Specifications

4 - 4 12/16/97
from other directions. The most common form is called a Yagi-Uda or just Yagi, after its inventors. The
antenna lobe, or focussed beam, lies in the direction in which the antenna boom is pointed.
The Yagi antenna’s beam width is expressed in degrees of angle between the two directions at which the
signal strength has fallen to –3dB compared with the center value. For example, the Davis 8.5 dB 900 MHz
antenna has a beam width of 65º when vertically-polarized. This means that the range will be reduced to
71% at angles of 32º either side of the aimed direction.
The antenna focus is not perfect, so it is possible for a Yagi antenna to communicate with antennas that lie
outside its main beam, but the effective range will be much lower.
Polarity. A Yagi antenna may be mounted in either of two orientations; the choice determines the polarity of
the transmitted signal:
a. If the antenna elements are vertical, the E-plane and the Polarity of the antenna are Vertical.
b. If the antenna elements lie in the horizontal plane, the E-plane and Polarity are Horizontal.
If two Yagi antennas are communicating with each other, either polarity may be used, but both must have the
same polarity. If another signal source is interfering, it may help to change the polarity. If a Yagi is
communicating with an omni, the polarity must be vertical.
ANTENNA HEIGHT
915 MHz signals (and, to a lesser degree, 450 MHz signals) tend to travel like visible light, in a straight path.
So there should be a line-of-sight unobstructed path between antennas of communicating stations. Because
the earth’s surface is curved, antennas must be elevated above the ground if they are to communicate at a
significant distance. A formula for estimating required antenna height above level ground is given in
References 1 and 2: H = D
2
/2, where D = Distance in miles and H = height in feet. This formula gives the
following estimates:
Transmission distance _Antenna Height_
3 miles 4,8 km 4.5 feet 1,4 m
4 7,3 8 2,5
8 14,6 32 9,8
Alternatives for antenna placement are discussed in the Site and System Considerations section.
RANGE
Table 1 gives estimated ranges for combinations of Davis-supplied antennas with line-of-sight paths between
them. Range estimates for other antenna gains may be found by using Figure 4 on page 11.
Table 1. Transmission distance, line-of-sight, vs. antenna type
--------------------------- Estimated Range ----------------------------
Omni-to-Omni Omni-to-Yagi Yagi-to-Yagi__
YDI RM910 8.5 dB Yagi 0.9 mi 1.4 km 1.6 mi. 2,6 km 3 mi. 4,8 km
12 dB Yagi 2.3 3,7 7 11,2
Neulink 9600 4 mi. 6,5 km 10 mi. 16 km 25 mi. 40 km
POWER
AC POWER
If AC “mains” power is available at the field and base stations, the radios may be on continuously and the
WeatherLink operated in its normal modes.
SOLAR POWER
If the field station is operating on solar/battery power, it is necessary to switch power to the radio so it is off
most of the time in order to conserve the charge drained from the battery. The radio’s power is switched on
by the Timer or the Alarm Output Module for brief pre-determined intervals to allow communication.