Base Station Antennas We Mean That Each Element Is Fed in Phase
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ENGINEERING ANTENNAS ments is the half-wavelength dipole. The element pattern for a half-wave dipole — obtained from Equation 2 Making gains in the — when rotated around a vertical axis forms the “doughnut” pattern familiar to most readers. The array vertical plane factor for a uniformly excited linear array can be obtained from Equa- Some theory and a summary of recent advances tions 3 and 4. By uniformly excited, in base station antennas we mean that each element is fed in phase. Sometimes it is desirable By Jay M. Jacobsmeyer, P.E. to alter the element phase to create null fill or electrical beam tilt at the ase station (i.e., repeater site) antennas tend to expense of some loss in gain. be vertical linear arrays of dipole or similar ele- An example of the use of Equa- Bments. A vertical array is preferred because the tions 1 through 4 is shown in Figure array creates gain in the vertical plane, thereby focus- 1, where we have plotted the ele- ing energy toward the horizon where it is needed. If a ment, array and antenna patterns directional antenna is required, a combination of a ver- for an eight-element linear array of tical array and a reflector is used to focus energy into vertical half-wave dipoles. a narrow sector, typically 65, 90 or 120 degrees wide at For directional antennas, the the 3 dB points of the azimuth pattern. In this article, azimuth pattern is as important we will examine some of the characteristics of antenna as the vertical pattern. One of the arrays for base-station applications and review some important characteristics of the recent technology advances, including smart antennas azimuth pattern is the front-to- and multiple-input, multiple-output (MIMO) antenna back ratio, which is the difference arrays. Let’s start with antenna patterns. (in dB) between the antenna gain Antenna patterns of vertical arrays. In the far field, in the main antenna lobe and the the pattern factor, f(φ), is the product of the element pat- gain directly behind the antenna. tern factor, fe(φ), and the array pattern factor, fa(φ). See Front-to-back ratio is important for Equation 1. One of the most widely used antenna ele- co-channel interference rejection and mitigation of time delay inter- FIGURE 1 PATTERN MULTIPLICATION ference (TDI) in simulcast networks. FOR LINEAR ARRAYS See Figure 2 for the azimuth pattern Eight-element linear array of λ/2 vertical dipoles, λ element spacing of a typical panel antenna. 1.0 Gain. The two-dimensional antenna patterns in the previous sec- 0.9 tion give some clues to the antenna 0.8 gain, but gain is measured in three 0.7 dimensions and is not found directly from the antenna pattern. Antenna 0.6 gain is defined in Equation 5. 0.5 In general, antenna gain is dif- Element Pattern 0.4 Array Pattern ficult to calculate except for the Far Field Pattern simplest antenna elements and Relative field 0.3 arrays. For the special case of 0.2 a uniformly fed linear array of 0.1 half-wave dipoles spaced one wave- length apart, the gain of the array 0 10 20 30 40 50 60 70 80 90 is equal to N times the gain of a Angle below horizontal, degrees dipole, where N is the number of 32 URGENT COMMUNICATIONS MARCH 10 ENGINEERING ANTENNAS elements in the array. For example, an 8-el- EQUATION 1 PATTERN MULTIPLICATION ement array has a gain of 8*1.64 = 13.1, or 11.2 dBi. In land mobile radio, it is customary f (φ) = fe (φ) fa (φ) to specify gain relative to a half-wave dipole. In this case, the gain is simply equal to the number of dipole elements, which in this EQUATION 2 ELEMENT PATTERN FOR A example is 8 or 9 dBd. HALF-WAVE DIPOLE Decreasing the element spacing to less π than one wavelength sometimes is desirable, cos sin φ because it creates favorable changes in the ( 2 ) fe (φ) = pattern side lobes, but the gain always will cos (φ) be less (at least for dipole elements). In other words, the number of array elements alone EQUATIONS 3 & 4 PATTERN FOR UNIFORMLY does not determine the gain; the length of the EXCITED ARRAY array is equally important. sin(N ψ/2) Beamwidth. The horizontal and verti- fa (φ) = cal beamwidth (measured at the 3 dB pattern N sin(ψ/2) points) are major determinants of how well the antenna will cover the service area. Gain and Where N is the number of array elements and ψ is given by beamwidth are tightly coupled; the higher the π -φ gain, the narrower the beamwidth. For tall sites, 2πl cos 2 ψ = ( ) high-gain omnidirectional antennas rarely are λ advisable because narrow beamwidth means the antenna will overshoot the service area. Where l is the element spacing and λ is the carrier wavelength. Beam tilt. Beam tilt comes in two flavors: electrical and mechanical. Electrical beam tilt AZIMUTH PATTERN FOR is created by altering the phase between the FIGURE 2 A PANEL ANTENNA upper half and lower half of the antenna array. 