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A helical antenna is an antenna consisting of one or more conducting wires (monofilar, bifilar, or quadrifilar with 1, 2, or 4 wires respectively) wound in the form of a helix. In most cases, directional helical antennas are mounted over a ground plane, while omnidirectional designs may not be. The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes — normal mode or axial mode.

In the normal mode or broadside helical antenna, the diameter and the pitch of the aerial are small compared with the wavelength. The antenna acts similarly to an electrically shortdipole or monopole, equivalent to a 1/4 wave vertical and the radiation pattern,^{[citation needed]} similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. For monofilar designs the radiation is linearly polarized parallel to the helix axis. These are used for compact antennas for portable hand held as well as mobile vehicle mount two-way radios, and in larger scale for UHF television broadcasting antennas. In bifilar or quadrifilar implementations, broadside circularly polarized radiation can be realized.

In the axial mode or end-fire helical antenna, the diameter and pitch of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarized radio waves. These are used for satellite communication. Axial mode operation was discovered by physicist John D. Kraus^[1]

• In terms of its specification a typical log periodic antenna might provide between 3 and 6 dB gain over dipole for a bandwidth of 2:1 while retaining an VSWR level of better than 1.3:1. With this level of performance it is ideal for many applications, although a log periodic antenna will be much larger than a Yagi of equivalent gain.
• The waves from the multiple elements superpose and interfere to enhance radiation in a single direction, achieving a very substantial increase in the antenna's gain compared to a simple dipole. Also called a 'beam antenna', 4 or 'parasitic array', the Yagi is very widely used as a high-gain antenna on the HF, VHF and UHF bands.

• 1Normal-mode helical

Normal-mode helical[edit]

If the circumference of the helix is significantly less than a wavelength and its pitch (axial distance between successive turns) is significantly less than a quarter wavelength, the antenna is called a normal-mode helix. The antenna acts similar to a monopole antenna, with an omnidirectionalradiation pattern, radiating equal power in all directions perpendicular to the antenna's axis. However, because of the inductance added by the helical shape, the antenna acts like a inductively loaded monopole; at its resonant frequency it is shorter than a quarter-wavelength long. Therefore, normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big. As with other electrically short antennas, the gain, and thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes 'helicals' useful as antennas for mobile and portable communications equipment on the HF, VHF, and UHF bands.^{[citation needed]}

^{[citation needed]}The loading provided by the helix allows the antenna to be physically shorter than its electrical length of a quarter-wavelength. This means that for example a 1/4 wave antenna at 27MHz is 2.7 m (108”) long and is physically quite unsuitable for mobile applications. The reduced size of a helical provides the same radiation pattern in a much more compact physical size with only a slight reduction in signal performance.

An effect of using a helical conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohms to between 25 and 35 ohms base impedance. This does not seem to be adverse to operation or matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wavelength at the frequency of operation.^{[citation needed]}

Mobile HF helicals[edit]

Another example of the type as used in mobile communications is 'spaced constant turn' in which one or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance and inductance for the radiating element at a particular resonant frequency. Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s. To date many millions of these ‘helical antennas’ have been mass-produced for mainly mobile vehicle use and reached peak production during the CB Radio boom-times during the 1970s to late 1980s and used worldwide. Multi-frequency versions with manual plug-in taps have become the mainstay for multi-band single-sideband modulation (SSB) HF communications with frequency coverage over the whole HF spectrum from 1mHz to 30 MHz with from 2 to 6 dedicated frequency tap points tuned at dedicated and allocated frequencies in the land mobile, marine and aircraft bands. Recently these antennas have been superseded by electronicly tuned antenna matching devices.^{[citation needed]}Most examples were wound with copper wire using a fiberglass rod as a former. The usually flexible or ridged radiator is then covered with a PVC or polyolefin heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna. The fibreglass rod was then usually glued and/or crimped to a brass fitting and screw mounted onto an insulated base affixed to a vehicle roof, guard or bull-bar mount. This mounting provided a ground plane or reflector (provided by the vehicle) for an effective vertical radiation pattern.^{[citation needed]}

