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Understanding Line-of-Sight Microwave Systems

System Planning for Outdoor Microwave Communications Systems
From the U.S. Department of Transportation, Federal Highway Administration
Wireless Systems Planning Guidelines Document

Planning for Wireless Systems

System planning is critical to the successful installation, operation and proper performance of any communication system, wireless systems are no exception, and this is especially true for line-of-sight (microwave) wireless. Unless your proposed microwave link will be operating over a very long path, you should be able to confirm whether a visible line-of-sight path exists between the two proposed antenna sites. This is only a first-step process, and is often accomplished by using a combination of strobe lights, mirrors (which reflect the sun), binoculars and spotting scopes. Being able to see one site from the other will not guarantee that the visible path is appropriate for a microwave signal, but at least you who know that the possibility of such a path exists.

"Line-of-sight" is a term used in radio system design to describe a condition in which radio device antennas can actually see each other. High frequency radios, such as those used in Spread Spectrum Radio require line-of-sight between antennas.

In many instances there may be obstacles to overcome such as buildings, trees, small hills and elevated roads, and it may not be possible to confirm that line-of-sight exists without additional aid. Keep in mind that even a "perfectly clear" visual path may not actually be so. As an example, small branches of deciduous trees, barren in the winter, may not be visible until spring or summer when growth appears. Even the skeleton of a new building may not be visible until the sides go up! Establishing line-of-sight for traffic signal systems should be easy to accomplish because of the short distances involved (a few blocks).

When establishing line-of-sight, it is extremely important to plan for the future. In urban areas, new building construction may result in total path obstruction. In areas where construction is not anticipated, the rapid growth of trees or foliage may severely affect the path over time. While a number of software products are available for assisting with path work, combining a topographical mapping of the path with a subsequent path walk or drive is often an excellent way to start the line-of-sight confirmation process.

Assuming an appropriate line-of-sight path from radio site to radio site can be established, both the feasibility and viability of a point-to-point microwave radio link will be dependent upon the gains, losses and receiver sensitivity corresponding with the system. Gains are associated with the transmitter power output of the radio, and the gains of both the transmitting and receiving antennas. Losses are associated with the cabling between the radios and their respective antennas, and with the path between the antennas. Other losses can also occur if the path is partially obstructed, or if path reflections cancel a portion of the normal receive signal. Manufacturers will state respective RF power output and gain for each of their products.

Radio Transmitter Power

Radio transmitters are described in terms of power output expressed in watts. The power output may also be expressed in terms of decibels of gain (dB). Radio receivers are rated in terms of sensitivity (ability to receive a minimal signal). The rating is listed in terms of milliwatts (mW), or decibels of gain (dB). Antenna cable is rated in terms of signal loss per foot and expressed as dB of loss per foot. The antenna is rated in terms of gain (dB). There are a number of software programs that will calculate path loss by frequency and use the specifications of the system hardware to help determine the overall system feasibility.

One of the first items to consider for any microwave path is the actual distance from antenna to antenna. The further a microwave signal must travel, the greater the signal loss. This form of attenuation is termed free space loss (FSL). Assuming an unobstructed path, only two variables need to be considered in FSL calculations:

  • The frequency of the microwave signal – numerically higher frequencies require more power to cover a given distance.
  • The actual path distance – the greater the distance the greater the signal loss.

A signal transmitted at a frequency of 6 GHz will have more available power than a signal transmitted at 11 GHz. For example, a microwave system at 6 GHz can expect to cover about 25 miles between communication points. The same system using a frequency of 11 GHz will only cover about 10 miles.

When RF energy is transmitted from a parabolic antenna, the energy spreads outward, much like the beam from a flashlight. This microwave beam can be influenced by the terrain between the antennas, as well as by objects in or along the path. When the centerline of a beam from one antenna to another antenna just grazes an obstacle along the path, some level of signal loss will occur due to diffraction. The amount of signal loss can vary dramatically, influenced by the physical characteristics and the distance of the object from the antenna.

A microwave beam can also be reflected by water or relatively smooth terrain, very much in the same way a light beam can be reflected from a mirror. Again, since the wavelength of a microwave beam is much longer than that of a visible light beam, the criteria for defining "smooth terrain" is quite different between the two. While a light beam may not reflect well off of an asphalt road, a dirt field, a billboard, or the side of a building, to a microwave beam these can all be highly reflective surfaces. Even gently rolling country can prove to be a good reflector.

