Antenna Fundamentals Antenna Theory and Measurements with the Model 8092 System
FEATURES • Low cost complete antenna measurement system. • No anechoic chamber required to obtain good results. • Covers several antenna technologies. • 1GHz and 10GHz operation. • Usable from 1GHz to 30GHz with separate generators. • Software can be used in stand-alone mode. • Unique options: phasing system, Rotman lens based multi-beam array antenna.
Background • Maxwell (1831-79) Fundamental equations. (Scottish) • Hertz (1857-94) First aerial propagation (German) • Marconi (1874-1937) Transatlantic transmission (Italian) • DeForest (Triode tube 1920) Signal generators (American) • World War II (1939-45) Intense war-driven development
Definitions • An antenna is a device for radiating or receiving radio waves. Antennas act as the transition between waveguides or transmission lines and free space.
Definitions • An isotropic source is a hypothetical antenna which is non(all)directional, that is, which has equal radiation intensity in all directions • The dipole antenna is a simple type of antenna consisting of two rods or wires. The length of this antenna is L. The dipole is connected at the center to the transmitter through a transmission line.
The Dipole Antenna Note balanced transmission line
Definitions • The current distribution, that is the magnitude of the alternating current along the length of a dipole antenna, is not necessarily uniform. Instead, it is zero at the ends, and may be highest at the center or at other points.
Definitions • An ideal dipole is another hypothetical antenna which is useful in the study of antennas. It can be considered to be a dipole of infinitesimal length with a uniform current distribution. The theoretical characteristics of an ideal dipole approximate those of electrically small dipole antennas.
Definitions • A radiation pattern is a three-dimensional, graphical representation of the far-field radiation properties of an antenna as a function of space coordinates. The far-field region is a region far enough for the radiation pattern to be independent of the distance from the antenna. The radiation pattern of a particular antenna can be measured by experiment or can be calculated, if the current distribution is known.
Definitions • Although the term "radiation" pattern is used, it applies just as well to receiving antennas. The reception pattern of an antenna is identical to its radiation (transmission) pattern. This is a general rule, known as the reciprocity theorem.
Definitions • Although the complete radiation pattern is a three-dimensional function, a pair of two-dimensional patterns are usually sufficient to characterize the directional properties of an antenna. In most cases, the two radiation patterns are measured in planes which are perpendicular to each other. A plane parallel to the electric field is chosen as one plane and the plane parallel to the magnetic field as the other. The two planes are called the E-plane and the H-plane, respectively.
Definitions • The half-power beamwidth (HPBW) of an antenna is the angular separation of the points in the main beam where the power equals one-half (-3 dB) the power radiated in the direction of maximum power
Theoretical E-plane Radiation Pattern of an Ideal Dipole Showing the Half-power Beamwidth
Radiation Pattern Characteristics • 3 dB beamwidth • Sidelobes • Nulls • Front-to-back ratio • Gain (approximate) • Maximum signal position
Far Field Conditions • How far is far enough? • If D >2.5λ then • If D<λ/3 then • If λ/3 < D < 2.5λ then rff>5D
Antenna Input Impedance • Input Impedance (resistance + reactance) • Radiation Resistance (corresponds to energy that is transmitted) • Loss Resistance
Antenna Input Impedance Input resistance (red line) and reactance (green line) of a dipole antenna as a function of antenna length
Baluns • BALanced to UNbalanced device • Similar to transformers • Used at RF • Usually band-limited • Improve matching and prevent unwanted currents on coaxial cable shields
Baluns Balun for connecting a center-fed dipole to a coaxial cable
Baluns As Impedance Transformers Transition from a 50Ώ coaxial cable to a 300 Ώ half-wave folded dipole through a four-to-one impedance transformation balun
Yagi (Parasitic) Antennas • Remember the near-field equations • The near-fields can be used to induce currents in adjacent antenna elements • With the proper delay (spacing and lengths), phases can be obtained that will add in certain directions and cancel in others • Reflector elements and director elements are thus used to give directivity to an antenna
Graph of gain versus the total number of elements of a typical Yagi antenna
Gain of a Yagi antenna for different numbers of elements (spacing = 0.5 λ)
Other Antennas • Other dipoles • Monopole • Helical • Loop (electrically small & large)
Image Theory (a) Dipole over a perfectly conducting plane (b) Equivalent model with image theory
Helical Antenna (d~λ) Axial-mode helical antenna : (a) geometry, (b) pencil-beam radiation pattern
Helical Antenna (d<<λ) Normal-mode helical antenna: (a) geometry, (b) radiation pattern
Patch Antenna Microstrip patch with microstrip transmission-line feed
Overview of Lab-Volt Antenna Measurement System • Complete system to build antennas and to measure antenna patterns • 2 ranges of operation (1 GHz and 10GHz) • With separate generator, can be used from 1GHz to over 30GHz • Contains wire antennas & aperture antennas
Wire Antennas versus Aperture Antennas • Wire antennas are usually operated at frequencies where the wavelength is larger than or approximately equal to the antenna length. Ex: dipoles, monopoles, folded dipoles…(λ≥L) • Aperture antennas are usually operated at frequencies where the wavelength is smaller than the antenna dimension. Ex: Horn antennas, parabolic reflectors, patch antennas…(λ≤L)