Folded unipole antenna

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".

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.

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.

Depending on desired recipients and the surrounding terrain, and particularly depending on locations of spacious expanses of open water, a longer antenna may tend to send signals out in directions that are increasingly more advantageous, up to the point that the antenna's electrical height exceeds about ⁠5/ 8 ⁠ wavelengths tall.

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.

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 causing the unbalanced currents in the unipole to function like a "cage antenna".

There are two possible paths that unbalanced current can take in response to the voltage difference between the top and the bottom: Either down (or up) through the mast, or down (or up) through the skirt wires.

Consequently, the flow of balanced current will tend to be larger in magnitude than its unbalanced counterpart, and the difference becomes greater the closer the conductors are spaced.

The electrical design of a unipole antenna lies in choosing the sizes and number of the skirt wires, their lengths, and (if possible) the size of the central mast, in order to adjust the relative impedances (or admittances) of the balanced and unbalanced current, in order to maximize radiation and to present a reactance-free balanced feedpoint impedance for the feedline.

(Other design considerations, like cost of materials and ease of erection, may lead to choices sub-optimal for electrical performance.)

That being said, however, field testing discussed below shows that when just considering antenna efficiency, the power radiated per power fed to a unipole is very nearly the same as an ordinary monopole antenna with the same height: Other than the advantage of being able to tailor the feedpoint impedance, there appears to be no inherent superior performance for unipoles' when compared to a basic monopole.

The only unipole design advantage boils down to it having an elaborately configurable built-in feedpoint impedance matching system.

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, and with nearly equivalent radiator widths, show essentially no difference in radiation pattern in actual measurements by Rackley, Cox, Moser, & King (1996)[4] and by Cox & Moser (2002).

[citation needed] Such improvements may have provoked conjectures that folded-unipole antennas had power gains, or other wonderful characteristics, but those suppositions are not borne out by radio engineering calculations.

Modern folded unipole antenna with six skirt wires surrounding a round solid metal mast. The skirt wires are held away from the mast by stand-off posts with insulated ends.
Folded unipole with only three skirt wires mounted off the corners of a standard triangular frame mast. (The close-in wire on the left side of the image is an incidental guy wire in the foreground.)
Modern folded unipole feed point, where the radio frequency excitation current will be measured. The connection ring that attaches to the skirt wires is a simple loop of fairly thin wire cutting across the top edge of the image. The stout vertical bars tighten the skirt: Near the top of the picture, the bars' connections to the skirt wires are insulated; their other ends fasten to the support truss anchored on the ground, near the bottom of the image.