A microwave beam arriving at an antenna could effectively be canceled by its own "mid-path" reflection, causing tremendous signal loss. Long microwave paths can also be affected by atmospheric refraction, the result of variations in the dielectric constant of the atmosphere.

For relatively short 2.4GHz microwave paths, only reflection points and obstructions are usually of real concern. The effects of atmosphere and earth curvature will not usually come into play, so the engineering of these paths is quite straightforward. For long or unusual paths, however, all aspects of path engineering must be considered.

Interference Issues – Spread spectrum microwave radio systems are among the most interference tolerant communication networks in use today. Spread spectrum signals are very difficult to detect and, by their nature, are highly resistant to jamming and interference. As more and more signals are transmitted, the "noise level" in the band increases accordingly. Once the noise reaches an identified level, communication in the band is effectively negated.

In the U.S., the 2.4GHz band is license free, making it very difficult to know whether or not another spread spectrum radio is operating in a manner which could possibly interfere with one's own link. While these links are usually able to spread narrow band interference, other spread spectrum signals in the 2.4 GHz band could possibly interfere if they are of the proper frequency and amplitude. It is extremely difficult to predict the effect of an interfering signal unless specific information is known about the interferer. In general, other spread spectrum signals in the 2.4 GHz band tend to raise the band's noise floor. For this reason, even when working with paths which are very short and not subject to any sort of fading condition, a fade margin of 15 dB or greater should always be maintained for the path.

A Word About Antennas

All RF systems have an antenna (or several in an array). The transponder used in a vehicle for toll collection has an antenna. The fact that it can't be seen doesn't mean that it isn't present. The antenna is built in-to the package. A cost comparison of all the elements that make up a radio system would show the antenna as the lowest cost piece. However, most of the problems that a radio system may have can be traced to either improper installation, or improper selection, of the antenna. Follow the recommendations listed below for proper installation.

All antennas have similar characteristics. They are designed with vertical and horizontal polarity. The manipulation of these characteristics creates a specific antenna coverage pattern. Some antennas are designed to provide a circular pattern referred to as omni-directional. Others have an elliptical pattern referred to as uni-directional. Antenna manufacturers will routinely provide the horizontal plane pattern as part of their product literature. An engineer can request a copy of the vertical pattern if necessary. The antenna pattern display is for an Antenna Specialists, Inc., 2.4 GHz Spread Spectrum radio system. The antenna projects two highly directional lobes. When setting up a radio system, it is critical that the installers match the pattern lobes to the system design. If the direction of the lobes is off by just a few degrees, that may cause the system to have a marginal performance.

Guidelines for Handling & Installation of Wireless Antenna and Transmission Cable

RF Transmission cable should be treated with the same care as fiber optic communication cable. This is important. To prevent interference with other radio systems on the tower the transmission cable is constructed with an internal shield of copper or copper foil. If this shield is broken, your system could cause interference with other systems at the site. Also, once the shield is cracked, your system is subject to interference. Some radio transmission cable uses a hollow copper tube to act as a "wave guide". If the hollow tube becomes damaged your system might not function properly. The following guidelines apply:

  • All cable should be inspected and tested when received.
  • All test results should be compared with factory pre-shipping tests.
  • Inspect the cable nomenclature to make certain that you received the correct product.
  • Notify the supplier (or manufacturer) of all discrepancies as quickly as possible.
  • Follow the manufacturer recommendations for installation
  • Cover all exposed cable ends to make certain that moisture does not penetrate the cable assembly.

Mounting cable on a pole or tower structure requires the use of qualified personnel, test equipment, and care to prevent damage to the transmission line:

  • Use a hoist line that supports the total weight of the cable – refer to manufacturer specifications
  • Use pulleys at both the top and bottom of the pole (or tower) to guide the hoist line.
  • Support the cable reel on an axle so that the cable can be freely pulled from the reel. Have a crew member control the rotation of the reel.
  • Short lengths of cable coiled and tied. Uncoil the cable on the ground away from the pole before hoisting.
  • After raising the cable to the top of the pole, anchor it to the support structure from the top down.
  • Never anchor the cable to an electrical (or lighting) conduit.
  • The top and bottom of the cable attached to the pole should be electrically grounded to the pole with a grounding kit.
  • The antenna input connection cannot be used as the cable ground at the top of the pole.
  • Test all connectors to make certain that they do not "leak" RF power.