Campbell Scientific RF320-Series Ritron VHF/UHF Radios - Manual (Page 18)

RF320-Series Ritron VHF/UHF Radios
radiated in the same direction by an isotropic antenna is defined as the antenna
gain and is expressed in dBi.
An isotropic antenna is a theoretical concept and doesn’t exist in nature. When
actually measuring antenna gain in the laboratory, a half-wave dipole is used as
the reference antenna. A half-wave dipole is known to have a directive gain of
1.64 times that of an isotropic radiator. Expressed as a ratio this equates to a
gain of 2.15 dBi. Gain referenced relative to that of dipole is expressed in dBd.
The relationship between dBi and dBd is: Gain in dBd = Gain in dBi – 2.15 dB.
Manufacturers may use either of these designations on an antenna data sheet. It
is important that one understands the relationship. When used in link budget
calculations, convert gain in dBd to gain in dBi.
The most common types of antennas used in an RF telemetry system are the
omnidirectional, vertical dipole and highly directional Yagi. While the dipole
will exhibit some gain in an omnidirectional pattern (re: the beach ball
analogy), the Yagi will concentrate its gain in a single direction. The choice of
an omnidirectional vs. a high-gain, directional antenna for a given installation
depends on the network topology and link distance. In a point-to-point
topology, a high-gain, directional antenna affords greater link distances. A
point-to-multipoint topology usually requires an omnidirectional antenna at the
base and repeater sites, with possible use of a directional antenna at the remote
sites.
6.3 Installation Best Practices
• General
o System Pretest
- The importance of ensuring that all system components are fully
functional BEFORE taking them to the field cannot be
overemphasized! Prior to installation, interconnect the radio with
as many of the actual system components as possible and perform
the functional checks listed in Section 8.3, Radio Functional
Checks. Record the results for later reference.
• Wire and Cable
o Routing
- Route all conductors and cables in a neat, orderly fashion. Avoid
routing directly over or across system components.
- Avoid routing conductors carrying low level analog signals in
close proximity and parallel to conductors carrying digital signals
or switched voltage levels.
o Bend Radius
- The RF cable used to interconnect the radio and antenna has a
specified minimum bend radius. Exceeding it will lead to a
degradation of system performance: extra losses, high VSWR,
etc.
o Strain Relief
- Avoid cable chaffing and connector fatigue by strain relieving all
conductors and cables that span a distance of more than 12 in. or
have a potential for relative motion due to vibration or wind.
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