A random wire antenna is a

radio antenna In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...

consisting of a long wire suspended above the ground, whose length does not bear a particular relation to the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

of the radio waves used, but is typically chosen more for convenience. The wire may be straight or it may be strung back and forth between trees or walls just to get enough wire into the air. Due to the great variability of the (unplanned) antenna structure, effectiveness can vary wildly from one installation to another. Random wire antennas are typically fed at one end against a suitable counterpoise (such as earth ground). They are widely used as receiving antennas on the

long wave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...

, medium wave, and

short wave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the High frequency, high frequency band (HF), which extends from 3 to 30 MHz (10 ...

bands, as well as transmitting antennas on these bands for small outdoor, temporary or emergency transmitting stations, as well as in situations where more permanent antennas cannot be installed.

Random wire and long wire

Often random wire antennas are also (inaccurately) referred to as long-wire antenna. Actual long-wire antennas require a length greater than a quarter-wavelength (λ/4) or half (λ/2) of the radio waves (most consider a true long wire to be least one wavelength), whereas random wire antennas have no such constraint. When the length of the wire is near an even multiple of quarter-wavelengths, its feedpoint impedance may take on extreme values (thousands of

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (b ...

s) due to

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

, which can cause low efficiency with popular

impedance matching In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...

schemes. The radiation efficiency of the wire is not affected; if the antenna is worked against a resistive counterpoise (such as a poor ground) and a suitable matching scheme is used, the efficiency of the antenna system may actually increase significantly. For the

amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...

HF bands, operator W0IPL developed a table of usable odd multiple lengths, and then down-selected these to pick an ideal work-any-band length of .

Radiation pattern

The

radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “A ...

of a thin wire antenna is easily predictable using antenna modeling. For a straight wire, the radiation pattern can be described by

axially symmetric In geometry, circular symmetry is a type of continuous symmetry for a Plane (geometry), planar object that can be rotational symmetry, rotated by any arbitrary angle and map onto itself. Rotational circular symmetry is isomorphic with the circl ...

multipole moments A multipole expansion is a mathematical series representing a function that depends on angles—usually the two angles used in the spherical coordinate system (the polar and azimuthal angles) for three-dimensional Euclidean space, \R^3. Similarly ...

with no component along the wire direction; as the length of the wire is increased, higher multipole contributions become more prominent and multiple lobes (maxima) at angles to the antenna axis develop. Under about 0.6λ a wire antenna will have a single lobe with a maximum at right angles to the axis. Above this the lobe will split into two conical lobes with their maximum directed at equal angles to the wire, and a

null Null may refer to: Science, technology, and mathematics Computing * Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value * Null character, the zero-valued ASCII character, also designated by , often use ...

between them. This results in four azimuth angles at which the gain is maximum. As the length of wire in wavelengths increases, the number of lobes increases and the maxima become increasingly sharp. Any unpredictability of the radiation pattern is caused by uncontrolled interaction with nearby matter (such as soil or structures). For example, a long wire antenna close to the ground will form a leaky two-conductor

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

and therefore also act somewhat as a

traveling wave antenna In radio and telecommunication, a traveling-wave antenna is a class of antenna that uses a traveling wave on a guiding structure as the main radiating mechanism. Its distinguishing feature is that the radio-frequency current that generates the ra ...

, with reception off the end of the wire (the system is no longer axially symmetric). A folded or zig-zag antenna may exhibit a more complex pattern as there are even fewer symmetry constraints on the dipole moments that may contribute. Long wire antennas are reported to be more effective for reception than multielement antennas such as Yagi or quad antennas with the same length of wire.

Construction

A random wire antenna usually consists of a long (at least one quarter wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Ideally, it is a straight wire strung as high as possible between trees or buildings, the ends insulated from supports with

strain insulator A strain insulator is an electrical insulator that is designed to work in mechanical tension (strain), to withstand the pull of a suspended electrical wire or cable. They are used in overhead electrical wiring, to support radio antennas and over ...

s. Typically it is made from number 12 or 14 AWG ( diameter) copperclad wire. Folding the wire into a zigzag pattern to fit in a limited space such as an apartment or attic will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) If used for transmitting, a random wire antenna usually will also require an

antenna tuner An antenna tuner (and any of the names in the list below) is a device that is inserted between a transmitter, radio transmitter and its antenna (radio), antenna; when placed close by the antenna and properly adjusted (tuned) it optimizes power ...

, as it has an unpredictable impedance that varies with frequency. One side of the output of the tuner is connected directly to the antenna, without a

, the other to a good earth ground. A quarter-

sized wire works best, and unless fed through an unun, a half-wavelength will exceed the matching ability of most tuners. Even without a good earth, the antenna will also radiate, but it will do so by coupling capacitively to any nearby conducting material; this is not recommended.The End Fed Half Wave Antenna (Steve Yates, AA5TB) http://www.aa5tb.com/efha.html The ground for a random wire antenna may be chosen by experimentation. Grounds could be returned to a nearby cold water pipe or a wire approximately one-quarter wavelength long, or can be replaced by randomly laid-out quarter-wavelength counterpoise wires attached to the ground connection.

References

{{DEFAULTSORT:Random Wire Antenna Radio frequency antenna types