DAVIS CELLULAR TELEPHONE ANTENNA KIT User Manual download pdf (Page 8)

the voltage drops below 12.0V, it is suggested that no communication be initiated until there is reason to

believe that the station has received charging sunlight for an hour or more.

Site and System Considerations

This section provides a brief discussion of some of the alternatives for the physical design of the radio

communications link.

Propagation Path

The radio path between antennas should be unobstructed. Spread-spectrum 915 MHz transmissions require a

clear line-of-sight path. Such a path is highly desirable for UHF (450-470 MHz) signals. Achieving the clear

path may require positioning the antennas at a considerable height (as discussed above) or horizontal distance

from the weather station or base station computer. If this proves necessary, two alternatives are available:

1. Use a long coaxial cable between the radio and the antenna. This may not be particularly desirable,

because signal strength is lost at the rate of 0.1 to 0.16 dB per foot of cable length.

2. Place the radio close to the antenna, and use a long data cable between the Weather Station (or the Base

Station Computer) and the radio. The RS-232 interface will drive a cable up to 50 feet in length; a longer

cable will require use of a short-range modem pair.

The 6-foot cables attached to Davis-provided antennas permit the antenna to be mounted up to two feet

(0,6m) above a Sensor Arm and System Shelter. Greater elevation can be obtained by using a 50-Ohm co-ax

extension cable (BNC female to BNC male). The connection should be protected from weather. The 12-foot

cable provided for the YDI 12-dB antenna permits a height of eight feet (2,4m) above the Arm.

Figure 1a. Schematic of “compact” field and base stations.

WeatherLink Radio Communications 8

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DAVIS CELLULAR TELEPHONE ANTENNA KIT User Manual Page 8