How to select the proper mobile antenna
Do your users complain about coverage problems? Do your dispatchers frequently complain of partial traffic or missed calls? There are numerous reasons why this can occur, but a problem often observed in the field is a poor subscriber unit mobile antenna or antenna installation. No matter how good the transmitter or receiver, the performance of the radio will always be limited without a properly functioning antenna.
Antennas selection, placement and tuning are critical to radio performance. Remove any one of these three parameters and performance is compromised. While this is true for a fixed radio site, it is also crucial for a mobile radio antenna. Many mobile-radio problems are often mistaken for system or fixed-site problems. It is usually assumed that the antenna is the easiest part of the mobile-radio installation. As radio professionals, we go through great lengths to select the proper radio equipment, the proper mounting location for the operator and we verify that the transmitter and receiver meet manufacturer specifications. The mobile antenna installation requires the same care.
First, you must select the proper antenna for the task. There are many different types of mobile antennas. Some antennas offer more gain than others.
For example, a 5/8 wave antenna at VHF has more gain than a 1/4 wave antenna. Review the service area you are covering for your end users. Does the antenna really need 3 dB of gain at VHF with 100 watts of output power? If your system has satellite receivers and voters or was designed for walkie-talkie coverage, you probably do not need gain antennas for most systems.
Do your users frequently operate very close to the radio site? Do they operate on the fringe areas? Where does your license allow you to operate your mobile radios? Remember that the VHF band is not structured in a band plan the way other frequency bands are. As a result, your high-powered mobile, in conjunction with a high-gain mobile antenna, may cause unnecessary interference to users in another area or contour.
Is a quarter-wave antenna sufficient? Always consider service area, terrain, frequency band and mobile transmitter power in your selection of the proper antenna.
Next: Determining proper antenna placement.
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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