Antenna (radio) - Wikipedia
Frequency is the oscillation speed of an electromagnetic wave, which governs We like to use "SI" units in science (length measured in meters,time in seconds The equation that relates frequency, wavelength and the speed of light can be. Only a full wave antenne length will create an optimal radiation pattern. You can play with the antenne length by adding capacitors or inductors. Can any body help to know the relation between antenna size and frequency? and according to frequecny- wavelength relationship.
There is a relationship between gain and directivity. We see the phenomena of increased directivity when comparing a light bulb to a spotlight. A watt spotlight will provide more light in a particular direction than a watt light bulb and less light in other directions. The spotlight is comparable to an antenna with increased directivity.
Gain is the practical value of the directivity. This is known as a gain transfer technique. At higher frequencies, it is common to use a calibrated gain horn as a gain standard with gain typically expressed in dBi.
Another method for measuring gain is the 3-antenna method. Transmitted and received powers at the antenna terminal are measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas. Pulse-Larsen uses both methods for measurement of gain.
The method is selected based on antenna type, frequency and customer requirement. Use the following conversion factor to convert between dBd and dBi: The radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner. It is also time-consuming to measure a three-dimensional radiation pattern. Often radiation patterns measured are a slice of the three-dimensional pattern, resulting in a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper.
These pattern measurements are presented in either a rectangular or a polar format. Omnidirectional antennas radiate and receive equally well in all horizontal directions. The gain of an omnidirectional antenna can be increased by narrowing the beamwidth in the vertical or elevation plane. Selecting the right antenna gain for the application is the subject of much analysis and investigation.
Gain is achieved at the expense of beamwidth. Higher-gain antennas feature narrow beamwidths while the opposite is also true. Omnidirectional antennas with different gains are used to improve reception and transmission in certain types of terrain.
A 0 dBd gain antenna radiates more energy higher in the vertical plane to reach radio communication sites located in higher places. Therefore they are more useful in mountainous and metropolitan areas with tall buildings.
A 3 dBd gain antenna is a good compromise for use in suburban and general settings. A 5 dBd gain antenna radiates more energy toward the horizon compared to the 0 and 3 dBd antennas.
This allows the signal to reach radio communication sites further apart and less obstructed. Therefore they are best used in deserts, plains, flatlands and open farm areas. Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired. Point-to-point links also benefit from directional antennas. Yagi and panel antennas are directional antennas. For example, for a 0 dB gain antenna, 3 db beamwidth is the area where the gain is higher than —3 dB.
The far-field is also called the radiation field, and is what is most commonly of interest. The nearfield is called the induction field although it also has a radiation component. Ordinarily, it is the radiated power that is of interest so antenna patterns are usually measured in the far-field region. For pattern measurement, it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength.
The accepted formula for this distance is: Two often-used special cases of elliptical polarization are linear polarization and circular polarization.
Initial polarization of a radio wave is determined by the antenna launching the waves into space. The environment through which the radio wave passes on its way from the transmit antenna to the receiving antenna may cause a change in polarization.
With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. The rotation may be right-hand or left-hand. Choice of polarization is one of the design choices available to the RF system designer.
Mobile radio system waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications there may be some possible advantage in one or the other.
Circular polarization has also been found to be of advantage in satellite applications such as GPS. Circular polarization can also be used to reduce multipath.
Theoretically, a whip provides an omnidirectional pattern in the horizontal plane and a dipolar pattern in the elevation plane.
In practice, this condition is never achieved. Common effects of reduction of the size of the ground plane are: Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.
Radio waves are electromagnetic waves which carry signals through the air or through space at the speed of light with almost no transmission loss. Radio transmitters and receivers are used to convey signals in broadcast audio radio, televisionmobile telephonesWi-Fi WLAN data networks, and remote control devices among many others.
Radio waves are also used directly for measurements in radar GPSand radio astronomy. Transmitters and receivers require antennas, although these are sometimes hidden such as the antenna inside an AM radio or inside a laptop computer equipped with Wi-Fi.
