天线的性能-毕业论文外文翻译.docx

目录 PROPERTIES OF ANTENNAS22.1 ANTENNA RADIATION32.2 GAIN52.3 EFFECTIVE AREA82.4 PATH LOSS82.5 RADAR RANGE EQUATION AND CROSS SECTION102.6 WHY USE AN ANTENNA?12天线的功能122.1天线辐射132.2 天线增益152.3天线有效面积172.4路径损耗182.5雷达距离方程和截面202.6为什么使用一个天线？21英文原文：PROPERTIES OF ANTENNASOne approach to an antenna book starts with a discussion of how antennas radiate. Beginning with Maxwells equations, we derive electromagnetic waves. After that lengthy discussion, which contains a lot of mathematics, we discuss how these waves excite currents on conductors. The second half of the story is that currents radiate and produce electromagnetic waves. You may already have studied that subject, or if you wish to further your background, consult books on electromagnetics.The study of electromagnetics gives insight into the mathematics describing antenna radiation and provides the rigor to prevent mistakes. We skip the discussion of those equations and move directly to practical aspects. It is important to realize that antennas radiate from currents. Design consists of controlling currents to produce the desired radiation distribution, called its pattern .In many situations the problem is how to prevent radiation from currents, such as in circuits. Whenever a current becomes separated in distance from its return current, it radiates. Simply stated, we design to keep the two currents close together, to reduce radiation. Some discussions will ignore the current distribution and instead, consider derived quantities, such as fields in an aperture or magnetic currents in a slot or around the edges of a microstrip patch. You will discover that we use any concept that provides insight or simplifies the mathematics. An antenna converts bound circuit fields into propagating electromagnetic waves and, by reciprocity, collects power from passing electromagnetic waves. Maxwells equations predict that any time-varying electric or magnetic field produces the opposite field and forms an electromagnetic wave. The wave has its two fields oriented orthogonally, and it propagates in the direction normal to the plane defined by the perpendicular electric and magnetic fields. The electric field, the magnetic field, and the direction of propagation form a right-handed coordinate system. The propagating wave field intensity decreases by 1/R away from the source, whereas a static field drops off by 1/. Any circuit with time-varying fields has the capability of radiating to some extent. We consider only time-harmonic fields and use phasor notation with time dependence . An outward-propagating wave is given by , where k, the wave number, is given by 2/. is the wavelength of the wave given by c/f , where c is the velocity of light (3 × m/s in free space) and f is the frequency. Increasing the distance from the source decreases the phase of the wave. Consider a two-wire transmission line with fields bound to it. The currents on a single wire will radiate, but as long as the ground return path is near, its radiation will nearly cancel the other lines radiation because the two are 180°out of phase and the waves travel about the same distance. As the lines become farther and farther apart, in terms of wavelengths, the fields produced by the two currents will no longer cancel in all directions. In some directions the phase delay is different for radiation from the current on each line, and power escapes from the line. We keep circuits from radiating by providing close ground returns. Hence, high-speed logic requires ground planes to reduce radiation and its unwanted crosstalk. 2.1 ANTENNA RADIATIONAntennas radiate spherical waves that propagate in the radial direction for a coordinate system centered on the antenna. At large distances, spherical waves can be approximated by plane waves. Plane waves are useful because they simplify the problem. They are not physical, however, because they require infinite power. The Poynting vector describes both the direction of propagation and the power density of the electromagnetic wave. It is found from the vector cross product of the electric and magnetic fields and is denoted S: S = E ×H* W/ Root mean square (RMS) values are used to express the magnitude of the fields. H* is the complex conjugate of the magnetic field phasor. The magnetic field is proportional to the electric field in the far field. The constant of proportion is , the impedance of free space ( = 376.73): W/ (1.1) Because the Poynting vector is the vector product of the two fields, it is orthogonal to both fields and the triplet defines a right-handed coordinate system: (E, H, S). Consider a pair of concentric spheres centered on the antenna. The fields around the antenna decrease as 1/R, 1/, 1/, and so on. Constant-order terms would require that the power radiated grow with distance and power would not be conserved. For field terms proportional to 1/, 1/, and higher, the power density decreases with distance faster than the area increases. The energy on the inner sphere is larger than that on the outer sphere. The energies are not radiated but are instead concentrated around the antenna; they are near-field terms. Only the 1/ term of the Poynting vector (1/R field terms) represents radiated power because the sphere area grows as and gives a constant product. All the radiated power flowing through the inner sphere will propagate to the outer sphere. The sign of the input reactance depends on the near-field predominance of field type: electric (capacitive) or magnetic (inductive). At resonance (zero reactance) the stored energies due to the near fields are equal. Increasing the stored fields increases the circuit Q and narrows the impedance bandwidth. Far from the antenna we consider only the radiated fields and power density. The power flow is the same through concentric spheres: The average power density is proportional to 1/. Consider differential areas on the two spheres at the same coordinate angles. The antenna radiates only in the radial direction; therefore, no power may travel in the or direction. Power travels in flux tubes between areas, and it follows that not only the average Poynting vector but also every part of the power density is proportional to 1/: Since in a radiated wave S is proportional to 1/, E is proportional to 1/R. It is convenient to define radiation intensity to remove the 1/ dependence: U(, ) = S(R, , ) W/solid angle Radiation intensity depends only on the direction of radiation and remains the same at all distances. A probe antenna measures the relative radiation intensity (pattern) by moving in a circle (constant R) around the antenna. Often, of course, the antenna rotates and the probe is stationary. Some patterns have established names. Patterns along constant angles of the spherical coordinates are called either conical (constant ) or great circle (constant ). The great circle cuts when = 0 °or = 90°are the principal plane patterns. Other named cuts are also used, but their names depend on the particular measurement positioner, and it is necessary to annotate these patterns carefully to avoid confusion between people measuring patterns on different positioners. Patterns are measured by using three scales: (1) linear (power), (2) square root (field intensity), and (3) decibels (dB). The dB scale is used the most because it reveals more of the low-level responses (sidelobes). Figure 1.1 demonstrates many characteristics of patterns. The half-power beamwidth is sometimes called just the beamwidth. The tenth-power and null beamwidths are used in some applications. This pattern comes from a parabolic reflector whose feed is moved off the axis. The vestigial lobe occurs when the first sidelobe becomes joined to the main beam and forms a shoulder. For a feed located on the axis of the parabola, the first sidelobes are equal. 