Whip antenna

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Whip antenna on portable FM radio receiver DEGEN DE-1129 (2) closeup.jpg
Whip antenna on portable FM radio receiver
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Whip antenna on car

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 (35 feet) long.

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The length of a whip antenna is determined by the wavelength of the radio waves it is used with. Their length varies from compact electrically short antennas 1/ 10  wavelength long, up to  5 /8  wavelength to improve directivity. The most common type is the quarter-wave whip, which is approximately  1 /4 wavelength long.

Whips are the most common type of monopole antenna, and are used in the higher frequency HF, VHF and UHF radio bands. They are widely used as the antennas for hand-held radios, cordless phones, walkie-talkies, FM radios, boom boxes, and Wi-Fi enabled devices, and are attached to vehicles as the antennas for car radios and two-way radios for wheeled vehicles and for aircraft. Larger versions mounted on roofs, balconies and radio masts are used as base station antennas for amateur radio and police, fire, ambulance, taxi, and other vehicle dispatchers.

Radiation pattern

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(left) Three large fiberglass whips mounted on a mast. (right) Fiberglass whip for 2 m and 70 cm amateur radio bands.

The whip antenna is a monopole antenna, and like a vertical dipole has an omnidirectional radiation pattern, radiating equal radio power in all azimuthal directions (perpendicular to the antenna's axis), with the radiated power falling off with elevation angle to zero on the antenna's axis. [1] Whip antennas less than one-half wavelength long, including the common quarter wave whip, have a single main lobe, and with a perfectly conducting ground plane under it maximum field strength is in horizontal directions, falling monotonically to zero on the axis. With a small or imperfectly conducting ground plane or no ground plane under it, the general result is to tilt the main lobe up so maximum power is no longer radiated horizontally but at an angle into the sky.

Antennas longer than a half-wavelength have patterns consisting of several conical "lobes"; with radiation maxima at several elevation angles; the longer the electrical length of the antenna, the more lobes the pattern has.

A vertical whip radiates vertically polarized radio waves, with the electric field vertical and the magnetic field horizontal.

Vertical whip antennas are widely used for nondirectional radio communication on the surface of the Earth, where the direction to the transmitter (or the receiver) is unknown or constantly changing, for example in portable FM radio receivers, walkie-talkies, and two-way radios in vehicles. This is because they transmit (or receive) equally well in all horizontal directions, while radiating little radio energy up into the sky where it is wasted.

Length

Firefighter using a handheld (walkie-talkie) with a whip antenna Miramar Fire Department Firefighters Conduct Prescribed Burns 140612-M-RB277-066.jpg
Firefighter using a handheld (walkie-talkie) with a whip antenna

Whip antennas are normally designed as resonant antennas; the rod acts as a resonator for radio waves, with standing waves of voltage and current reflected back and forth from its ends. Therefore, the length of the antenna rod is determined by the wavelength () of the radio waves used. The most common length is approximately one-quarter of the wavelength (), called a "quarter-wave whip" (although often shortened by the use of a loading coil; see Electrically short whips below). For example, the common quarter-wave whip antennas used on FM radios in the USA are approximately 75 cm (2.5 feet) long, which is roughly one-quarter the length of radio waves in the FM radio band, which are 2.78 to 3.41 m (9 to 11 feet) long.

Half-wave whips ( long) which have greater gain, and five-eighth wave whips ( long) which have the maximum horizontal gain achievable by a monopole, are also common lengths.

Gain and radiation resistance

The gain and input impedance of the antenna is dependent on the length of the whip element, compared to a wavelength, but also on the size and shape of the ground plane used (if any). A quarter wave vertical antenna working against a perfectly conducting, infinite ground will have a gain of 5.19  dBi and a radiation resistance of about 36.8  ohms. However this gain is never approached in actual antennas unless the ground plane is many wavelengths in diameter. 2 dBi is more typical for a whip with a ground plane of Whips mounted on vehicles use the metal skin of the vehicle as a ground plane. In hand-held devices usually no explicit ground plane is provided, and the ground side of the antenna's feed line is just connected to the ground (common) on the device's circuit board. [2] Therefore, the radio itself serves as a rudimentary ground plane. If the radio chassis is not a good deal larger than the antenna itself, the combination of whip and radio functions more as an asymmetrical dipole antenna than as a monopole antenna. [3] [4] The gain will be somewhat lower than a dipole, or a quarter-wave whip with an adequate size ground plane.

