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Appendix B
Radio Range
When planning the installation of a radio net it is useful to consider a number of different arrangements and
compare their relative merits before deciding on a final layout. One aspect to be considered is the useable signal
strength at the receiver input represented by the Received Signal Strength Indicator (RSSI) figure.
RSSI is a negative number related to signal strength in dBm; a smaller number represents a stronger signal so -70
is much better than -80. To obtain a reliable link using Rad24 radios RSSI must be no worse than -85 to -90,
beyond this figure packet loss rate increases and link quality drops off sharply. A table illustrating the relationship
between receiver input power and RSSI is reproduced at Table 1 below.
Radiated energy diminishes over distance with an inverse square law; signal power is also lost due to absorption
and scattering in the air between the transmit and receive antennas, these losses are referred to as Free Space
Path Loss and vary according to the wavelength of the signal. At 2.4GHz Free Space Path Loss is given by:
Path Loss = 32.4+20Log
Where:
Path Loss is expressed in dB
d = Path length in metres
Example 1
Path Loss over 80metres = 32.4+20Log
20Log
80 = 38.06
10
Path Loss = 32.4+38.06 = 70.46 dB
For convenience, Table 2 below gives path losses at range intervals of 5 metres.
Example 2
An estimate of signal power at the receiver input can be made by considering two Rad24i radios in the open
separated by a distance of 80m. Rad24i output power is 0dBm and the peak gain of the integral antenna is 3dB as
shown at Figure 1 below.
G
=
T
3dB
0dBm
0dBm
Tx
G
= Gain of transmit antenna
T
G
= Gain of receive antenna
R
Calculation of signal power at the receiver input is achieved by summing all the gains and then subtracting the
sum of all the losses, in this case:
Signal power at the receiver input = (G
or
6 – 70.46 = -64.46dBm
Signal power at the input of the receiver = -64.46dBm
Note that although the manufacturer's data sheet for the chip antenna gives gain as 3dBi Peak, this figure is
rarely achieved in practice; between 1 and 1.5dBi is more usual.
Mantracourt Electronics Limited T24 Technical Manual
d
10
80
10
Path loss = 70.46dB
+G
) – Path Loss
T
R
80m
Figure 1
G
= 3dB
R
Rx
82
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