These popular designs are still in common use as of 2018 and the ‘constant turn’ design originating in Australia have been universally adapted as standard FM receiving antennas for many factory produced motor vehicles as well as the existing basic style of aftermarket HF and VHF mobile helical. Another common use for broadside helixes is in the 'rubber ducky antenna' found on most portable VHF and UHF radios using a steel or copper conductor as the radiating element and usually terminated to a BNC / TNC style or screw on connector for quick removal.^{[citation needed]}

Helical broadcasting antennas[edit]

Specialized enlarged normal-mode helical antennas are used for Base Station transmitters for FM radio and television broadcasting stations on the VHF and UHF bands.^{[citation needed]}

Axial-mode helical[edit]

When the helix circumference is near the wavelength of operation, the antenna operates in axial mode. This is a nonresonanttraveling wave mode, in which instead of standing waves, the waves of current and voltage travel in one direction, up the helix. Instead of radiating linearly polarized waves normal to the antenna's axis, it radiates a beam of radio waves with circular polarisation along the axis, off the ends of the antenna. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.

In radio transmission, circular polarisation is often used where the relative orientation of the transmitting and receiving antennas cannot be easily controlled, such as in animal tracking and spacecraft communications, or where the polarisation of the signal may change, so end-fire helical antennas are frequently used for these applications. Since large helices are difficult to build and unwieldy to steer and aim, the design is commonly employed only at higher frequencies, ranging from VHF up to microwave.

The helix of the antenna can twist in two possible directions: right-handed or left-handed, the former having the same form as that of a common corkscrew. The 4-helix array in the first illustration uses left-handed helices, while all other illustrations show right-handed helices. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the emitted wave. Two mutually incompatible conventions are in use for describing waves with circular polarisation, so the relationship between the handedness (left or right) of a helical antenna, and the type of circularly-polarized radiation it emits is often described in ways that appear to be ambiguous. However, Kraus (the inventor of the helical antenna) states 'The left-handed helix responds to left-circular polarisation, and the right handed helix to right-circular polarisation (IEEE definition)' ^[2]. The IEEE defines the sense of polarisation as 'the sense of polarization, or handedness ... is called right handed (left handed) if the direction of rotation is clockwise (anti-clockwise) for an observer looking in the direction of propagation' ^[3] Thus a right-handed helix radiates a wave which is right-handed, the electric field vector rotating clockwise looking in the direction of propagation.

Helical antennas can receive signals with any type of linear polarisation, such as horizontal or vertical polarisation, but when receiving circularly polarized signals the handedness of the receiving antenna must be the same as the transmitting antenna; left-hand polarized antennas suffer a severe loss of gain when receiving right-circularly-polarized signals, and vice versa.

The dimensions of the helix are determined by the wavelength λ of the radio waves used, which depends on the frequency. In order to operate in axial-mode, the circumference should be equal to the wavelength.^[4] The pitch angle should be 13 degrees, which is a pitch distance (distance between each turn) of 0.23 times the circumference, which means the spacing between the coils should be approximately one-quarter of the wavelength (λ/4).^{[citation needed]} The number of turns in the helix determines how directional the antenna is: more turns improves the gain in the direction of its axis at both ends (or at 1 end when a ground plate is used), at a cost of gain in the other directions. When C<λ it operates more in normal mode where the gain direction is a donut shape to the sides instead of out the ends.

Terminal impedance in axial mode ranges between 100 and 200 ohms, approximately^{[citation needed]}

${\displaystyle Z\simeq 140\left(\frac{C}{\lambda }\right)}$

where C is the circumference of the helix, and λ is the wavelength. Impedance matching (when C=λ) to standard 50 or 75 ohm coaxial cable is often done by a quarter wave stripline section acting as an impedance transformer between the helix and the ground plate.

