Microwave antennas An antenna is a component that radiates and receives the RF or microwave power. It is a reciprocal device, and the same antenna can serve as a receiving or transmitting device. Antennas are structures that provide transitions between guided and free-space waves. Guided waves are confined to the boundaries of a transmission line to transport signals from one point to another , while free-space waves radiate unbounded. A transmission line is designed to have very little radiation loss, while the antenna is designed to have maximum radiation. The antenna is a key component in any wireless system, as shown in below
The RF/microwave signal is transmitted to free space through the antenna. The signal propagates in space, and a small portion is picked up by a receiving antenna. The signal will then be amplified, down converted, and processed to recover the information. Different categories of antennas Wire antennas: These include dipoles, monopoles, loops, yagi yuda arrays etc. wire antennas are characterized by low gains and used at lower frequencies .They have advantage of light weight, low cost and simple design Aperture antennas: Include open ended waveguides, rectangular or circular horns, reflectors etc. Aperture antennas are most commonly used at microwave frequencies and have moderate to high gains Antenna arrays: They consist of a regular arrangement of antenna elements with feed network. They are motivated by two reason: Beam steering and beam nulling. Pattern characteristics such as beam pointing angle and sidelobe levels can be controlled by adjusting the amplitude and phase distribution of the array elements.
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Antenna characteristics and parameters These parameters provide information about the properties and characteristics of an antenna 1. Radiation pattern An antenna radiation pattern or antenna pattern is defined as “a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. In most cases, the radiation pattern is determined in the far field region and is represented as a function of the directional coordinates. Radiation properties include power flux density, radiation intensity, field strength, directivity etc. For an antenna, the a. Field pattern ( in linear scale) typically represents a plot of the magnitude of the electric or magnetic field as a function of the angular space. b. Power pattern ( in linear scale) typically represents a plot of the square of the magnitude of the electric or magnetic field as a function of the angular space. c. Power pattern ( in dB) represents the magnitude of the electric or magnetic field, in decibels, as a function of the angular space.
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A major lobe (also called main beam) is defined as “the radiation lobe containing the direction of maximum radiation A minor lobe is any lobe except a major lobe A side lobe is “a radiation lobe in any direction other than the intended lobe.” (Usually a side lobe is adjacent to the main lobe and occupies the hemisphere in the direction of the main beam.)
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A back lobe is “a radiation lobe whose axis makes an angle of approximately 180◦ with respect to the beam of an antenna.” Usually it refers to a minor lobe that occupies the hemisphere in a direction opposite to that of the major (main) lobe. Minor lobes usually represent radiation in undesired directions, and they should be minimized. Side lobes are normally the largest of the minor lobes. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is often termed the side lobe ratio or side lobe level Isotropic,Directional And Omnidirectional Patterns An isotropic radiator is defined as “a hypothetical lossless antenna having equal radiation in all directions.” Although it is ideal and not physically realizable, it is often taken as a reference for expressing the directive properties of actual antennas. A directional antenna is one “having the property of radiating or receiving electromagnetic waves more effectively in some directions than in others This type of a pattern is designated as omnidirectional, and it is defined as one “having an essentially nondirectional pattern in a given plane (in this case in azimuth) and a directional pattern in any orthogonal plane (in this case in elevation).” An omnidirectional pattern is then a special type of a directional pattern Field Regions The space surrounding an antenna is usually subdivided into three regions: (a) reactive nearfield, (b) radiating near-field (Fresnel) and (c) far-field (Fraunhofer) regions as shown in Figure below. Although no abrupt changes in the field configurations are noted as the boundaries are crossed, there are distinct differences among them. The boundaries separating these regions are not unique, although various criteria have been established and are commonly used to identify the regions. Reactive near-field region is defined as “that portion of the near-field region immediately surrounding the antenna wherein the reactive field predominates.” For most antennas, the outer boundary of this region is commonly taken to exist at a distance R < 0.62 D3 / from the antenna surface, where λ is the wavelength and D is the largest dimension of the antenna. “For a very short dipole, or equivalent radiator, the outer boundary is commonly taken to exist at a distance λ/2π from the antenna surface.” Radiating near-field (Fresnel) region is defined as “that region of the field of an antenna between the reactive near-field region and the far-field region wherein radiation fields predominate and wherein the angular field distribution is dependent upon the distance from the antenna. If the antenna has a maximum dimension that is not large compared to the wavelength, this region may not exist. Far-field (Fraunhofer) region is defined as “that region of the field of an antenna where the angular field distribution is essentially independent of the distance from the antenna. If the kyu
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antenna has a maximum overall dimension D, the far-field region is commonly taken to exist at distances greater than 2D2/λ from the antenna, λ being the wavelength.
