Analysis of Antenna Beam-tilt and Broadcast Coverage
Myron D. Fanton, PE Electronics Research, Inc.
Abstract—Antenna beam tilt affects areas covered by strong signals. The energy density, S, at a distance, R, from the antenna is given Improvements greater than 10dB are realized using greater beam- by the following equation [2], tilts offered in end-fed arrays. P Index Terms—Antenna Arrays, Antenna Beam Tilt, Coverage, S = (1), Propagation 4πR 2
I. INTRODUCTION where P is the ERP of the antenna at the elevation angle of interest. overage is the key parameter in broadcast system engineering. A great deal of effort is exerted to position the C 0.00 antenna and select antenna patterns to enhance the signal at the 1.5 Deg. Beam Tilt receiver. Many analyses of propagation to date have not 0.5 Deg. Beam Tilt accounted for antenna beam-tilt, though now technology, 0 Deg. Beam Tilt products, and analytical techniques [1] can optimize the coverage with increased beam-tilt. -5.00
II. SMOOTH EARTH PROPAGATION
Amplitude on Ground (dB) Associating the elevation pattern of an antenna with the -10.00 distance from the transmitter site provides an understanding of the key areas of the pattern. A mapping of elevation angle and distance from the transmitter is shown in Figure 1. The Standard (4/3) Radio Horizon is also noted on the plot. -15.00 For typical tower heights, the radio horizon occurs at 90 80 70 60 50 40 30 20 10 0 Elevation Angle (Degrees) elevation angles between 0.1 and 1 degrees and distances of 10 to 100 miles. The distance from 1 to 10 miles from the Figure 2a: Energy Density Elevation Angle Scale transmitter site corresponds to elevation angle between 1 and 10 degrees, and the region less than one mile from the tower is 0.00 greater than 10 degrees in elevation angle.
Placing the antenna peak of beam on the radio horizon -5.00 does not ensure optimum coverage on the surface of the earth. The energy transmitted from an antenna propagates as a spherical wave away from the phase center. -10.00
100 -15.00 Ht=10000 ft Ht=6400 ft Amplitude on Ground (dB) Ht=4000 ft Ht=2500 ft 1.5 Deg. Beam Tilt Ht=1600 ft 0.5 Deg. Beam Tilt Ht=1000 ft -20.00 Ht=650 ft 0 Deg. Beam Tilt Ht=400 ft 10 s Ht=250 ft Ht=150 ft Ht=100 ft -25.00 Radio Horizon 0.01 0.1 1 10 100 Distance From Transmitter (mi) Figure 2b: Energy Density Log Distance Scale Elevation Angle (Degree 1 The antenna pattern must overcome the inverse-square nature of path loss. Tilting the beam below the radio horizon is an effective method of improving coverage [3]. Figure 2 shows
0.1 the normalized energy density at the surface of the earth for an 0.01 0.1 1 10 100 1000 Distance From Transmitter (mi) antenna 1000ft above the surface. The radio horizon for this case is 44mi or 0.5 deg. elevation angle. With no tilting of the Figure 1: Elevation Angle and Distance From Transmitter antenna beam, the signal energy density at the 10mi radius is
ERI Technical Series, Vol. 6, April 2006 7 more than 10dB below the energy near the transmitter. By tilting the beam 1.5 degrees, the energy is balanced between this 10mi region and the tower site, an improvement of 10dB.
0.00
1.5 Deg. Beam Tilt
0.5 Deg. Beam Tilt
-5.00 0 Deg. Beam Tilt
-10.00
-15.00 Amplitude on Ground (dB)
-20.00
-25.00 Figure 3b: Energy Density Map: 0.5 Deg. Beam Tilt 0 5 10 15 20 25 30 35 40 45 50 Distance From Transmitter (mi) Figure 2c: Energy Density Linear Distance Scale
Note that from Figure 2b this coverage improvement includes the entire area. Figure 2c gives an indication that areas significant signal change occur at about 10% of the distance from the transmitter to the radio horizon, while looking on a linear-scale map.
