Antenna Fundamentals Basics antenna theory and concepts

Transcription

1 Antenna Fundamentals Basics antenna theory and concepts M. Haridim Brno University of Technology, Brno February

2 Topics What is antenna Antenna types Antenna parameters: radiation pattern, directivity, efficiency, gain, radiation resistance, bandwidth, beamwidth Antenna array Dipole Monopole Loop antennas 2

3 What is antenna? A passive device acting as transition between some form of transmission line and the space. Signals from an RF transmitter are delivered via a transmission line to the antenna, which launches them as electromagnetic waves in the space, or vice versa. The shape and size of the current carrying structure determine how much energy is radiated, direction of propagation and how well the radiation is captured. Reciprocity property: The same antenna can be used in both transmitting and receiving modes. 3

4 Antenna types Antennas can be of either wire or aperture types. Wire type: dipole, monopole, loop, helix, Yagi-Uda. Aperture type: horn, reflector. 4

5 Common types of antennas: -1 (p. 389) ingle-element antennas. 5

6 Radiation intensity I(, ) [W/str] is the average power radiated per unit solid angle in direction (, ). Integration of I over the whole space (4 Str) gives the total radiated power. Power density [w/m 2 ] is the time-average of the Poynting vector 6

7 Near/far field regions Radiating Far- Field Region ( Fraunhofer Zone) Radiating Near- Field Region (Fresnel Zone) Reactive Near- Field Region ( Rayleigh Zone) infinity 7

8 Near/far field regions The boundary of near/far field zones is commonly accepted as r 2 2D D is the largest dimension on the antenna element, and assumed D>λ.

9 Wire Antenna Near/Far Field Spatial Distribution To feature the spatial dependence of the field intensity, we substitute all antenna and media parameters with constants H ja 1 a 1 r r E b jb b jb r r r r r 1 1 E jc1 c2 jc 2 3 r 3 r r Radiating Antenna + I ant E rad H rad F I E L D Z O N E S I II III Near-Field Reactive Near-Field Radiating Far-Field radiating 9

10 Near/Far Field of Aperture Antennas The fictional HARD field limits between the near and far field zones 10

11 Radiation pattern 2D vs. 3D E-plane (E and boresight direction) and H-plane Main, side and back lobes Isotropic Radiation Pattern, constant in both azimuth and elevation planes. Omni Directional Radiation Pattern. Directional Radiation Pattern- contains one main beam (lobe) in both azimuth and elevation planes. ( a ) 11

12 Radiation patters shown in polar plots

13 Directivity D(, ) An antenna s directivity is determined by the directionality of its radiation pattern. It is defined as the ratio between its radiation intensity and that of an isotropic antenna, radiating the same total power. Proportional to the radiation intensity. Max. directivity D max is the Max. value of D(, ). Not related to matching level 13

14 Efficiency The average power delivered to the antenna s input is P inc = P rad + P loss Antenna s efficiency is defined as Prad P rad P P P inc rad loss Loss mechanisms: dielectric losses, conduction losses, and reflection (mismatch) losses 14

15 Gain G(, ) Antenna s directivity has no information about the antenna s efficiency. The gain of an antenna is a function of both directivity and efficiency. The max gain occurs when the directivity is max. It s expressed in dbi, with respect to an isotropic antenna, or in dbd. 15

16 Antenna's Input Impedance and Radiation Resistance a Ra jxa Ra Rrad Rloss The real part of the antenna impedance consists of radiation resistance R rad and a dissipative resistance R loss (ohmic and dielectric losses). R rad at represents the useful part of the input power. In order to avoid reflection of the incident power, the antenna input impedance must be equal to the line's characteristic impedance, for the whole operation bandwidth. The mismatch is measured by the Voltage Standing Wave Ratio- VSWR. 16

17 Example A /2 dipole with a total input impedance of 75 +j40 and loss resistance 2 is fed by a power amplifier whose output voltage and impedance are 5cos 0 t and 50+j30, respectively. Find the power delivered to antenna, the radiated power and the amount of power dissipated by the antenna (ohmic losses). Find the power delivered to antenna, the radiated power and the amount of power dissipated by the antenna (ohmic losses). 17

18 Solution First, we find the current (phasor) in the equivalent circuit I V 5 Z Z R 50 j30 75 j40 s ant loss o I ma The power delivered from the power amplifier is 1 * 1 p Re{ v in s I } Re{.. }. mw 2 2 The radiated power rad prad I R (. ). mw 2 2 And the dissipated power 2 1 P 1 diss I R loss.. mw 2 18

