Afar AR-24010E, Operator'S Manual

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pulsAR radio Operator's Manual
The RF link budget calculations are made a lot easier by using “deciBel” units (dB). The deciBel is a
logarithmic scale that compares a parameter value against a specific reference. The advantage of
working in dB is that you can simply add all the parameters that boost your signal and subtract the
ones that attenuate it.
The following paragraphs follow an RF signal from the transmit radio to the receive radio, explaining
the various parameters and how they apply to the
pulsAR radio
Transmit Power
The RF signal starts at the output of the radio at Site 1 with a specific transmit power. In the pulsAR
radio you can configure that power from 0 to 23 dBm (the “m” in the dBm unit indicates that this
power is measured relative to 1 milliwatt).
Cable Losses
The radio is connected to the antenna through an RF coaxial cable. As the signal propagates through
this cable it is attenuated. The total attenuation (loss) depends on the frequency, cable type, cable
length and number of connectors. You can use the “Cable Loss Calculator” (at the bottom of the RF
Link Budget calculator), which includes the characteristics for several RF cable types. If your cable
is not listed you can also enter its “loss per 100 ft” (or loss per meter) at 2.4 GHz and the calculator
computes the total loss. Note that each connector along the way introduces additional attenuation,
typically around 0.2 dB per connection.
The p ulsAR radio is housed in a watertight enclosure so that you can mount it in very close proximity
to the antenna. That way you can keep the RF coaxial cable very short and therefore reduce these
losses.
Antenna Gain
The transmit signal is radiated through the antenna at Site 1. The antenna focuses the radiated energy
in a specific direction or plane, boosting your signal strength in that specific direction. That boost is
measured by the “antenna gain” in dBi (the “i” in the dBi unit indicates that the antenna gain is
measured in relation to an
i sotropic radiating element).
Distance and Free Space Loss
Once the signal is in the air it propagates towards the receiver but suffers attenuation as it radiates
away from the transmitter. If there are no obstructions the total attenuation is called the Free-Space-
Loss (FSL). This loss is a function of the frequency, f, and the distance, d. It can be computed, in
dB, from the following expressions:
FSL = 32.4 + 20 log f + 20 log d (with f in MHz and d in km)
or
FSL = 36.6 + 20 log f + 20 log d (with f in MHz and d in miles)
The calculator computes this loss for you and displays it in the output panel. An easy rule to
remember is that the free space loss increases by 6 dB every time you double the distance.
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