Whips not mounted on the radio itself are usually fed with coaxial cable feedline of 50 ohm or 75 ohm impedance. In transmitting antennas the impedance of the antenna must be matched to the feedline for maximum power transfer.

A half wave whip antenna (length of ) has somewhat higher gain than a quarter wave whip, but it has a current node at its feedpoint at the base of the rod so it has very high input impedance. If it was infinitely thin the antenna would have an infinite input impedance, but the finite width gives typical, practical half wave whips an impedance of 800–1,500 ohms. These are usually fed through an impedance matching transformer or a quarter wave stub matching section (e.g. the J-pole antenna). An advantage is that because it acts as a dipole it does not need a ground plane.

The maximum horizontal gain of a monopole antenna is achieved at a length of five eighths of a wavelength so this is also a popular length for whips. However at this length the radiation pattern is split into a horizontal lobe and a small second lobe at a 60° angle, so high angle radiation is poor. The input impedance is around 40 ohms.

Ground plane antenna

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Ground plane antennas

In a whip antenna not mounted on a conductive surface, such as one mounted on a mast, the lack of reflected radio waves from the ground plane causes the lobe of the radiation pattern to be tilted up toward the sky so less power is radiated in horizontal directions, undesirable for terrestrial communication. [5] Also the unbalanced impedance of the monopole element causes RF currents in the supporting mast and on the outside of the ground shield conductor of the coaxial feedline, causing these structures to radiate radio waves, which usually has a deleterious effect on the radiation pattern.

To prevent this, with stationary whips mounted on structures, an artificial "ground plane" consisting of three or four rods a quarter-wavelength long connected to the opposite side of the feedline, extending horizontally from the base of the whip, is often used. [5] This is called a ground plane antenna. [6] These few short wire elements serve to receive the displacement current from the driven element and return it to the ground conductor of the transmission line, making the antenna behave somewhat as if it has a continuous conducting plane under it.

The radiation resistance of a quarter wave ground plane antenna with horizontal ground wires is around 22 ohms, a poor match to coaxial cable feedline, and the main lobe of the radiation pattern is still tilted up toward the sky. Often (see pictures) the ground plane rods are sloped downward at a 45-degree angle, which has the effect of lowering the main lobe of the radiation pattern so more of the power is radiated in horizontal directions, and increases the input impedance for a good match to standard 50-ohm coaxial cable. To match 75-ohm coaxial cable, the ends of the ground plane can be turned downward or a folded monopole driven element can be used.

Electrically short whips

A rubber ducky antenna, a common type of electrically short whip, on a handheld UHF CB transceiver. With rubber sheath (left) removed. UHF CB with rubber ducky exposed.jpg
A rubber ducky antenna, a common type of electrically short whip, on a handheld UHF CB transceiver. With rubber sheath (left) removed.

To reduce the length of a whip antenna to make it less cumbersome, an inductor (loading coil) is often added in series with it. This allows the antenna to be made much shorter than the normal length of a quarter-wavelength, and still be resonant, by cancelling out the capacitive reactance of the short antenna. This is called an electrically short whip. The coil is added at the base of the whip (called a base-loaded whip) or occasionally in the middle (center-loaded whip). In the most widely used form, the rubber ducky antenna, the loading coil is integrated with the antenna itself by making the whip out of a narrow helix of springy wire. The helix distributes the inductance along the antenna's length, improving the radiation pattern, and also makes it more flexible. Another alternative occasionally used to shorten the antenna is to add a "capacity hat", a metal screen or radiating wires, at the end. However all these electrically short whips have lower gain than a full-length quarter-wave whip.