2.2 GAINGain is a measure of the ability of the antenna to direct the input power into radiation in a particular direction and is measured at the peak radiation intensity. Consider the power density radiated by an isotropic antenna with input power Po at a distance R: S = Po/4. An isotropic antenna radiates equally in all directions, and its radiated power density S is found by dividing the radiated power by the area of the sphere 4. The isotropic radiator is considered to be 100% efficient. The gain of an actual antenna increases the power density in the direction of the peak radiation: or (1.2)Gain is achieved by directing the radiation away from other parts of the radiation sphere. In general, gain is defined as the gain-biased pattern of the antenna: power density radiation intensity (1.3) FIGURE 1.1 Antenna pattern characteristics. The surface integral of the radiation intensity over the radiation sphere divided by the input power Po is a measure of the relative power radiated by the antenna, or the antenna efficiency: efficiency where Pr is the radiated power. Material losses in the antenna or reflected power due to poor impedance match reduce the radiated power. In this book, integrals in the equation above and those that follow express concepts more than operations we perform during design. Only for theoretical simplifications of the real world can we find closed-form solutions that would call for actual integration. We solve most integrals by using numerical methods that involve breaking the integrand into small segments and performing a weighted sum. However, it is helpful that integrals using measured values reduce the random errors by averaging, which improves the result. In a system the transmitter output impedance or the receiver input impedance may not match the antenna input impedance. Peak gain occurs for a receiver impedance conjugate matched to the antenna, which means that the resistive parts are the same and the reactive parts are the same magnitude but have opposite signs. Precision gain measurements require a tuner between the antenna and receiver to conjugate-match the two. Alternatively, the mismatch loss must be removed by calculation after the measurement. Either the effect of mismatches is considered separately for a given system, or the antennas are measured into the system impedance and mismatch loss is considered to be part of the efficiency. Example Compute the peak power density at 10 km of an antenna with an input power of 3 W and a gain of 15 dB. First convert dB gain to a ratio: G = = 31.62. The power spreads over the sphere area with radius 10 km or an area of 4 . The power density is We calculate the electric field intensity using Eq. (1-2): Although gain is usually relative to an isotropic antenna, some antenna gains are referred to a /2 dipole with an isotropic gain of 2.14 dB. If we approximate the antenna as a point source, we compute the electric field radiated by using Eq. (1.2): (1.4) This requires only that the antenna be small compared to the radial distance R. Equation (1.4) ignores the direction of the electric field, which we define as polarization. The units of the electric field are volts/meter. We determine the far-field pattern by multiplying Eq. (1.4) by R and removing the phase term since phase has meaning only when referred to another point in the far field. The far-field electric field unit is volts: or (1.5) During analysis, we often normalize input power to 1 W and can compute gain easily from the electric field by multiplying by a constant = 0.1826374. 1.3 EFFECTIVE AREAAntennas capture power from passing waves and deliver some of it to the terminals. Given the power density of the incident wave and the effective area of the antenna, the power delivered to the terminals is the product. (1.6) For an aperture antenna such as a horn, parabolic reflector, or flat-plate array, effective area is physical area multiplied by aperture efficiency. In general, losses due to material, distribution, and mismatch reduce the ratio of the effective area to the physical area. Typical estimated aperture efficiency for a parabolic reflector is 55%. Even antennas with infinitesimal physical areas, such as dipoles, have effective areas because they remove power from passing waves. 2.4 PATH LOSSWe combine the gain of the transmitting antenna with the effective area of the receiving antenna to determine delivered power and path loss. The power density at the receiving antenna is given by Eq. (1.3), and the received power is given by Eq. (1.6). By combining the two, we obtain the path loss: Antenna 1 transmits, and antenna 2 receives. If the materials in the antennas are linear and isotropic, the transmitting and receiving patterns are identical (reciprocal) 2, p. 116. When we consider antenna 2 as the transmitting antenna and antenna 1 as the receiving antenna, the path loss is Since the responses are reciprocal, the path losses are equal and we can gather and eliminate terms: = constant Because the antennas were arbitrary, this quotient must equal a constant. This constant was found by considering the radiation between two large apertures 3: (1.7) We substitute this equation into path loss to express it in terms of the gains or effective areas: (1.8) We make quick evaluations of path loss for various units of distance R and for frequency f in megahertz using the formula. path loss(dB)= (1.9) where Ku depends on the length units: Example Compute the gain of a 3-m-diameter parabolic reflector at 4 GHz assuming 55% aperture efficiency. Gain is related to effective area by Eq. (1.7): We calculate the area of a circular aperture by. By combining these equations, we have (1.10) where D is the diameter and is the aperture efficiency. On substituting the values above, we obtain the gain: (39.4dB)Example Calculate the path loss of a 50-km communication link at 2.2 GHz using a transmitter antenna with a gain of 25 dB and a receiver antenna with