The maximum directive gain is approximately:

${\displaystyle Gain\simeq 15{\left(\frac{C}{\lambda }\right)}^2\left(\frac{NS}{\lambda }\right)}$^[5]

where N is the number of turns and S is the spacing between turns. Most designs use C=λ and S=0.23*C, so the gain is typically G=3.45*N. In decibels, the gain is ${\displaystyle G_{dBi}=10\cdot {\log }_{10}\left(0.8N\right)}$.

The half-power beamwidth is:

${\displaystyle \text{HPBW}\simeq \frac{52}{\frac{C}{\lambda }\sqrt{\frac{NS}{\lambda }}}\text{degrees}}$^[5]

The beamwidth between nulls is:

${\displaystyle \text{FNBW}\simeq \frac{115{\lambda }^{3/2}}{C\sqrt{NS}}\text{degrees}}$

The gain of the helical antenna strongly depends on the reflector.^[6] The above classical formulas assume that the reflector has the form of a circular resonator (a circular plate with a rim) and the pitch angle is optimal for this type of reflector. Nevertheless, these formulas overestimate the gain for several dB.^[7] The optimal pitch that maximizes the gain for a flat ground plane is in the range from 3° to 10° and it depends on the wire radius and antenna length.^[7]

See also[edit]

References[edit]

1. ^Proceedings of the I.R.E., March 1949, P.263
2. ^Kraus, J.D. Antennas 2nd Ed, MacGraw Hill, 1988
3. ^IEEE Std 149-1979 (R2008), 'IEEE Standard Test Procedures for Antennas'. Reaffirmed December 10, 2008, Approved December 15, 1977, IEEE-SA Standards Board. Approved October 9, 2003, American National Standards Institute. ISBN0-471-08032-2. doi:10.1109/IEEESTD.1979.120310, sec. 11.1, p. 61.
4. ^https://www.cv.nrao.edu/~demerson/helixgain/helix.htm
5. ^ ^abTomasi, Wayne (2004). Electronic Communication Systems - Fundamentals Through Advanced. Jurong, Singapore: Pearson Education SE Asia Ltd. ISBN981-247-093-X.
6. ^Djordjević, A.R., Zajić, A.G., and Ilić, M.M., “Enhancing the gain of helical antennas by shaping the ground conductor”, IEEE Antennas and Wireless Propagation Letters, Vol. 5, 2006, pp. 138-140
7. ^ ^abDjordjević, A.R., Zajić, A.G., Ilić, M.M., and Stueber, G.L., “Optimization of helical antennas“, IEEE Antennas and Propagation Magazine, vol. 48, no. 6, December 2006, pp. 107-115

General

• John D. Kraus and Ronald J. Marhefka, 'Antennas: For All Applications, Third Edition', 2002, McGraw-Hill Higher Education
• Constantine Balanis, 'Antenna Theory, Analysis and Design', 1982, John Wiley and Sons
• Warren Stutzman and Gary Thiele, 'Antenna Theory and Design, 2nd. Ed.', 1998, John Wiley and Sons

External links[edit]

• Helical Antennas Antenna-Theory
• The Basics of Quadrifilar Helix Antennas by Bill Slade, Orban Microwave

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.

The monopole is a resonant antenna; the rod functions as an open resonator for radio waves, oscillating with standing waves of voltage and current along its length. Therefore, the length of the antenna is determined by the wavelength of the radio waves it is used with. The most common form is the quarter-wave monopole, in which the antenna is approximately one quarter of the wavelength of the radio waves. However in broadcasting monopole antennas 5/8 = 0.625 wavelength long are also popular, because at this length a monopole radiates a maximum amount of its power in horizontal directions. The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi; for this reason it is sometimes called the Marconi antenna.^[1][2][3] Common types of monopole antenna are the whip, rubber ducky, helical, random wire, umbrella, inverted-L and T-antenna, inverted-F, mast radiator, and ground plane antennas.