Multi-band operation is possible with coils at about one-half or one-third and two-thirds that do not affect the aerial much at the lowest band, but it creates the effect of stacked dipoles at a higher band (usually ×2 or ×3 frequency).

At higher frequencies [lower-alpha 1] the feed coax can go up the centre of a tube. The insulated junction of the tube and whip is fed from the coax and the lower tube end where coax cable enters has an insulated mount. This kind of vertical whip is a full dipole and thus needs no ground plane. It generally works better several wavelengths above ground, hence the limitation normally to microwave bands.

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Cellphone whip antenna with base loading coil on car
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Collection of walkie-talkies with electrically short whips. Units on ends and small one in foreground have “rubber ducky” antennas.
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Whip antenna on portable AM/FM receiver
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Tethered fiberglass whip on a military jeep

See also

Footnotes

  1. Feedlines can be run up through a metal mast at any radio frequency, but a center-routed feedline not common for HF band antennas. Feedlines are more commonly run up through the center of the antenna's radiator at 2.4 GHz, but military whips for 50 MHz to 80 MHz band exist, and are standard issue for the SINCGARS radio in the 30–88 MHz range.

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

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<span class="mw-page-title-main">Omnidirectional antenna</span> Radio antenna that sends signals in every direction

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

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<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 one of the two simplest and most widely-used types of antenna; the other is the monopole. 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 far 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. All dipoles are electrically equivalent to two monopoles mounted end-to-end and fed with opposite phases, with the ground plane between them made virtual by the opposing monopole.

<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 operators, shortwave listeners, longwave radio DXers and for 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. The shape of the antenna resembles the letter "T", hence the name. The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a ground connection.

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

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 by 1943 it was named the J antenna. 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.

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

<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.

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

An umbrella antenna is a capacitively top-loaded wire monopole antenna, consisting in most cases of a mast fed at the ground end, to which a number of radial wires are connected at the top, sloping downwards. One side of the feedline supplying power from the transmitter is connected to the mast, and the other side to a ground (Earthing) system of radial wires buried in the earth under the antenna. They are used as transmitting antennas below 1 MHz, in the MF, LF and particularly the VLF bands, at frequencies sufficiently low that it is impractical or infeasible to build a full size quarter-wave monopole antenna. The outer end of each radial wire, sloping down from the top of the antenna, is connected by an insulator to a supporting rope or cable anchored to the ground; the radial wires can also support the mast as guy wires. The radial wires make the antenna look like the wire frame of a giant umbrella hence the name.

<span class="mw-page-title-main">Dual-band blade antenna</span>

A dual-band blade antenna is a type of blade antenna, which is a monopole whip antenna mounted on the outside of an aircraft in the form of a blade-shaped aerodynamic fairing to reduce air drag. It is used by avionics radio communication systems. The dual-band type uses a "plane and slot" design to allow efficient omni-directional azimuth coverage, enabling it to operate on two different radio bands.

<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.

In radio systems, many different antenna types are used whose properties are especially crafted for particular applications.

References

  1. Hickman, Ian (2006). Practical RF Handbook, 4th Ed. Elsevier. pp. 16.20–16.22. ISBN   0-7506-8039-3.
  2. Chen, Zhi Ning (2007). Antennas for Portable Devices. Chichester, UK: John Wiley.
  3. Seybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. pp. 48–49. ISBN   9780471743682.
  4. Fujimoto, Kyohei (2008). Mobile Antenna Systems Handbook. Artech House. pp. 216, 220. ISBN   9781596931275.
  5. 1 2 Kraus, John D. (1988). Antennas, 2nd Ed. Tata McGraw-Hill. pp. 721–724. ISBN   0-07-463219-1.
  6. Wallace, Richard; Andreasson, Krister (2005). Introduction to RF and Microwave Passive Components. Artech House. pp. 85–87. ISBN   1-63081-009-6.