Electrical and Mechanical Downtilt and their Effects on Horizontal Pattern Performance
By Louis (Lou) J. Meyer, P.E. Director, Applications Engineering
Inter-sector interference has been a problem for wireless operators ever since RF engineers deployed the first sector array antennas. Radiation patterns that bleed outside the antenna's defined sector affect not only thequality of service in adjacent sectors within the same cell, but can disrupt service in adjacent cells as well. In an attempt to confine the signal to its specific sector, operators have employed a variety of techniques, including physically downtilting the antenna. Known as mechanical downtilt, this technique has been effective to some degree, but has also caused additional problems. Physicallydowntilting the antenna occurs along a single horizontal plane. As the front of the antenna is tilted down, the back is, by default, tilted up. This limitation creates a variety of radiation pattern irregularities, such as pattern blooming, that are a major source of inter-sector interference. Even still, mechanical downtilt has become an accepted practice. To help compensate for the inherentlimitations of mechanical downtilt, the industry has developed certain general guidelines. The introduction of electrically downtilted antennas gave network operators greater flexibility in tilting the antenna beam and manipulating the radiation pattern. The electrically downtilted antenna enables the operator to tilt the antenna pattern along an infinite number of angles, in effect, creating athree-dimensional “cone of coverage.” As a result, electrical downtilt allows the operator greater freedom in shaping the antenna's horizontal radiation pattern to minimize intersector interference and maximize quality of service within the specified sector. The use of electrically downtilted antennas has increased significantly since the technology was first introduced. RF engineers, however, continue toapply the same basic guidelines initially developed to help compensate for the limitations of mechanical downtilt antennas. Additionally, many operators have begun to use mechanical downtilt in tandem with electrical downtilt. While combining the two methods can be effective in very limited applications, data suggests that overall this practice leads to horizontal pattern deformations that canaltogether offset the benefits of electrical downtilt. This paper has been developed in order to demonstrate the horizontal pattern-shaping abilities of sector array antennas using electrical versus mechanical downtilt. Specifically, it illustrates how electrical downtilt can be used to minimize interference in the horizontal plane by systematically lowering gain — at boresite, 180° behind boresite,and at ±90° to boresite. Additionally, this paper seeks to quantify the negative effects of attempting to combine the two technologies. It also provides a revised and improved guideline for antennas using electric downtilt in order to help operators reduce horizontal pattern irregularities such as pattern blooming, beam squint and front-to-back ratios to acceptable levels.
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© 2010 CommScope, Inc. All rights reserved. Andrew Solutions is a trademark of CommScope. All trademarks identified by ® or ™ are registered trademarks or trademarks, respectively, of CommScope. This document is for planning purposes only and is not intended to modify or supplement anyspecifications or warranties relating to Andrew products or services. WP-103755-EN (6/10) • Page 1 of 15
SECTORIZATION AND INTRA-CELL INTERFERENCE
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