The load impedance of the quarter-wave monopole is half that of the dipole antenna or 37.5+j21.25 ohms.

History[edit]

The monopole antenna was invented in 1895 and patented 1896^[4] by radio pioneer Guglielmo Marconi during his historic first experiments in radio communication. He began by using dipole antennas invented by Heinrich Hertz consisting of two identical horizontal wires ending in metal plates. He found by experiment that if instead of the dipole, one side of the transmitter and receiver was connected to a wire suspended overhead, and the other side was connected to the Earth, he could transmit for longer distances. For this reason the monopole is also called a Marconi antenna,^[1][2][3] although Alexander Popov independently invented it at about the same time.^[5][6][7][8]

Vertical Dipole Radiation Pattern

Radiation pattern[edit]

Like a dipole antenna, a monopole has an omnidirectionalradiation pattern: it radiates with equal power in all azimuthal directions perpendicular to the antenna. However, the radiated power varies with elevation angle, with the radiation dropping off to zero at the zenith of the antenna axis. It radiates vertically polarized radio waves.

A monopole can be visualized (right) as being formed by replacing the bottom half of a vertical dipole antenna(c) with a conducting plane (ground plane) at right-angles to the remaining half. If the ground plane is large enough, the radio waves from the remaining upper half of the dipole (a) reflected from the ground plane will seem to come from an image antenna(b) forming the missing half of the dipole, which adds to the direct radiation to form a dipole radiation pattern. So the pattern of a monopole with a perfectly conducting, infinite ground plane is identical to the top half of a dipole pattern, with its maximum radiation in the horizontal direction, perpendicular to the antenna. Because it radiates only into the space above the ground plane, or half the space of a dipole antenna, a monopole antenna will have a gain of twice (3 dB greater than) the gain of a similar dipole antenna, and a radiation resistance half that of a dipole. Since a half-wave dipole has a gain of 2.19 dBi and a radiation resistance of 73 ohms, a quarter-wave monopole, the most common type, will have a gain of 2.19 + 3 = 5.19 dBi and a radiation resistance of about 36.8 ohms if it is mounted above a good ground plane.^[9]

The general effect of electrically small ground planes, as well as imperfectly conducting earth grounds, is to tilt the direction of maximum radiation up to higher elevation angles.^[10]

Types[edit]

The ground plane used with a monopole may be the actual earth; in this case the antenna is mounted on the ground and one side of the feedline is connected to an earth ground at the base of the antenna. This design is used for the mast radiator antennas employed in radio broadcasting at low frequencies, as well as other low frequency antennas such as the T-antenna and umbrella antenna. At VHF and UHF frequencies the size of the ground plane needed is smaller, so artificial ground planes are used to allow the antenna to be mounted above the ground. A common type of monopole antenna at these frequencies consists of a quarter-wave whip antenna with a ground plane consisting of several wires or rods radiating horizontally or diagonally from its base; this is called a ground-plane antenna. At gigahertz frequencies the metal surface of a car roof or airplane body makes a good ground plane, so car cell phone antennas consist of short whips mounted on the roof, and aircraft communication antennas frequently consist of a short conductor in an aerodynamic fairing projecting from the fuselage; this is called a blade antenna.^[9] The most common antenna used in mobile phones is the inverted-F antenna, which is a variant of the inverted-L monopole. Bending over the antenna saves space and keeps the it within the bounds of the mobile's case but the antenna then has a very low impedance. To improve the match the antenna is not fed from the end, rather some intermediate point, and the end is grounded instead. The quarter-wave whip and rubber ducky antennas used with handheld radios such as walkie-talkies and cell phones are also monopole antennas. These don't use a ground plane, and the ground side of the transmitter is just connected to the ground connection on its circuit board. The hand and body of the person holding them may function as a rudimentary ground plane.

