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3.5.1 Line-of-sight propagation
Line of sight propagation (also known as free space propagation) is the most common
mode of propagation for radio waves in the Very High Frequency (VHF) section and above
of the radio spectrum. The radio waves travel in a direct path, propagating outwards from
the transmitting antenna towards the receiving antenna at the speed of light, with the
signal strength reducing as it moves further away from the transmitter. This is known as
Free Space Path Loss and assumes no reflections or obstacles between the transmitter
and receiver.
The rate at which the signal strength falls is shown by the equation below:
Where ‘k’ is a constant and ‘d’ is the distance from the transmitter. This formula indicates
that the rate at which the strength of a signal falls as it travels further away from the
transmitter is inversely proportional to the square of the distance from the transmitter.
For example, the strength of the signal at two meters away from the transmitter will be a
quarter of the signal strength at one meter away from the transmitter.
The basic formula above can be altered to take into account other factors that can
influence the radio signal.
The formula for Free Space path loss is shown below:
Where:
d = Distance between antennas in meters
f = Frequency in Hertz
G
Tx
= Gain of transmitting antenna
G
Rx
= Gain of receiving antenna
c = Speed of light in vacuum in meters per second (i.e. 3 x 108 m/s)
It is important to note that this formula provides only an estimation of signal loss through
free space as reflections or obstacles are not considered. This value can be used to
calculate the RF Link Budget, which is a summary of the transmitted power with all the
gains and losses in the system and provides an estimation of the strength of the received
signal. Depending on the estimated strength of the received signal, the transmitted power
and antenna gains can then be adjusted accordingly. It is essential for the link budget to
be calculated during planning of a radio communications system to ensure that the signal
strength is sufficient to allow the system to meet its operational requirements once it is
installed.