Far field properties The EM field in far field satisfies the following properties: 1. The Electric and magnetic fields are orthogonal 2. The ratio of the E and H fields is a constant and equal to the intrinsic impedance of the medium E Thus H 3. The fields in far field region are plannar. Radian and Steradian The measure of a plane angle is a radian. One radian is defined as the plane angle with its vertex at the center of a circle of radius r that is subtended by an arc whose length is r. Since the circumference of a circle of radius r is C = 2πr, there are 2π rad (2πr/r) in a full circle. The measure of a solid angle is a steradian. One steradian is defined as the solid angle with its vertex at the center of a sphere of radius r that is subtended by a spherical surface area equal to that of a square with each side of length r. Since the area of a sphere of radius r is A = 4πr2, there are 4π sr (4πr2/r2) in a closed sphere. Radiation power density Electromagnetic waves are used to transport information through a wireless medium or a guiding structure, from one point to the other. It is then natural to assume that power and energy are
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associated with electromagnetic fields. The quantity used to describe the power associated with an electromagnetic wave is the instantaneous Poynting vector defined as W=EXH Where W = instantaneous Poynting vector (W/m2) E= instantaneous electric-field intensity (V/m) H= instantaneous magnetic-field intensity (A/m) Since the Poynting vector is a power density, the total power crossing a closed surface can be obtained by integrating the normal component of the Poynting vector over the entire surface. In equation form,
The time average Poynting vector (average power density) can be written as
Based upon the definition of (2-8), the average power radiated by an antenna (radiated power) can be written as
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Radiation Intensity Radiation intensity in a given direction is defined as “the power radiated from an antenna per unit solid angle.” The radiation intensity is a far-field parameter, and it can be obtained by simply multiplying the radiation density by the square of the distance. In mathematical form it is expressed as
The total power is obtained by integrating the radiation intensity over the entire solid angle of 4π. Thus
Beam Width The beam width of a pattern is defined as the angular separation between two identical points on opposite side of the pattern maximum. One of the most widely used beamwidths is the Half-Power Beamwidth (HPBW ) HPBW definition: “In a plane containing the direction of the maximum of a beam, the angle between the two directions in which the radiation Intensity is one-half value of the beam.”. Another important beamwidth is the angular separation between the first nulls of the pattern, and it is referred to as the First-Null Beamwidth (FNBW).
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DIRECTIVITY The directivity of an antenna is defined as “the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4π. If the direction is not specified, the direction of maximum radiation intensity is implied.” Stated more simply, the directivity of a non isotropic source is equal to the ratio of its radiation Intensity in a given direction over that of an isotropic source. In mathematical form, it can be written as
If the direction is not specified, it implies the direction of maximum radiation intensity (maximum directivity) expressed as
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Antenna Efficiency Resistive losses due to non perfect and dielectric materials exist in all antennas. Such losses result into a difference between the power delivered to the input of the antenna and the power radiated by that antenna We define radiation efficiency of an antenna as the ratio of the desired output power to the supplied input power
rad
prad pin ploss p 1 loss pin pin Pin
The total antenna efficiency e0 is used to take into account losses at the input terminals and within the structure of the transmission line antenna system. Such losses may be due, 1. Reflections because of the mismatch between the transmission line and the antenna 2. I 2R losses (conduction and dielectric losses)
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Gain Another useful measure describing the performance of an antenna is the gain. Although the gain of the antenna is closely related to the directivity, it is a measure that takes into the antenna efficiency as well as its directional capabilities Gain of an antenna (in a given direction) is defined as “the ratio of the intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. The radiation intensity corresponding to the isotropically radiated power is equal to the power accepted (input) by the antenna divided by 4π.” In equation form this can be expressed as
Note that the total radiated power (Prad) is related to the total input power (Pin ) by Prad=ecd Pin where ecd is the antenna radiation efficiency.
Effective area/Aperture: Effective area (aperture), in a given direction is defined as “the ratio of the available power at the terminals of a receiving antenna to the power flux density of a plane wave incident on the antenna from that direction, the wave being polarization matched to the antenna. If the direction is not specified, the direction of maximum radiation intensity is implied.”
