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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

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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.

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.

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.

Fresnel zone formula

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The image below is a Java applet for calculating the Fresnel zone. Clicking the image starts the calculation screen.

Field testing values

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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"

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"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

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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

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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 image below is a Java applet for calculating the Height pattern. Clicking the image starts the calculation screen.

Height pattern calculation result

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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.

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

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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

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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.


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