Random wire antenna

A random wire antenna is a radio antenna consisting of a long wire suspended above the ground, whose length does not bear a relation to the wavelength 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; this type of antenna sometimes is called a zig-zag antenna. Such antennas are usually not as effective as antennas whose length is adjusted to resonate at the wavelength to be used. Random wire antennas are a type of monopole antenna and the other side of the receiver or transmitter antenna terminal must be connected to an earth ground.

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 referred to as long-wire antenna. 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.

However, problems can occur when the so-called random wire happens to be an even number of half-wavelengths on the transmitting frequency (odd multiples do not have this problem). This reduces the efficiency of the antenna, and should be avoided. For the Amateur radio 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 74 feet.

The radiation pattern of a straight random wire antenna is unpredictable and depends on its electrical length; its length measured in wavelengths (λ) of the radio waves used. The radiation will drop off to zero on the axis; however it may have several lobes (maxima) at angles to the antenna axis. 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 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. A folded or zig-zag antenna will exhibit a pattern that is even more complicated and difficult to predict.

...
Wikipedia