The antenna must be designed for the frequency bands deployed in the regions that the wireless modem is being used. For fixed locations this may be dual bands (for example E-GSM900/GSM1800 in Europe; GSM850/GSM1900 in North America). For applications which are mobile, users should consider whether three or all four GSM bands could be encountered.

Other factors in choosing antenna are equally important:

•Impedance of the antenna and antenna cable must be 50 ohms at all frequencies being used

•Antenna output-power handling capability must be a minimum of 2 W

•Antenna VSWR value should be less than 3:1 to avoid damage to the radio device

7.5.3Antenna Placement

The antenna should be placed away from electronic devices or other antennas. The recommended minimum distance between adjacent antennas, operating in a similar radio frequency band, is at least 50 cm.

If signal strength is weak, it is useful to face a directional antenna at the closest radio base station. This can increase the strength of the signal received by the wireless modem.

The wireless modem’s peak output power can reach 2 W. RF field strength varies with antenna type and distance. At 10 cm from the antenna the field strength may be up to 70 V/m and at 1m it will have reduced to 7 V/m.

In general, CE-marked products for residential and commercial areas, and light industry can withstand a minimum of 3 V/m.

7.5.4 The Antenna Cable

Use 50 ohm impedance low-loss cable and high-quality 50 ohm impedance connectors (frequency range up to at least 2 GHz) to avoid RF losses. Ensure that the antenna cable is as short as possible.

The effectiveness of the antenna, cable and connectors is determined by their quality. All connectors, adaptors and cables should be of the highest quality, lowest loss, lowest VSWR rating that is affordable to the user.

Minimize the use of extension cables, connectors and adapters. Each additional cable, connector or adapter will result in additional loss of signal power.

7.5.5 Possible Communication Disturbances

Communication disturbances can adversely effect the quality of wireless links, including the following causes:

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