Random wire antenna
Encyclopedia
A random wire antenna is a radio frequency antenna
consisting of a wire whose length does not bear a relation to the wavelength
of the radio waves used, but is typically chosen more for convenience. This type of antenna sometimes is called the zig-zag antenna, as it may be strung back and forth between trees just to get enough wire into the air. For example, an antenna for 3MHz might be 20 m (66 ft) - 40 m (131 ft) long. Such antennas are usually not as effective as antennas whose length is adjusted to resonate
at the wavelength to be used. 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. 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
.
of a straight random wire antenna is unpredictable and depends on its electrical length
, it may have several lobes at angles to the antenna axis. The radiation will drop off to zero on the axis. A folded or zig-zag antenna will have an even more unpredictable pattern.
s. Typically it is made from number 12 or 14 AWG
(1.6 to 2 mm (0.078740157480315 in) diameter) copperclad wire. Folding (to fit in space available) 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
, 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 transmission line
, the other to a good earth ground
. One-quarter wavelength
works best, and one half wavelength will work poorly with most tuners. Since the antenna is located very close to the transmitter, RF
feedback can be an issue. RF feedback can be minimized by selecting a wire length that causes the low feed-point impedance at a current loop to occur at the transmitter. Alternately, a remote tuner can be fed with feedline, and the tuner located on the antenna.
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 that's approximately one-quarter wavelength long.
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
consisting of a wire whose length does not bear a relation to the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the radio waves used, but is typically chosen more for convenience. This type of antenna sometimes is called the zig-zag antenna, as it may be strung back and forth between trees just to get enough wire into the air. For example, an antenna for 3MHz might be 20 m (66 ft) - 40 m (131 ft) long. Such antennas are usually not as effective as antennas whose length is adjusted to resonate
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
at the wavelength to be used. 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. Random wire antennas are a type of monopole antenna
Monopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
and the other side of the receiver or transmitter antenna terminal must be connected to an earth ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
.
Radiation pattern
The radiation patternRadiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
of a straight random wire antenna is unpredictable and depends on its electrical length
Electrical length
In telecommunications, electrical length is the length of a transmission medium or antenna element expressed as the number of wavelengths of the signal propagating in the medium....
, it may have several lobes at angles to the antenna axis. The radiation will drop off to zero on the axis. A folded or zig-zag antenna will have an even more unpredictable pattern.
Construction
Usually, it 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 insulatorStrain insulator
A strain insulator is an insulator that provides both large electrical insulation and a large load-bearing capacity. Strain insulators were first used in telegraph systems to isolate the signal wire from ground while still supporting the wire...
s. Typically it is made from number 12 or 14 AWG
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...
(1.6 to 2 mm (0.078740157480315 in) diameter) copperclad wire. Folding (to fit in space available) 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
Antenna tuner
An antenna tuner, transmatch or antenna tuning unit is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna...
, as it has an unpredictable impedance
Impedance
Impedance may refer to:*Electrical impedance, the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit**Characteristic impedance of a transmission line...
that varies with frequency. One side of the output of the tuner is connected directly to the antenna, without a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, the other to a good earth ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. One-quarter wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
works best, and one half wavelength will work poorly with most tuners. Since the antenna is located very close to the transmitter, RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
feedback can be an issue. RF feedback can be minimized by selecting a wire length that causes the low feed-point impedance at a current loop to occur at the transmitter. Alternately, a remote tuner can be fed with feedline, and the tuner located on the antenna.
The ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
for a random wire antenna may be chosen by experimentation. Grounds could be returned to a nearby cold water pipe or a wire that's approximately one-quarter wavelength long.