Campbell Scientific RF320-Series Ritron VHF/UHF Radios - Manual (Page 17)

RF320-Series Ritron VHF/UHF Radios
6.2.2 Antenna
The antenna and all interconnecting cables, connectors, and adapters are
critical elements of an RF link. It is essential that these components be selected
to match the operational parameters of the intended system. The primary
considerations when selecting an antenna are bandwidth and gain.
6.2.2.1 Antenna Bandwidth
Antenna bandwidth can be defined in a number of ways. For the purposes of
this document, antenna bandwidth is defined as the range of frequencies over
which the antenna will transfer maximum RF energy with maximum
efficiency.
As with any system, the maximum transfer of energy occurs when the
impedance of the load matches the impedance of the source. A properly
matched radio/antenna system will radiate (and, conversely, receive) the
maximum amount of RF energy. An improperly “matched” system will
significantly degrade the range and performance of an RF link and, in extreme
cases, could damage the radio.
An indicator of how well an antenna system is matched to the radio’s 50-ohm
source impedance, is the Voltage Standing Wave Ratio (VSWR). An
impedance mismatch will cause a portion of the voltage waveform traveling
toward the antenna to be reflected back toward the source. The interaction of
the reflected waveform with the incident waveform will cause the formation of
standing waves in the transmission line (antenna cable). The ratio of maximum
to minimum voltage in the standing waveform is the VSWR. On some data
sheets this is denoted as simply SWR. In a perfectly matched system, the
VSWR is 1:1. In practice, a VSWR from 2:1 to 1.5:1 is achievable. In the latter
case, 96% of the radio’s output power is being radiated, with only 4% being
lost to the mismatch.
The de facto industry standard impedance for RF
communications equipment is 50 ohms (50 Ω), but there are
commonly available coaxial cables and connectors, designed for
use in video and CATV applications, that have a characteristic
impedance of 75 ohms. The difference may not be visually
obvious. NOTE
As impedance is directly related to frequency, one should select an antenna that
exhibits a minimum SWR over the operating frequencies of the radio.
6.2.2.2 Antenna Gain
Antenna gain is an often misunderstood concept. The antenna is a passive
element and does not generate nor add any power to the system. Simply stated,
gain refers to the directivity of an antenna; its ability to direct or concentrate
the RF energy in a particular direction. A hypothetical, idealized antenna is an
isotropic source, radiating energy equally in all directions. One can imagine the
spherical radiation pattern of an isotropic source as being represented by a
beach ball. If one sits on a beach ball, the spherical shape is deformed into a
toroid; the beach ball (i.e., radiation pattern) is compressed in the vertical
direction and expanded, or directed, in the horizontal direction. The ratio of
energy directed by an antenna in a given direction relative to the energy
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