Broadcast Writer

By Victoria W. Kipp

Site Management & Technology, Aug 1, 2002      

The next time you spot a communications tower while driving down the road, take a moment to classify it. Towers can be identified by their means of structural support (guyed or self-supporting) and their shape (steel-lattice or monopole).

Guyed towers

Guyed towers are held in place by the competing tensions of high-strength braided or stranded steel guy cables. The cables are strategically placed on each side of the tower to anchor the tower to the earth. Typically tall and slender, guyed towers are supported by one or more levels of guyed cables. As a general rule, the wider the face of the tower, the stiffer the tower is. Thus, a tower with a wider cross section requires fewer guy levels than a tower with a narrower face. A large tower may require more guy tension and greater guy sizes.

Guy cables are anchored to the ground at a distance from the base that is roughly equivalent to 80% of tower height. For guy cable calculations, tower height is considered the distance between the ground and the bottom of the top-mounted antenna. If the guy anchor radius is shorter than 70 percent to 80 percent of tower height, then a greater guy wire size is required, and a greater stress is placed on the tower shaft and foundation by the guy anchors.

Guyed towers require much more land than self-supporting towers. When a new guyed tower site is chosen, the soil type must be appropriate for holding guy anchors. A study of the subsurface soil situation will reveal frost protection requirements and buoyancy effects of close water tables.

While land costs for guyed towers are higher than those for self-supporting towers, material costs are lower because guyed towers require less steel. The foundations for guyed towers generally cost less than foundations for self-supporting towers because the guyed tower foundation is smaller, using less concrete. Guyed towers tend to have a shorter construction period than self-supporting towers because they use less steel.

Anchors are another aspect of a guyed tower that should be considered. For maximum cost-effectiveness, towers as high as 500 feet or 600 feet can be constructed with just one anchor point per tower cross-section. However, tower designers can protect against the risk of tower collapse due to anchor failure by placing multiple anchors along each azimuth.

Guyed towers require more recurrent maintenance than self-supporting towers. For example, guy cables need to be examined for proper tension and to uncover corrosion. When the guy tension is set properly, tower twisting due to high winds will cause minimal tower deflection, and the tower will be supported properly.

Guyed towers are versatile. From less taxing applications (light-duty microwave, cellular and land mobile radio) to heavy functions (heavy cellular, medium-to-heavy microwave, and broadcast), they can handle a broad range of loading conditions.

Self-supported towers

A self-supported tower can be classified as either a steel-lattice structure or a cylindrical monopole. Typically, self-supported towers require much less land than guyed towers because they only require space for the base of the tower.

Self-supporting steel towers have three or four sides and form a lattice pyramid or box. Towers with triangular bases are usually favored over towers with square bases because they are lighter and less expensive to assemble. Triangular towers are ideal for lightweight applications such as mobile two-way radio and cellular, and situations where land space is limited and costly.

Monopoles, (single-pole, self-supporting towers) can be either a tubular design or a multisided tapered pole. The monopole pillar can be smooth or a polygon shape with multiple faces.

Ranging from 75 feet to 150 feet high, monopoles require far less land than other tower types. The cost of materials and installation for a monopole is higher than the cost for a latticed self-supporting tower.

Steel is by far the strongest and most popular material for monopoles, but they can also be constructed from concrete, fiberglass, or treated wood. With a concrete monopole, there is the risk that the tower could shatter if struck by lightning due to rapid thermal expansion.

Many zoning boards consider monopoles better looking and less obtrusive in the skyline than other tower types. Monopoles are generally used for cellular applications.

Kipp is a broadcast engineer.

© 2002, PRIMEDIA Business Magazines & Media Inc. All rights reserved. This article is protected by United States copyright and other intellectual property laws and may not be reproduced, rewritten, distributed, redisseminated, transmitted, displayed, published or broadcast, directly or indirectly, in any medium without the prior written permission of PRIMEDIA Business Magazines & Media Inc.