Electric
field intensity at the receiving point Fresnel zone and height pattern
Introduction
When the radio system has reached the level of
successful operational checks in the laboratory, it is time to move on to
field testing.
In the design stage of radio systems, it is normal to order samples from
manufacturers and to carry out tests of communication range, communication
speed and other performance aspects. But when field testing is actually
carried out at the installation site, often more problems arise than
anticipated, with both hardware and software. These problems are caused by
the differences between the test environment and the environment where the
system is to be installed.
What sort of environment is the radio equipment installed in? Have you
actually measured the electric field intensity at the receiving point?
When the system is built with radio equipment that does not have a
function for measuring electric field intensity (received power), is the
system really installed appropriately?
Radio waves are undoubtedly transmitted by slipping between walls, houses
and buildings and by passing over hills and rows of building. But radio
equipment cannot be installed hastily. Even if there is no problem when
the equipment is installed, communication may become impossible after a
while. This may be because the electric field intensity at the
installation site is insufficient. Normally when you install radio
equipment, it is necessary to install it at a distance assumed to have
sufficient electric field intensity. For radio equipment with receive
sensitivity of -110 dBm, stable communication will not be possible unless
received power of about -90 dBm is obtained, with a margin of at least 20
dB.
It is important to know in advance what conditions determine the electric
field intensity of the place where the receiver is located. This involves
understanding radio wave propagation. The radio wave propagation loss
aspect of this is covered elsewhere, but in this article we will look at
the Fresnel zone and height pattern.
Please also use the Java applet we provide for calculating the Fresnel
zone and height pattern.
Fresnel zone
For radio communication, the radio equipment must be
installed so as to establish a Fresnel zone. If there are any obstacles inside
this Fresnel zone, propagation loss occurs, so that the actual received power
differs from the theoretical value for radio wave propagation in free space,
reducing communication ability accordingly. This reduced communication ability
might involve reduced communication range, frequent errors and reduced
transmission speed, or it might mean that communication becomes impossible
altogether.
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What is a 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.
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The 1st Fresnel zone
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.
Incidentally, radio waves that pass through the 1st Fresnel zone reinforce
each other at the receiving point.
Assuming that a 1st Fresnel zone can be established, received power is close
to the theoretical value for radio wave propagation in free space. In
addition, as long as there are no obstacles within a 60% radius of the 1st
Fresnel zone, it is permissible to apply the formula for radio wave
propagation in free space.
So, if there are obstacles within the zone, the question arises to what extent
attenuation will occur, but this is not the kind of problem that can easily be
solved theoretically. This is because the environmental parameters are too
complex.
It is important to explain to your customers the importance of establishing a
Fresnel zone and to propose and install suitable systems.
Obstacle
zone
The figure above is based on the situation of an actual field test.
The radio equipment has a frequency of 429.25 MHz, output is 10 mW,
antenna gain is 2.14 dBi for both transmitting and receiving, and the
height of the antennas is 10 m for the transmitter and 49 m for the
receiver. The communication range was measured at 6,500 m.
The result of measurement was a received signal intensity of -96 dBm.
At this distance free space propagation loss is 101 dB, electric field
intensity at the receiving point is 40.6 dBμV/m, and received power is
-87.1 dBm.
However, clearly there is ground reflection rather than free space.
Therefore if we apply a formula for radio wave propagation with a 2-wave
model, electric field intensity at the receiving point is 42.6 dBμV/m,
and received power is -85.1 dBm.
Whatever the result, it is probably best to achieve received power of
about -86 dm, but in fact it is about 20 dB lower than the theoretical
value.
In this test, a 49 m high art gallery could be seen from the radio
equipment located at 10 m on a utility pole, so while so-called line of
sight was achieved, propagation loss was significant due to the fact
that a Fresnel zone was not established. In addition, in this test the
directivity of the antennas was not optimized and measured, so it is
likely that several decibels of loss were added.
If the Fresnel zone radius at the middle point is calculated with the
parameters of the test environment in this example, the communication
distance is 6,500 m,
At the location d1 = 1,625 m, the following result is obtained.
Even when the fact that the height of the radio equipment on the art
gallery is 49 m is taken into account, it is clear that a Fresnel zone
is not achieved by the middle point, and this can be thought to be the
case of the propagation loss.
