Random wire antenna

Last updated
A wire antenna kit, with a coil of wire, strain insulators and a balun. When installed the wire is supported by buildings or trees using the insulators to prevent a short circuit to ground. Long-wire-and-balun-0a.jpg
A wire antenna kit, with a coil of wire, strain insulators and a balun. When installed the wire is supported by buildings or trees using the insulators to prevent a short circuit to ground.

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 enough wire into the air. Due to the great variability of the (unplanned) antenna structure, effectiveness can vary widely from one installation to another. Random wire antennas are typically fed at one end against a suitable counterpoise (such as earth ground).

Contents

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 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 ( 1 /4 λ) or half ( 1 /2 λ) of the radio waves (most consider a "true" long wire to be least one wavelength [1] ), whereas random wire antennas have no such constraint. [2]

When the length of the wire is near an even multiple of quarter-wavelengths, its feedpoint impedance may take on extreme values (thousands of ohms) due to resonance, which can cause low efficiency with popular impedance matching 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 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 (22.56 m). [3]

Radiation pattern

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. [4] Under about 0.6  λ 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.

Construction

A typical permanent wire antenna strung between two buildings and then run off to a remote post. This example shows a lightning switch on the window frame, to disconnect the radio and ground the antenna for safety during electrical storms. Random wire antenna example picture.png
A typical permanent wire antenna strung between two buildings and then run off to a remote post. This example shows a lightning switch on the window frame, to disconnect the radio and ground the antenna for safety during electrical storms.

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 insulators. Ideally, it is made from AWG  12 or 14 { diameter 1.6 to 2.0 mm (0.063 to 0.079 in) } copper-clad steel 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, however, the added length typically helps more than the folding 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. [1] 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. [1]

See also

Related Research Articles

<span class="mw-page-title-main">Antenna (radio)</span> Electrical device

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 electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.

Radiation resistance is that part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna. In radio transmission, a radio transmitter is connected to an antenna. The transmitter generates a radio frequency alternating current which is applied to the antenna, and the antenna radiates the energy in the alternating current as radio waves. Because the antenna is absorbing the energy it is radiating from the transmitter, the antenna's input terminals present a resistance to the current from the transmitter.

<span class="mw-page-title-main">Rhombic antenna</span> Rhombus-shaped antenna

A rhombic antenna is made of four sections of wire suspended parallel to the ground in a diamond or "rhombus" shape. Each of the four sides is the same length – about a quarter-wavelength to one wavelength per section – converging but not touching at an angle of about 42° at the fed end and at the far end. The length is not critical, typically from one to two wavelengths (λ), but there is an optimum angle for any given length and frequency. A horizontal rhombic antenna radiates horizontally polarized radio waves at a low elevation angle off the pointy ends of the antenna.

<span class="mw-page-title-main">Helical antenna</span> Type of antenna

A helical antenna is an antenna consisting of one or more conducting wires wound in the form of a helix. A helical antenna made of one helical wire, the most common type, is called monofilar, while antennas with two or four wires in a helix are called bifilar, or quadrifilar, respectively.

<span class="mw-page-title-main">Dipole antenna</span> Antenna consisting of two rod shaped conductors

In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each end. A dipole antenna commonly consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, and the other side connected to some type of ground. A common example of a dipole is the "rabbit ears" television antenna found on broadcast television sets.

<span class="mw-page-title-main">Whip antenna</span> Type of radio antenna

A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer whips, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 11 m long.

<span class="mw-page-title-main">Antenna tuner</span> Telecommunications device

An antenna tuner is an electronic device inserted into the feedline between a radio transmitter and its antenna. Its purpose is to optimize power transfer by matching the impedance of the radio to the impedance of the end of the feedline connecting the antenna to the transmitter.

<span class="mw-page-title-main">Beverage antenna</span> Type of radio antenna

The Beverage antenna or "wave antenna" is a long-wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It is used by amateur radio, shortwave listening, and longwave radio DXers and military applications.

<span class="mw-page-title-main">Mast radiator</span> Type of radio frequency antenna

A mast radiator is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations. The conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.

<span class="mw-page-title-main">T-antenna</span> Type of radio antenna

A ‘T’-antenna, ‘T’-aerial, or flat-top antenna is a monopole radio antenna consisting of one or more horizontal wires suspended between two supporting radio masts or buildings and insulated from them at the ends. A vertical wire is connected to the center of the horizontal wires and hangs down close to the ground, connected to the transmitter or receiver. Combined, the top and vertical sections form a ‘T’ shape, hence the name. The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a ground connection. ‘T’-antennas are typically used in the VLF, LF, MF, and shortwave bands, and are widely used as transmitting antennas for amateur radio stations, and long wave and medium wave AM broadcasting stations. They can also be used as receiving antennas for shortwave listening.

