|
This program is an applet for calculating the Fresnel zone.
A particularly common problem with radio systems is communication errors. The
occurrence of communication errors changes significantly depending on the
method of placement of the radio equipment. With radio
communication, it is important to ensure "line of sight" between the
transceivers.
When we can see the receiving antenna we use the expression "line of sight"
without much thought, but with radio waves, it can be said that "line of
sight" is only achieved when the Fresnel zone indicated below is established.
With radio systems used at about 1.5 m above ground, if a Fresnel zone is not
established, multipath interference will occur, so that the true electric
field intensity cannot be obtained. If operation of the equipment is unstable,
it is necessary to consider whether the fixed station can be set up in a
higher place.
Radio system errors
Radio system errors occur when the bit strings of 0's and 1's that make up the
data cannot be identified by the electronic circuits inside the receiver, due
to corruption of the transmission data.
Viewed on a temporal basis, assessment of the receive data is performed in the
middle position of the receive bit width, and in terms of level, the bit is
determined to be 1 or 0 by whether the received voltage of the median value
(threshold value) is bigger or smaller. This identified signal is the signal
after FSK demodulation. Because the signal and noise are a composite waveform
to begin with, if the difference between the signal level and noise level
(inside the receiver) is low, with certain sampling timing, since the decision
is made with a waveform in which the threshold value is exceeded due to noise,
an error results. Therefore in general, the signal level is required to be
about 20 dB (100 times the voltage ratio) greater than the level of noise.
FSK radio equipment is immune to the influence of amplitude noise, but if
there is an obstruction between the antennas (the ground, buildings, natural
objects and so on), the radio waves will be reflected resulting in multipath
interference. At the receiving point, the composite wave incorporating the
delayed radio waves is distorted in amplitude, resulting in an error. In
addition, the ratio of the noise component of signals that enter the
demodulation circuit in a location with weak electric field intensity is high
in relation to the signal, and this causes errors. Of course, signals
generated by the antenna due to external noise will also have an impact on
amplitude and frequency.
Fresnel zone
In order for radio waves emitted from the transmitter to reach the receiver
without attenuation of power, a certain amount of space is required. The
energy cannot reach the receiver via one straight line in space. It is easy to
understand for example that the waves will not get there through a hole the
size of a needle in a concrete wall.
The space required is a spheroid with its center along the shortest distance
between antennas, and this is called the Fresnel zone. In fact this space
expands indefinitely, but the part that principally contributes to
communicating the energy is called the 1st Fresnel zone.
If there are obstacles inside the Fresnel zone, insufficient energy is
transmitted so that received field intensity becomes weak. If the received
field intensity is weak, the probability that errors will occur becomes
gradually higher.
The receive sensitivity of the receiver is absolute, and propagation loss
which depends on the distance traveled by the radio waves cannot be avoided.
Therefore in order to prevent errors from occurring, it is important to ensure
that the received radio waves are as close as possible to the theoretical
value.
The 1st Fresnel zone is a spheroid space formed within the trajectory of the
path when the path difference when radio wave energy reaches the receiver by
the shortest distance, and when it gets there by another route, is within λ/2.
In this case, λ is the wave length of the radio wave (wave length = speed of
light / frequency) which at 400 MHz is 0.75 m.
When positioning radio systems
The distance between the Fresnel zone boundary and a straight line running the
shortest distance between the antennas is called the Fresnel radius, and if
there are no obstacles in the space forming 60% of this distance, propagation
characteristics are said to be the same as in free space.
|
Fresnel zone formula
