Folded unipole antenna

A typical AM broadcast antenna is a series-fed monopole antenna mounted on top of 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. To minimize corrosion, all the ground system components are bonded together, usually by using brazing or coin silver solder. Quarter-wave monopole antennas ordinarilly have insulated bases, so the ground system and antenna mast are electrically separate, and each of these constitutes one of the 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 electrically separate segments, linked by insulators, to ensure all segments are too short to resonte 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.

${\displaystyle P=I^{2}R}$

Electrically short monopole antennas have low resistance and high 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 5/ 8  wavelengths tall. In every case, there is an electrical network in a small hut at the base of the tower that matches the antenna to its transmission line. If the tower is too short or too tall for the frequency, it will have its capacitive or inductive reactance tuned out by the matching network.

The combined limitations of the matching network, ground wires, and tower can cause the system to have a narrow bandwidth, possibly severely enough to limit the audio 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 RF resistance of the ground system and matching network(s), all of which are in series with the antenna current.

A well-designed folded unipole will increase the bandwidth of electrically short antennas, because its feed-point presents less reactance that needs to be tuned out over a broader range of frequencies than a tuned simple monopole.

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.[1][lower-alpha 1]

Folded unipole with skirt wires connected to a midpoint on the mast, rather than its top. The connection point can be altered to adjust the feedpoint reactance behavior for optimum bandwidth.

Heuristically, the unipole's skirt wires can either be thought of as attached segments of several long skinny loop antennas, with the mast completing the final side of each loop, or equivalently as a closed parallel wire stub (with one of the parallel wires being the central mast) that makes a stub inductor. Either way, the effect of the skirt wires is to add inductive reactance to the antenna mast, which helps neutralize the short mast's capacitive reactance.

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".

Modern folded unipole feed point, where the radio frequency excitation current will be measured. The skirt wires are attached by insulated connections near the bottom of the image; the connection ring is a simple loop of wire running between the skirt wires cutting across the image top edge.

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.[1][2]
• 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.[1][2]

Most commonly, folded-unipole designs were used to replace a shunt-fed antenna – a different-design broadcast antenna 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.[1][lower-alpha 2][lower-alpha 3]

When the well-made folded-unipole antenna replaced the decrepit slant-wire 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.

Sites of ground-mounted monopole antennas require landscape maintenance: Keeping weeds and grass covering the antenna's ground-plane wiring as short as possible, since weeds will dissipate radio frequency power, severely reducing antenna efficiency. Folded-unipole antenna sites were alleged to be less affected by disturbances near the ground that cause attenuation in other monopole antenna designs, but measurements show no such advantage.[3][4]

Some claims have been made that other ground-system losses are reduced for the folded unipole antenna. However, such claims do not consider the general principle that for the same conditions, the total displacement current in the ground system around a folded unipole is essentially identical to a conventional series-fed monopole using the same ground system. Therefore it is no surprise that ground losses for the two radiator configurations are very similar.[4] Experiments by Rackley, Cox, Moser, & King (1996) have shown that claims of superior folded-unipole performance are incorrect.[3]