"Line of sight"
"Line of
sight" is a frequently used expression, but for radio wave propagation
"line of sight" means that a Fresnel zone has been established. Note
that when you test on flat terrain there is often an illusion of "line
of sight", but in fact line of sight does not apply.
In addition, note that even if there are no obstacles when the equipment
is installed, with the passage of time buildings may appear and the
foliage of trees may begin to obstruct the Fresnel zone.
Height pattern
Height pattern indicates the
relationship between antenna height and electric field intensity when
the communication conditions other than the height of the antennas are
fixed.
Although it is generally advisable to set the antennas in a high place
in order to achieve satisfactory communication conditions, in fact
electric field intensity at the receiving point depends on the
frequency, communication distance, and antenna height, varying
significantly. The higher the frequency, the closer the communication
distance, and the higher the antenna, the more the composite electric
field varies due to the phase relationship of the reflected waves and
direct waves.
The results for height pattern shown in this article are calculated with
reflected waves from 1 surface (the ground), and they are different from
the electric field intensity attenuation that actually occurs in the
field. In the actual field, since the radio waves arrive by different
routes (multipath), differences in the times that direct waves and
reflected waves arrive occur (phase difference), and the composite waves
are distorted in amplitude and over time (fading). Therefore, points
where there is marked attenuation in received field intensity and points
where it increases occur. If radio equipment is located at the
attenuation points, frequent errors will occur.
Height pattern formula
The height pattern formula is
shown below. However, please note that since the reflecting surfaces are
complete reflectors in this calculation, the results should be viewed
with caution. In an actual environment, it is necessary to assume
reflection loss in relation to reflected waves in any calculation.
The following graph shows the
height pattern under the conditions below. It expresses the received
field intensity when the transmitting antenna height is fixed at 10 m
and the communication distance is 200 m, and when the height of the
receiving antenna is varied between 1 m and 30 m. The X axis is the
height, and the Y axis is the electric field intensity and received
power.
The graph shows that under these conditions, as the receiving antenna is
raised, first there is a peak at 3.8 m, then around 7.6 m comes the
maximum attenuation, followed by repetition of the pattern thereafter.
Conditions:
Frequency : 400 MHz, Transmitted power : 10 dBm, Transmitting antenna
gain: Gt = 2.14 dBi, Receiving antenna gain: Gr = 2.14 dBi,
Communication distance: d = 200 m,
Transmitting antenna height: Ht = 10m, Receiving antenna height: Hr =
changing from 1 - 30 m
The diagram below shows the
height pattern when the distance is changed to 3,200 m and the receiving
antenna is changed to 100 m.
You can see that up to a height of 60m, as the antenna gets higher the
electric field intensity increases.
Conditions:
Frequency: 400 MHz, Transmitted power: 10 dBm, Transmitting antenna
gain: Gt = 2.14 dBi, Receiving antenna gain: Gr = 2.14 dBi,
Communication distance: d = 3,200 m, Transmitting antenna height: Ht =
10 m, Receiving antenna height: Hr = changing from 1 - 100m
When considering radio equipment systems
At the design stage of radio
systems it is important to carry out onsite tests in the field, and to
select an installation site and consider the configuration taking into
account the receive sensitivity of the radio equipment. If the results
of the measurements show significant propagation loss, please reconsider
the installation site from the point of view of the Fresnel zone and
height pattern.
If only the base unit can be located in a high place, the situation
begins to change significantly.
1. Measure the field intensity at the actual location of use.
2. Allow a sufficient margin with regard to received power (about 20 dB)
3. Ensure line of sight
4. Locate the equipment in as a high a position as possible. However,
check the height pattern.
5. Consider whether obstacles will appear in the communication space
with the passage of time.
In conclusion
Under the title "Electric
field intensity at the receiving point", we looked at the Fresnel zone
and height pattern as knowledge required before designing a system.
Although radio waves are commonly viewed as nebulous and troublesome
entities, in fact if you measure the electric field intensity and allow
a sufficient margin, it is easier to get a handle on them.
This article has covered only a small part of the knowledge involved in
setting up radio equipment, but we hope it has been useful to designers
of equipment.
And we hope that you will use the knowledge to provide your customers
with reliable and stable systems.
Introduction
Fresnel zone formula
