Specifications

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drops too low. It is suggested that communication be limited whenever the battery voltage is less than 12.5
Volts; if 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:
a. 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.
b. 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.