III. SPECIFIC PROPAGATION STUDIES
Numerical models of propagation with greater sophistication exist, accounting for terrain and population. Figure 3 shows graphic results using the Longley-Rice propagation model to predict signal strength from a 1000ft tower surrounding Chandler, Indiana. Increasing the beam tilt from 0 to 0.5 degrees significantly increases the area of signal strength from 5 to 10 mi Figure 3c: Energy Density Map: 1.0 Deg. Beam Tilt from the transmitter site. Note the increase in diameter of the central, yellow graduation.
Figure 3d: Energy Density Map: 1.5 Deg. Beam Tilt
Figure 3a: Energy Density Map: 0.0 Deg. Beam Tilt With additional increase in beam tilt, an area of increased signal is introduced within the 10mi area. Note the white ring within
ERI Technical Series, Vol. 6, April 2006 8 the yellow graduation in Figures 3c and 3d. With no shows the patterns of a center-fed array with the array pattern of degradation of signal to the outer limits of the coverage area, a the two sub-arrays and explains the existence of the large upper large area of viewers is delivered 10dB greater signal strength. side-lobes.
Note that the coverage maps of Figure 3 have an outer radius of 1 60mi and the color graduations are 10dB. 0.9
IV. ANTENNA LIMITATIONS 0.8
0.7 End-fed broadcast antennas have no practical limitation on the beam tilt. The slot radiators may be freely positioned in 0.6 the array to form a pattern that has optimal beam-tilts, high side- 0.5 lobes and high null-fill. The patterns generated by end-fed 0.4 arrays have low differential amplitude and phase characteristics,
Amplitude (Relative Field) (Relative Amplitude 0.3 important for digital modulation [4]. Center-fed arrays have patterns will low side-lobes and low null fill, and they have a 0.2 troublesome limitation on beam-tilt. The amplitude and phase 0.1 patterns of the two antennas are shown in Figure 4. 0 -20-18-16-14-12-10-8-6-4-202468101214161820 1 Elevation Angle
0.9 Figure 5: Center-Fed Patterns with Interference Pattern
0.8 V. CONCLUSIONS 0.7
0.6 Propagation analysis using smooth-earth and Longley- Rice models has shown that significant improvements in 0.5 coverage may be obtained by simply increasing the antenna 0.4 beam tilt. Gains greater than 10dB are realized when the beam
Amplitude (Relative Field) (Relative Amplitude 0.3 is tilted below the radio horizon. The greatest gains occur in the
0.2 region between 1 and 10 miles, a key area in metropolitan areas where interference and low-gain receive antennas abound. The 0.1 beam-tilt constraints on center-fed antennas make end-fed arrays 0 more attractive for realizing the coverage benefits of increased -20-18-16-14-12-10-8-6-4-202468101214161820 Elevation Angle beam tilt.
Figure 4a: End and Center-Fed Amplitude Patterns REFERENCES
180 [1] Smith, S.A., System and Method for Determining Optimal 150 Broadcast Area of an Antenna, US Patent No. 7006038, 28 Feb 120 2006. 90 [2] C.A. Balanis, Antenna Theory – Analysis and Design, Harper
60 & Row, New York, 1982. th 30 [3] Wittaker, Jerry, ed., NAB Engineering Handbook, 9 Center-Fed Edition, Washingtion, DC, 1999 0 End-Fed -30 [4] G.W. Collins, Fundamentals of Digital Television Phase (Degrees) Phase -60 Transmission, John Wiley & Sons, Inc., New York, 2001. -90 -120 For More Information Contact: -150 -180 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 [email protected] Elevation Angle [email protected]
Figure 4b: End and Center-Fed Phase Patterns www.eriinc.com
The beam-tilt in a center-fed antenna arises chiefly Electronics Research, Inc. from an offset in the feed-point. This causes a step function in 7777 Gardner Road phase as well as an increase in the spacing between the center- Chandler, IN 47610-9219 most bays. The center-fed array behaves like two small arrays USA with distinct phase-centers, separated by a large distance and interfering with each other. When the phase shift is increased to +1 812 925-6000 (tel) generate beam-tilts greater than 0.5 degrees, a large upper side- +1 812 925-4030 (fax) lobe is generated, and the gain of the array is degraded. Figure 5
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