Campbell Scientific RF320-Series Ritron VHF/UHF Radios - Manual (Page 21)

RF320-Series Ritron VHF/UHF Radios
7.1.1 Link Budget
Simply stated, the link budget is the difference, in dB, between the RF signal
level arriving at the input to the remote radio ( known as the received signal
strength) and the minimum signal level required by the receiver for proper
communication (known as the receiver sensitivity). This is also known as the
link fade margin. For example, if the received signal strength is -70 dBm and
the receiver sensitivity is -100 dBm, the link budget or fade margin is 30 dB.
To ensure link reliability in the event of intermittent fading of the received
signal strength, a properly designed RF link will have a link budget of between
20 and 30 dB.
The received signal strength can only truly be known by directly measuring it
with an RF field strength meter or spectrum analyzer, but an approximation can
be calculated using the following equation:
RSS = P
TX
– L
TX
+ G
TX
– L
FS
+ G
RX
– L
RX
Where:
RSS = Received Signal Strength in dBm
P
TX
= transmitting radio’s output power in dBm
L
TX
= system losses at the transmitter site (losses due to cables and connectors)
in dB
G
TX
= antenna gain at the transmitter site in dBi
L
FS
= free space propagation loss in dB
G
RX
= antenna gain at the receiver site in dBi
L
RX
= system losses at the receiver site (losses due to cables and connectors) in
dB
It is important to understand that the result of this calculation is only an
approximation and assumes a perfectly matched antenna system and an
unobstructed line-of-sight. It should only be relied upon for planning purposes.
It should also be noted that propagation losses and system losses are directly
dependent on the operating frequency. A web search for “link budget” should
yield a number of on-line calculators for determining a link budget based on a
user’s input of system parameters such as operating frequency, cable loss, path
distance and antenna gain.
For critical applications, it is strongly recommended that the link budget be
verified by measurement.
7.1.2 Line-of-Sight
An unobstructed, line-of-sight path of propagation is essential for a reliable RF
link. At the operating frequencies of the RF320-series radios, the
electromagnetic wave (EM-wave) emitted by the antenna travels in a
continuous, straight line from the point of emission. In practical terms, it is not
reflected by the ionosphere nor does it follow the curvature of the earth’s
surface. Obstacles in the line of travel can absorb, reflect, diffract, or even
block propagation of the EM-wave. Much like optical vision, the transmitting
antenna must be able to “see” the receiving antenna. For this reason, EM-wave
propagation at the operating frequencies of the RF320-series radios is said to
be restricted to line-of-sight.
It is important to understand that an obstruction to the line-of-sight propagation
does not need to appear within the cross-hairs, so to speak, in order to degrade
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