<span class="mw-page-title-main">J-pole antenna</span>

The J-pole antenna, more properly known as the J antenna, is a vertical omnidirectional transmitting antenna used in the shortwave frequency bands. It was invented by Hans Beggerow in 1909 for use in Zeppelin airships. Trailed behind the airship, it consisted of a single one half wavelength long wire radiator, in series with a quarter-wave parallel transmission line tuning stub that matches the antenna impedance to the feedline. By 1936 this antenna began to be used for land-based transmitters with the radiating element and the matching section mounted vertically, giving it the shape of the letter "J", and was named the J antenna by 1943. When the radiating half-wave section is mounted horizontally, at right-angles to the quarter-wave matching stub, the variation is usually called a Zepp antenna.

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this physical description there are two distinct types:

<span class="mw-page-title-main">Monopole antenna</span> Type of radio 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 signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver with the antenna and makes the two devices compatible. In a radio transmitter, the transmitter generates an alternating current of radio frequency, and the feed system feeds the current to the antenna, which converts the power in the current to radio waves. In a radio receiver, the incoming radio waves excite tiny alternating currents in the antenna, and the feed system delivers this current to the receiver, which processes the signal.

<span class="mw-page-title-main">Folded unipole antenna</span> Antenna used for radio broadcasts

The folded unipole antenna is a type of monopole mast radiator antenna used as a transmitting antenna mainly in the medium wave band for AM radio broadcasting stations. It consists of a vertical metal rod or mast mounted over and connected at its base to a grounding system consisting of buried wires. The mast is surrounded by a "skirt" of vertical wires electrically attached at or near the top of the mast. The skirt wires are connected by a metal ring near the mast base, and the feedline feeding power from the transmitter is connected between the ring and the ground.

A shortwave broadband antenna is a radio antenna that can be used for transmission of any shortwave radio band from among the greater part of the shortwave radio spectrum, without requiring any band-by-band adjustment of the antenna. Generally speaking, there is no difficulty in building an adequate receiving antenna; the challenge is designing an antenna which can be used for transmission without an adjustable impedance matching network.

<span class="mw-page-title-main">Counterpoise (ground system)</span> Substitute for a ground connection in a radio antenna system

In electronics and radio communication, a counterpoise is a network of suspended horizontal wires or cables, used as a substitute for an earth (ground) connection in a radio antenna system. It is used with radio transmitters or receivers when a normal earth ground cannot be used because of high soil resistance or when an antenna is mounted above ground level, for example, on a building. It usually consists of a single wire or network of horizontal wires, parallel to the ground, suspended above the ground under the antenna, connected to the receiver or transmitter's "ground" wire. The counterpoise functions as one plate of a large capacitor, with the conductive layers of the earth acting as the other plate.

<span class="mw-page-title-main">Halo antenna</span> Omnidirectional circular bent dipole antenna

A halo antenna, or halo, is a center-fed  1 /2 wavelength dipole antenna, which has been bent into a circle, with a break directly opposite the feed point. The dipole's ends are close, but do not touch, and their crossections may be broadened to form an air capacitor, whose spacing is used to adjust the antenna's resonant frequency. Most often mounted horizontally, this antenna's radiation is then approximately omnidirectional and horizontally polarized.

<span class="mw-page-title-main">Antenna array</span>

An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together to enhance the power radiated in desired directions, and cancelling to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements.

In radio systems, many different antenna types are used whose properties are especially crafted for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.

References

  1. 1 2 3 Straw, R. Dean; et al., eds. (2003). The ARRL Antenna Book (20th ed.). Newington, CT: The American Radio Relay League. p. 944. ISBN   0-87259-904-3.
  2. ↑ Carr, J. Practical Antenna Handbook. McGraw Hill. Chapter 9.
  3. ↑ ""Random" length". w0ipl.net. Wire antennas.
  4. ↑ Silver, Ward; et al., eds. (2007). The ARRL General Class License Manual (6th ed.). Newington, CT: The American Radio Relay League. p. 6.6. ISBN   978-087259996-3 – via Google Books.
  5. ↑ "The "almost random" wire antenna". kb6nu.com. 12 March 2007.
  6. ↑ "Random length wire". w0ipl.net. NVIS.
  7. ↑ Yates, Steve (AA5TB). "End-fed half wave antenna". aa5tb.com.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.