Last week we provided some insight regarding the selection of mobile antennas to ensure that they are appropriate for the task at hand. Today we examine antenna placement, which is crucial to its optimal performance.

Many times, the antenna is mounted in a location that is easiest to accommodate for the installation. In actuality, the antenna should always be located and positioned for optimal radiation and operator safety. In theory, antennas radiate best in the center of the roof of a vehicle, because the metal roof of the vehicle acts as the best ground plane. In reality, center-roof location is not always possible due to appearance, vehicle mission, roof materials, low hanging obstructions, or owner preference.

Most antennas must have an effective ground plane to radiate properly and for safety, which is often overlooked. There are some antenna designs that do not require a ground plane. Be sure that the type of antenna being installed matches the environment of the location.

Improperly grounded antennas that require a ground plane can radiate on the outside of the coax and introduce RF radiation into areas and devices within vehicles that it was never intended to manage. Modern vehicles supporting a variety of missions from utilities and law enforcement to military and EMS have very sophisticated computer modules that operate everything from engine management and gear selection to climate control, safety systems and braking.

Strong radio-frequency interference (RFI) introduced into a vehicle's power train or safety systems can cause the driver to lose control of the vehicle. Many vehicles that radio professionals install antennas upon are heavy and often are operated at greater speeds than most private vehicles. The safety of the driver, the vehicle occupants and others on the highway should never be forgotten. An accident that appeared to be the result of equipment failure or operator error could be the result of RFI, and the evidence vanishes when the culprit transmitter un-keys.

Antennas respond with extreme sensitivity to any conductive objects located near them. This can be verified by using an antenna analyzer sweeping a frequency range while observing SWR or return loss. As the sweep is performed, move your hand near the antenna — or simply move the antenna near various objects — and watch the antenna's optimum SWR or return loss at one frequency move to another frequency.

While conducting the install, cut the antenna close to the specified length using the supplied cutting chart or calculate the proper length, but slightly longer than the cutting chart recommends. Use an antenna analyzer for antenna optimization and fine tuning after the antenna is installed. Roof-top air conditioners, ladders, crane booms, dump bodies, other antennas, and surface metal all affect SWR. Consider these factors in choosing your antenna mounting location and final length.

Next: How to properly tune mobile antennas.

Ira Wiesenfeld, P.E., has been involved with commercial radio systems since 1966, and has experience with land-mobile-radio, paging and military communications systems. He holds an FCC general radiotelephone operator's license and is the author of Wiring for Wireless Sites, as well as many articles in various magazines. Wiesenfeld can be reached at iwiesenfel@aol.com.

Christopher Dalton has designed, staged and implemented virtually every kind of LMR system in his two-decade-long career, including conventional, trunked, simulcast, Project 25, single-site and multisite. He holds an FCC general radiotelephone operator's license. Dalton can be reached at cdalton@fairpoint.net.

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