In general, the maximum effective aperture (Ae) of receiving antenna is related to the gain of the antenna as,
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Ae Gr
2 4
Input Impedance Input impedance is defined as “the impedance presented by an antenna at its terminals or the ratio of the voltage to current at a pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a point.” The ratio of the voltage to current at these terminals, with no load attached, defines the impedance of the antenna as Z A RA jX A where Z A =Antenna impedance at input terminals
RA =Antenna resistance at input terminals X A =Antenna reactance at input terminals
Microwave Antennas Horn Antenna The horn antenna is a transition between a waveguide and free space. A rectangular waveguide feed is used to connect to a rectangular waveguide horn, and a circular waveguide feed is for the circular waveguide horn. The horn antenna is commonly used as a feed to a parabolic dish antenna, a gain standard for antenna gain measurements, and as compact medium-gain antennas for various systems. Its gain can be calculated to within 0.1 dB accuracy from its known dimensions and is therefore used as a gain standard in antenna measurements. For a rectangular pyramidal horn, shown in Fig. below, the dimensions of the horn for optimum gain can be designed by setting
where A and B are dimensions of the horn and le and lh are the slant lengths of the horn
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Parabolic Dish Antenna A parabolic dish is a high-gain antenna. It is the most commonly used reflector antenna for point-to-point satellites and wireless links. A parabolic dish is basically a metal dish illuminated by a source at its focal point. The spherical wave front illuminated by the source is converted into a planar wavefront by the dish For an illumination efficiency of 100%, the effective area equals the physical area
where D is the diameter of the dish.
Radiation from parabolic dish antenna Microwave propagation In free space, electromagnetic waves propagate in straight lines without attenuation or other adverse effects. Free space however is just an idealization that is only approximated when kyu
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microwave energy propagates through the atmosphere or in the presence of the earth. In practice, the performance of the communication system may be adversely be affected by effects such as reflection, refraction, attenuation or diffraction and scattering Attenuation: caused by absorption of microwave energy by water vapor and molecular oxygen Ground effects: the most obvious effect of the presence of the ground on microwave propagation is reflection from the earth’s surface. A receiver may be illuminated by both a direct wave from the transmitter and a wave reflected from the ground. The reflected wave is smaller in amplitude than the direct wave because of the larger distance it travels, the fact that it usually originates from the side lobe region of the transmit antenna. Plasma Effects: Plasma is a gas consisting of ionized particles. The ionosphere consists of ionized particles due to solar radiation. Depending on the density of ions and frequency, wave may be reflected, aborbed or transmitted by the plasma medium. Microwave Biological effects and safety The proven dangers of exposure to microwave radiation are due to thermal effects .The body absorbs RF and microwave energy and converts it to heat; as in the case of the microwave oven, this heating occurs within the body and may not be felt at low levels. such heating is more dangerous in the brain, the eye and stomach organs. Excessive radiation can lead to cataracts, sterility and cancer. This makes it important to determine the safe radiation levels so that users of microwave equipment will not be exposed to harmful power levels. The most recent standards for human exposure as given by IEEE: In the RF microwave frequency range of 100 MHz to 300 GHz, exposure limits are set on the power density (w/cm2) as a function of frequency. The recommended safe power density limit is as low as 0.2 mw/cm2 at the lower end of the frequency range since the fields penetrate the body more deeply at lower frequencies. At frequencies above 15 GHz the power density limit rises to 10 mW/cm2, since most of the power absorption at such frequencies occurs near the skin surface. Other countries have different exposure limits some of which are a function of exposure time. A separate standard applies to microwave ovens in the United States; Law requires that all microwave ovens be tested to ensure that the power level at 5cm from any point of the oven does not exceed 1 mW/cm2. Exercise 1. A parabolic reflector antenna used for reception with DBS system is 0.45m in diameter and operates at 12.4 GHz. Find the operating wavelength and the far field distance for this antenna. 2. Distinguish between reflection, refraction, scattering and diffraction
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A 6 GHz common carrier microwave communication link uses a tower mounted antenna with a gain of 40dB and a transmitter of power 5W.Evaluate the radiation hazard of this system at a distance of 20m from the antenna. sin 4. A microwave antenna is characterised by Electric field intensity E Ao r field.Calculate power radiated at apoint located in far field 3.
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