A random wire antenna is a radio antenna consisting of a long wire suspended above the ground, whose length does not bear a particular relation to the wavelength of the radio waves used, but is typically chosen more for convenient fit between the available supports, or the length of wire at hand, rather than selecting length to be resonant on any particular frequency. The wire may be straight or it may be strung back and forth between trees or walls just to get as much wire into the air as feasible. Due to the great variability of the (unplanned) antenna structure, the random wire’s effectiveness can vary erratically from one installation to another, and a single random wire antenna can have wildly different reception / transmission strength in one direction than it achieves in another azimuth direction about 70°~140° different, and finally reception / transmission strengths and directions can be wildly different on only moderately different frequencies. Random wire antennas are typically fed at one end against a suitable counterpoise (such as earth ground or a parallel wire hidden under the grass below the elevated antenna wire).
They are widely used as receiving antennas on the long wave, medium wave, and short wave 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.
Often random wire antennas are also (inaccurately) referred to as long-wire antennas. There is no accepted minimum size, but actual long-wire antennas must be greater than at least a quarter-wavelength () or perhaps greater than a half () at the frequency the long wire antenna is used for, and even a half-wave may only be considered "long-ish" rather than "truly" long; the required lower size limit depends on the book consulted. Most sources consider a "true" long wire to be at least one wavelength, whereas random wire antennas involve no such confusion.[1] Further confusing the issue, if an antenna is used over a wide range of frequencies, some writers may technically qualify it as a long wire at the shorter wavelengths but not at the longer. The term random wire raises no such quibbles: Its only requirement is that the antenna length was not planned for resonance at a particular interesting frequency.
Although antennas an odd numbers of quarter-wavelengths long are easy to feed, when the length of the wire is near an even multiple of a its feedpoint impedance may take on thousands of Ohms of both reactance and resistance due to antiresonance – extreme values which can exceed the ranges where conventional impedance matching schemes are adequate. The radiation efficiency of the antenna is not changed, despite high resistance; if the antenna had been fed transmit power against a resistive counterpoise (such as a poor ground) and an appropriate matching system is used, the efficiency of the antenna system may actually increase significantly. Based on these considerations, operator developed a table of usable odd multiple lengths and even multiple lengths for HF frequencies assigned to amateurs, and then selected a compromise among all of them as an ideal work-any-band length of 74feet for the amateur radio HF bands.[2] Hence ~74feet is a carefully selected non-random length preferred for an HF "random" length antenna. Another favored length is 100feet.
The radiation pattern of a thin wire antenna is easily predictable using antenna modeling. For a straight wire, the radiation pattern can be described by axially symmetric multipole moments 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.[3] Under about 0.6 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 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 and therefore also act somewhat as a traveling wave antenna, 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 multi-element antennas such as Yagi or quad antennas with the same length of wire.
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 strung as high as possible between trees or buildings, with any bends as obtuse as possible, and with the ends insulated from supports.
If used for transmitting, a random wire antenna usually will also require an antenna tuner, as it has an unpredictable impedance that varies with frequency.[4] One side of the output of the tuner is connected directly to the antenna, without a transmission line, the other to a good earth ground. A wire near an odd number of a quarter-waves in length works best; in contrast, a wire near an even number of quarter-wavelengths long, although fine for receiving, on transmit will exceed the matching ability of most small tuners, unless first fed through an unun with a large transform ratio.[5] [6] Even without a good earth, the antenna will still radiate, although poorly; it will do so by capacitively coupling to any nearby conducting material; this is not recommended.[7]
The ground for a random wire antenna may be chosen by experimentation. The antenna tuner ground could be connected to a nearby cold water pipe (if the pipe used reaches the soil via an all-steel or all-copper segment) or one or several wires laid on the floor or ground, one of which is approximately one-quarter wavelength long, or the ground can be connected to one or several randomly laid-out counterpoise wires under the antenna, at least one of which is a quarter-wavelength.[4]