Antenna gain - Wikipedia
A complete tutorial on Antenna Gain and Directivity. It is normal to refer to the directional patterns and gain in terms of the transmitted signal. It is often easier to visualise the RF antenna . How to Build: Cell Phone Jammer. When the antenna gain increases, the antenna radiation pattern becomes more directive that means the beamwidth becomes more narrow which makes the. Relation between frequency and wave length is given by λ = /f The 3- dimensional antenna gain pattern is usually described by a vertical.
Directivity - Wikipedia
And, if you remember multivariable calculus, Purcell's Electricity and Magnetism presents the topic in far more detail than I provided in the preceding discussion. Most antennas operate in the far field and transmit information over long distances through changing electric fields. Even though radio transmitters such as the nRF24 and Bluetooth devices have limited range, they still use far-field communication—the electric field is transmitting the information.
Radiation Patterns The animation above shows contours of constant radiation power density, propagating outward with time, traced in a plane that passes through a vertically oriented dipole antenna.
The contour surfaces are centered around an antenna and the contour lines are centered on orthogonal planes that intersect the antenna, often around a line of symmetry.
The Hertzian dipole above transmits very little to no energy in the vertical direction. Based on a Mathematica model found here. Different antenna designs produce different radiation patterns.
The complexity of the pattern depends on the antenna's design and construction. Antenna specification sheets sometimes come with three-dimensional projections. More often, we see a two-dimensional plot and must imagine the three-dimensional pattern.
Polar and Cartesian representations of a radiation pattern for a Yagi antenna. This phenomenon is due to charge polarization inside the dielectric medium.
Antenna Gain Explained
Permittivity is a measure of how readily those charges can align themselves polarization in the presence of an electric field. Higher permittivity indicates greater resistance to forming an electric field, and also slower propagation of a disturbance through the medium. A high-permittivity material that surrounds a low-permittivity material will not affect the frequency of oscillation, but the high-permittivity material reduces the speed of the wave's propagation. If we recall that wave speed is equal to the product of frequency and wavelength, we can see that if frequency remains the same, the reduction in speed must come with a corresponding reduction in wavelength.
When the wave exits the high-permittivity material, the wave speed and wavelength increase. When an antenna is embedded in a high-permittivity material, the size of the antenna can be reduced in accordance with the decreased wavelength of the electromagnetic waves in the immediate vicinity of the antenna.
Similar techniques are used to allow and cell phones to have resonant antennas that are substantially smaller than the wavelength associated with propagation in air. When waves transition between materials of different permittivity, energy is reflected. If the wave moves from a low-permittivity i. The reflected waves can combine with new waves to produce the various interference patterns seen in An Introduction to Antenna Basics.
Recall that the signals emitted from antennas are in the form of electromagnetic radiation—both electric and magnetic fields are involved. Thus, it is not surprising that permeability, like permittivity, affects the propagation of electromagnetic waves.
Indeed, both permittivity and permeability result in slower wave speed and decreased wavelength. To reinforce the idea that permittivity and permeability influence the speed and the wavelength of electromagnetic radiation, we can consider the "speed of light," which is actually the speed not only of light but of electromagnetic radiation in general. The speed of light in a vacuum—the fastest speed in the universe, denoted by c—is calculated using the permittivity and permeability of free space: The integral represents the theoretical total radiated power.
The energy from isotropic emitters spreads out evenly to cover this increasingly larger area, and thus the electromagnetic power flux density decreases in proportion to the square of the distance from the source. Since the power density of an isotropic emitter decreases rapidly with distance, antenna engineers manipulate the direction of energy radiated from real antennas so as to increase the power density in desired directions and reduce it in other directions.
An antenna that radiated equally well in all directions would have a directivity of 1 0 dB. The Hertzian dipole presented earlier has a directivity of 1. 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:Antenna Gain, Antenna Parameters in Antennas and Wave Propagation by Engineering Funda