Whip antenna on car, common example of an omnidirectional antenna Antennas can be classified as omnidirectionalradiating energy approximately equally in all directions, or Directionalwhere energy radiates more along one direction than others.
Antennas are reciprocal, so the same effect occurs for reception of radio waves. A completely uniform omnidirectional antenna is not physically possible. Many important antenna types have a uniform radiation pattern in the horizontal plane, but send little energy upward or downward.
A "directional" antenna usually is intended to maximize its coupling to the electromagnetic field in the direction of the other station. One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a metal rod. A dipole antenna is similar but consists of two such conductors extending in opposite directions. Dipoles are typically oriented horizontally in which case they are weakly directional: Half-wave dipole antenna Both the vertical and dipole antennas are simple in construction and relatively inexpensive.
The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals through a balanced transmission line or to a coaxial transmission line through a so-called balun.
The vertical antenna, on the other hand, is a monopole antenna. It is typically connected to the inner conductor of a coaxial transmission line or a matching network ; the shield of the transmission line is connected to ground.
In this way, the ground or any large conductive surface plays the role of the second conductor of a dipole, thereby forming a complete circuit. Since monopole antennas rely on a conductive ground, a so-called grounding structure may be employed to provide a better ground contact to the earth or which itself acts as a ground plane to perform that function regardless of or in absence of an actual contact with the earth.
Diagram of the electric fields blue and magnetic fields red radiated by a dipole antenna black rods during transmission. Antennas more complex than the dipole or vertical designs are usually intended to increase the directivity and consequently the gain of the antenna.
This can be accomplished in many different ways leading to a plethora of antenna designs. The vast majority of designs are fed with a balanced line unlike a monopole antenna and are based on the dipole antenna with additional components or elements which increase its directionality.
Antenna "gain" in this instance describes the concentration of radiated power into a particular solid angle of space, as opposed to the spherically uniform radiation of the ideal radiator. The increased power in the desired direction is at the expense of that in the undesired directions.
Power is conserved, and there is no net power increase over that delivered from the power source the transmitter. For instance, a phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves a number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" directional normal to a line connecting the elements or as an "end-fire array" directional along the line connecting the elements.
Antenna arrays may employ any basic omnidirectional or weakly directional antenna type, such as dipole, loop or slot antennas. These elements are often identical.
However a log-periodic dipole array consists of a number of dipole elements of different lengths in order to obtain a somewhat directional antenna having an extremely wide bandwidth: The dipole antennas composing it are all considered "active elements" since they are all electrically connected together and to the transmission line. On the other hand, a superficially similar dipole array, the Yagi-Uda Antenna or simply "Yagi"has only one dipole element with an electrical connection; the other so-called parasitic elements interact with the electromagnetic field in order to realize a fairly directional antenna but one which is limited to a rather narrow bandwidth.
The Yagi antenna has similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There may be a number of so-called "directors" in front of the active element in the direction of propagation, and usually a single but possibly more "reflector" on the opposite side of the active element.
Greater directionality can be obtained using beam-forming techniques such as a parabolic reflector or a horn. Since high directivity in an antenna depends on it being large compared to the wavelength, narrow beams of this type are more easily achieved at UHF and microwave frequencies. At low frequencies such as AM broadcastarrays of vertical towers are used to achieve directionality  and they will occupy large areas of land.
For reception, a long Beverage antenna can have significant directivity.
For non directional portable use, a short vertical antenna or small loop antenna works well, with the main design challenge being that of impedance matching. With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance ; small loop antennas are tuned with parallel capacitors for this purpose.
An antenna lead-in is the transmission lineor feed linewhich connects the antenna to a transmitter or receiver. The " antenna feed " may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called aperture antenna, such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system normally at the focus of the parabolic dish or at the throat of a horn which could be considered the one active element in that antenna system.
A microwave antenna may also be fed directly from a waveguide in place of a conductive transmission line. Cell phone base station antennas Cellular Mobile UHF Antenna Tower with multiple Antennas An antenna counterpoiseor ground planeis a structure of conductive material which improves or substitutes for the ground.