19 The Effective Area of a Receiving Antenna The antenna's gain applies to both transmitting and receiving modes. However, it is more convenient to characterize a receiving antenna by its effective area that is given by the ratio between the power flux incident on the antenna and the power delivered to a matched load. It can be shown that the following relationship between antenna's gain and effective area prevails: G 4 Ae 2 19

20 Beam-width The are some zeros and nulls in radiation pattern, indicating no radiations. A directional antenna directs radiation in one or more directions. The width of this beam is defined as the angle between its half-power points. A half-wave dipole has beam-widths of about 78º and 360º in horizontal and vertical directions. The directivity of an antenna increases as its beamwidth is made smaller, as the energy radiated is concentrated into a smaller solid angle 20

21 Polarization The polarization of EM waves is determined by the direction of the electric field, and that of an antenna is defined as the polarization of its far field.. A dipole antenna, for example, produces a linearly polarized wave aligned with its axis. It picks-up maximum power energy in co-pol (co-polarization) operation, i.e. when its axis is aligned to the polarization of the impinging waves. In the cross-pol operation, on the other hand, it receives no radiation, as its axis is orthogonal to the polarization of the incident waves. The cross-pol level (in db) of an antenna denotes the ratio between the maximum received power in the co-pol and cross-pol operations. 21

22 Antenna arrays Antenna arrays are formed by an assembly of identical (in most cases) radiating elements, arranged in either a one-dimensional (1D) or twodimensional (2D) structure. The design of a single antenna, such as a resonance wire antenna, allows for limited control of its properties, especially its gain and radiation pattern. Greater design flexibility as well as improved performance can be achieved by arranging multiple spatially separated antennas that are separately driven, at the cost of increased size and complexity. Antenna arrays are used for increasing the antenna gain, combatting interference from certain directions, steering the boresight direction, and direction finding. 22

23 Dipole Dipole is the simplest antenna consisting of two straight pieces of wire or metal rods, fed at the center. The length of the dipole arms determines its characteristics, e.g. impedance, operating frequency, etc. 23

24 The dipole antenna radiates energy in all directions perpendicular to its axis, and no radiation along its axis. Thus, dipole has some directionality. In the case of a Hertzian dipole it is given by D(, ) 15. sin 2 So the maximum directivity is D 15. max 24

25 For a half-wave dipole the directivity is slightly higher, that is D max =1.64, its HPBW is 78 degree and its radiation resistance is R rad

26 . Elementary dipole An elementary dipole or Hertzian dipole, is a wire segment whose length L is very small compared to wavelength Assuming a vertical Hertzian dipole placed at the origin and excited by a sinusoidal current I I0 sint jr I0Le 0 r E Hr H 0 E j sin 4 r Where E 0 H In free space is the intrinsic impedance of the medium

27 L I0 2 P av 05. sin ( 4 r ) L I0 2 I(, ) sin

28 Current distribution If the wire diameter d << λ, the wave pattern along the dipole arms is sinusoidal with a null at the end. For center-fed dipoles: L << L = /2 No phase reversal No phase reversal 28

29 No phase reversal Phase reversal /2 < L < < L < 3/2 29

30 Monopole Monopole are unbalanced antenna, and hence they can be fed with an unbalanced feed lines such as coaxial cables. Like dipoles, monopoles are omnidirectional, but in contrast to dipoles that are balanced antenna, monopoles are, 30

31 Whip Whip antenna is the most common example of a monopole radio antenna, almost always vertically mounted onto a base vehicle, resulting in vertical polarization. Whip antennas are easy to install and operate, but suffer low efficiency when they lack a stable ground. The length of the whip determines its operating wavelength. It is possible to shorten the whip by inserting a coil anywhere along the antenna. 31

32 Loop antenna Loop is a very versatile antenna. It has different forms: circle, rectangle, square, triangle, ellipse, etc. A small loops is equivalent to an infinitesimal dipole, whose axis is perpendicular to the loop s plane. Loop is used as field probes at both low and high frequencies. 32

33 Loop antenna Electrically small loop antennas are radiation resistances lower than their loss resistances, hence they have poor performance. The radiation pattern of electrically small loops is similar to that of an infinitesimal dipole. It has a null perpendicular to the loop s plane and its maximum is along the plane of the loop. As the loops perimeter increases and approaches, the maximum of the pattern shifts from the loop s plane to the axis of the loop which is perpendicular to its plane. 33