern in actual measurements by Rackley, Cox, Moser, & King (1996) and by Cox & Moser (2002).

The expected wider bandwidth was also not found during antenna range tests of several folded unipoles.

Replaced shunt-fed antenna

Most commonly, folded-unipole designs were used to replace a shunt-fed antenna – a different broadcast antenna design that also has a grounded base. A “shunt-fed” (or “slant-wire”) antenna comprises a grounded tower with the top of a sloping single-wire feed-line attached at a point on the mast that results in an approximate match to the impedance desired at the other end of the sloping feed-wire.

When the well-made folded-unipole antenna replaced the aged-out slant-fed antenna, a marked improvement of performance was often noticed. This improvement gave rise to the supposition that folded-unipole antennas had power gains, or other wonderful characteristics, not supported by radio engineering calculations.

Ground system maintenance

Sites of ground-mounted monopole antennas require landscape maintenance: Keeping weeds and grass covering the antenna's ground plane wires as short as possible, since green plants in between the antenna tower and the antenna ground system will dissipate power of the radio waves passing through them, reducing antenna efficiency. Folded-unipole antenna sites were alleged to be less affected by weeds and long grass on top of the ground wires that cause attenuation in other monopole antenna designs, but measurements show no such advantage.

Self-resonant unipole patents

A possible improvement over the basic folded-unipole antenna is the “self resonant” unipole antenna, described in .

Another possible improvement to the folded unipole is described in , which concerns a more carefully designed form of ground plane for use with all monopole types (only incidentally including folded unipoles).

See also

* Driven element
* Monopole antenna
* Omnidirectional antenna

Footnotes

References

External links

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{{Antenna Types

Radio frequency antenna types
Antennas (radio)