It may be connected to or insulated from the natural ground.Critical Frequency, Sky Wave Propagation in Antennas and Wave Propagation by Engineering Funda
In a monopole antenna, this aids in the function of the natural ground, particularly where variations or limitations of the characteristics of the natural ground interfere with its proper function. Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable. An electromagnetic wave refractor in some aperture antennas is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it.
The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This is the radio equivalent of an optical lens. An antenna coupling network is a passive network generally a combination of inductive and capacitive circuit elements used for impedance matching in between the antenna and the transmitter or receiver.
This may be used to improve the standing wave ratio in order to minimize losses in the transmission line and to present the transmitter or receiver with a standard resistive impedance that it expects to see for optimum operation.
Reciprocity[ edit ] It is a fundamental property of antennas that the electrical characteristics of an antenna described in the next section, such as gainradiation patternimpedancebandwidthresonant frequency and polarizationare the same whether the antenna is transmitting or receiving. This is a consequence of the reciprocity theorem of electromagnetics. A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal.
Reciprocal or bilateral means that the material has the same response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite direction. Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulatorsmade of nonreciprocal materials such as ferrite.
Antenna Basic Concepts – Pulse Electronics
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. January See also: Chief among these relate to the directional characteristics as depicted in the antenna's radiation pattern and the resulting gain.
Even in omnidirectional or weakly directional antennas, the gain can often be increased by concentrating more of its power in the horizontal directions, sacrificing power radiated toward the sky and ground. The antenna's power gain or simply "gain" also takes into account the antenna's efficiency, and is often the primary figure of merit.
Resonant antennas are expected to be used around a particular resonant frequency ; an antenna must therefore be built or ordered to match the frequency range of the intended application.
A particular antenna design will present a particular feedpoint impedance. While this may affect the choice of an antenna, an antenna's impedance can also be adapted to the desired impedance level of a system using a matching network while maintaining the other characteristics except for a possible loss of efficiency.
Although these parameters can be measured in principle, such measurements are difficult and require very specialized equipment. Beyond tuning a transmitting antenna using an SWR meter, the typical user will depend on theoretical predictions based on the antenna design or on claims of a vendor.
An antenna transmits and receives radio waves with a particular polarization which can be reoriented by tilting the axis of the antenna in many but not all cases. The physical size of an antenna is often a practical issue, particularly at lower frequencies longer wavelengths. Highly directional antennas need to be significantly larger than the wavelength. Resonant antennas usually use a linear conductor or elementor pair of such elements, each of which is about a quarter of the wavelength in length an odd multiple of quarter wavelengths will also be resonant.
Antennas that are required to be small compared to the wavelength sacrifice efficiency and cannot be very directional. At higher frequencies UHF, microwaves trading off performance to obtain a smaller physical size is usually not required. Resonant antennas[ edit ] Standing waves on a half wave dipole driven at its resonant frequency. The waves are shown graphically by bars of color red for voltage, V and blue for current, I whose width is proportional to the amplitude of the quantity at that point on the antenna.
The majority of antenna designs are based on the resonance principle. This relies on the behaviour of moving electrons, which reflect off surfaces where the dielectric constant changes, in a fashion similar to the way light reflects when optical properties change.
In these designs, the reflective surface is created by the end of a conductor, normally a thin metal wire or rod, which in the simplest case has a feed point at one end where it is connected to a transmission line. The conductor, or element, is aligned with the electrical field of the desired signal, normally meaning it is perpendicular to the line from the antenna to the source or receiver in the case of a broadcast antenna. This causes an electrical current to begin flowing in the direction of the signal's instantaneous field.
When the resulting current reaches the end of the conductor, it reflects, which is equivalent to a degree change in phase. That means it has undergone a total degree phase change, returning it to the original signal. The current in the element thus adds to the current being created from the source at that instant.
This process creates a standing wave in the conductor, with the maximum current at the feed. The physical arrangement of the two elements places them degrees out of phase, which means that at any given instant one of the elements is driving current into the transmission line while the other is pulling it out.
Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large.