Campbell Scientific RF500M Radio Modem - Manual (Page 12)

Section 2. Radiotelemetry Network Components
An omnidirectional antenna will transmit/ receive in a full 360 degree circle.
Generally, an omnidirectional antenna will be a straight cylindrical rod which
is to be mounted vertically at the top of a tripod.
A unidirectional antenna is designed to transmit/receive in a particular
direction, or in a specified sector. There are various shapes of unidirectional
antennas. The most common is the Yagi antenna. The elements of a Yagi
antenna can be mounted either vertically or horizontally, corresponding to
either vertical or horizontal polarization.
FIGURE 2-4. The PD237 Crossover Plate Antenna Mount
Normally, all antennas will be mounted with vertical polarization. Whichever
polarization is used, be sure to keep antennas at all sites identically polarized.
2.3.3 Antenna Cables and Connectors
The most common cable type to connect a radio to the antenna is a coaxial RG-
8A/U cable. Two connectors are required for each length of cable. The
connector for the radio is a BNC type connector. The connector for the
antenna is usually either a Type-NM or Type-NF. The BNC, Type-NM, and
Type-NF connectors are shown in Figure 2-6. The Type-NM (male) connector
is for antennas with a female receptacle, and Type-NF (female) for antennas
with male receptacles.
A Campbell Scientific antenna cable complete with connectors is specified as
either COAX NM-L or COAX NF-L. COAX NF-L is a coaxial RG-8A/U
cable with a BNC connector on one end and a Type-NF connector on the other.
See Table 2-2 for cable requirements for common antennas.
Due to power loss through the cable, the length of coax cable cannot be
extended to any desired length. The amount of power loss is dependent on the
radio frequency. RG-8A/U will lose approximately 3.1 dB/100 ft. at 200 MHz
and 5.0 dB/100 ft. at 400 MHz. Power loss calculations are reviewed in
Appendix B.
2-6