65-degree beamwidth and 35 dB front-to-back ratio Electrical beam tilt creates the same tilt in all directions. Mechanical beam tilt, on the other hand, varies from a maximum value in the tilt direction to 0° at +/– 90° azimuth to maximum uptilt behind the antenna. The expression (without derivation) for the mechanical beam tilt, γ, at an arbitrary angle, α, from the center of the beam when the beam tilt at center is θ degrees is obtained from Equation 6. For example, if the tilt at the main lobe is θ = –3°, the beam tilt at the edge of beam for a 120-degree panel antenna (at α = +/– 60°) is –1.5°. Total beam tilt is simply the sum of elec- trical beam tilt and mechanical tilt. Figure 3 depicts a plot of the combined beam tilt as a function of azimuth angle, α, for an antenna with –2° electrical tilt and –1.5° mechanical tilt in the direction of the main pattern lobe. Many antennas are ordered with no beam tilt and zero beam tilt is probably harm- less when the vertical beamwidth is greater than 15 degrees, but high-gain antennas (> 6 dBd) almost always should have some 34 URGENT COMMUNICATIONS MARCH 10 The Professional Tool for Radio Frequency Interference Analysis Identify and Prevent RF Interference downward beam tilt because radio energy directed above the horizon is wasted. Creative combinations of electrical beam tilt and mechanical beam tilt sometimes are used to limit co-channel interference in a direction away from the service area (in front . Maximize Inbound Radio Coverage of the antenna) while simultaneously providing good coverage in the ser- . Resolve Interference Issues Before They Affect System Capacity vice area (behind the antenna). Beam tilt also is a powerful tool in simulcast . Evaluate Non-Ionizing Radiation for MPE Compliance network design because it helps limit TDI within the network. Assess Rebanding Comparable Facilities Bandwidth. Antenna bandwidth Interference Objectives can be defined in several ways, but usually it is defined by a maxi- mum Voltage Standing Wave Ratio Sign up for a web seminar on (VSWR). Often, a maximum VSWR of “Solving RF Interference Issues” at www.rcc.com/csp 1.5 is used. Several antenna charac- teristics affect bandwidth, including RCC Consultants, Inc. 100 Woodbridge Center Dr. Woodbridge, NJ 07095 ph.800.247.4796 the type of element used and the www.rcc.com [email protected] size of the conductors. Generally speaking, the wider the conductor, the greater the bandwidth. When wide bandwidth is needed, log-pe- riodic elements sometimes are used rather than dipoles. The type of array feed also affects bandwidth. A series feed has nar- rower bandwidth because the phase difference between the center fre- quency and the edge of the bandwidth frequency increases proportionally to the number of elements. A branch feed, on the other hand, uses power dividers and equal length transmis- sion lines to each element, thereby limiting the phase error between the center frequency and the bandwidth edge. A typical antenna bandwidth is 12% of the center frequency. Polarization. In many land mobile radio bands, the FCC requires ver- tical polarization, so the licensee has no polarization option. Cellular phone operators, on the other hand, have no limits on polarization and 45-degree “slant” polarization is common, using two antenna arrays in one radome, with the antenna elements orthogonal to each other URGENTCOMM.COM 35 ENGINEERING ANTENNAS and at a 45-degree angle to the ing intermodulation a key concern. PIM performance over time. Good 7 vertical. One of the advantages Antenna manufacturers typically low-PIM designs employ /16 DIN of polarization diversity is that design for –150 dBc, 3rd-order PIM connectors, no coaxial cable har- it limits the number of required rejection with two, 20 W transmit- nesses, all-stripline construction, antennas, reducing the wind load ters applied at the connector. Many welding rather than fasteners, no on the tower and making zoning manufacturers claim to PIM test dissimilar metals and the use of approval more likely. For more on every antenna before it is shipped, mated materials with similar tem- antenna diversity, see the June 2006 but low-PIM design is more impor- perature-expansion coefficients. issue of Mobile Radio Technology. tant than factory tests. Low-PIM Recent advances. Smart antennas Passive Intermodulation Rejec- design is important because vibra- enhance coverage through range tion (PIM). Transmit antennas must tion, temperature variations, extension and hole-filling.