Sometimes, monopole antennas are printed on a dielectric substrate to make it less fragile and they may be fabricated easily using the printed circuit board technologies. Such antennas are known as printed monopole antennas. They are suitable for various applications such as RFID, WLAN ^[11].

Monopole broadcasting antennas[edit]

When used for radio broadcasting, the radio frequency power from the broadcasting transmitter is fed across the base insulator between the tower and a ground system. The ideal ground system for AM broadcasters comprises at least 120 buried copper or phosphor bronze radial wires at least one-quarter wavelength long and a ground-screen in the immediate vicinity of the tower. All the ground system components are bonded together, usually by welding, brazing or using coin silver solder to help reduce corrosion. Monopole antennas that use guy-wires for support are called masts in some countries. In the United States, the term “mast” is generally used to describe a pipe supporting a smaller antenna, so both self-supporting and guy-wire supported radio antennas are simply called monopoles if they stand alone. If multiple monopole antennas are used in order to control the direction of radio frequency (RF) propagation, they are called directional antenna arrays.

Are Dipole Antennas Directional

In the United States, the Federal Communications Commission (FCC) requires that the transmitter power input to the antenna be measured and maintained. The power input is calculated as the square of the measured current, ${\displaystyle i}$, flowing into the antenna from the transmission line multiplied by the real part of the antenna's feed-point impedance, ${\displaystyle r}$.

${\displaystyle P=i^{2}r}$

This impedance is periodically measured to verify the stability of the antenna and ground system. Normally, an impedance matching network matches the impedance of the antenna to the impedance of the transmission line feeding it.

Examples of monopole antennas are:

• the whip antenna
• the mast radiator (when isolated from the ground and bottom-fed)

Monopole antennas have become one of the components of mobile and Internet networks across the globe. Their relative low cost and fast installation makes them an obvious choice for developing countries.

See also[edit]

Half Dipole Antenna

References[edit]

1. ^ ^abDas, Sisir K. (2016). Antenna and Wave Propagation. Tata McGraw-Hill Education. p. 116. ISBN1259006328.
2. ^ ^abWong, K. Daniel (2011). Fundamentals of Wireless Communication Engineering Technologies. John Wiley and Sons. p. 94. ISBN1118121090.
3. ^ ^abKishore, Kamal (2009). Antenna and Wave Propagation. IK International Ltd. p. 93. ISBN9380026064.
4. ^ ^abUS patent 586193, Guglielmo Marconi Transmitting electrical signals, filed December 7, 1896, granted July 13, 1897
5. ^Visser, Hubregt J. (2006). Array and Phased Array Antenna Basics. John Wiley and Sons. p. 31. ISBN0470871180.
6. ^Howeth, L. S. (1963). The History of Communications - Electronics in the U.S. Navy. U.S. Navy. p. 19.
7. ^Meinel, Christoph; Sack, Harald (2014). Digital Communication: Communication, Multimedia, Security. Springer Science and Business Media. p. 55. ISBN3642543316.
8. ^Stutzman, Warren L.; Thiele, Gary A. (2012). Antenna Theory and Design. John Wiley and Sons. p. 8. ISBN0470576642.
9. ^ ^abMacnamara, Thereza (2010). Introduction to Antenna Placement and Installation. USA: John Wiley and Sons. p. 145. ISBN0-470-01981-6.
10. ^Weiner, Melvin M. Weiner (2003). Monopole antennas. USA: CRC Press. pp. vi. ISBN0-8247-4844-1.
11. ^J. R. Panda, A. S. R. Saladi, Rakhesh Singh Kshetrimayum, 'A Compact Printed Monopole Antenna for DualbandRFID and WLAN Applications, ' Radioengineering, vol. 20, no. 2, June 2011, pp. 464-467

Radiation Of A Dipole

External links[edit]

Dipole Antenna Radiation Pattern Diagrams