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Influence of Fresnel zones when installing 3G / 4G antennas

Fresnel Zone

Fresnel zone is alternating in the space of the maximums and minimums of the signal at the point of reception, which arise due to radio interference.

Such interference occurs:

  • the reflection of signal from buildings, the surface of the earth
  • due to reradiation,
  • due to the presence of humidity changes, dust clouds, etc.

Consider this in more detail.

Imagine the ideal case: in empty space at a distance from each other is the source signal (satellite № 1) and its receiver (satellite № 2).

The signal will propagate at the speed of light, without any distortion. We will not notice any changes when moving the receiving antenna around the transmission. When you remove the antennas from each other the signal is expected to decrease, without sudden jumps in power.
Make our perfect picture of one change: to add reflective surface. The situation has become similar to the real world in which, for example, the transmitting and receiving antenna is a base station and antenna for the 3G modem, and the surface is the ground or the roof of another house.

Now at the point of reception there are two signals:
Direct signal № 1, which is similar to the first signal in the ideal case.
The reflected signal № 2, which arose due to signal reflection from the earth's surface. This signal will be weaker than the signal № 1, but its existence can not be neglected.
The paths that the signals follow are different from each other, and because of this, the signals will come to the receiving point with different phases. If their phases differ by π, the signals will weaken each other. In the limiting case, the resulting signal will be quite weak.
If the signals come in phase (that is, 2π), then they mutually reinforce each other, and the resulting signal will be noticeably stronger than the signal in the first ideal case.
If the phase difference takes up some intermediate value, then the resulting signal can either be amplified or weakened.
It is called interference. And the periodic highs and lows formed in this case are Fresnel zones.

The result of interference in each case depends on the following parameters:

  • height of the transmitting antenna above the surface,
  • height of receiving antenna above the surface,
  • distance between the transmitting and receiving antennas,
  • frequency of the signal
  • reflection coefficient of the surface.

Thus, when moving the receiving antenna up and down, the signal will be either good or bad:

The figure shows an example of power distribution of the resulting signal when you move the receiving antenna up and down.

The distance between the highs and lows of the resulting signal may be several tens of centimeters (for example, when you installing antenna to enhance 4G signal).
For this reason, any comparison of the two antennas when they are installed next to each other but at different heights, completely meaningless, since one of the antennas may be located at the point where the resulting signal is maximum, and the other is where the minimum.

In practice, there are many reflective surfaces: the ground, roofs and walls of houses, hills, etc., That is, the main signal coming in straight line from transmitting antenna to the receiving antenna, and interferes with a huge number of secondary waves reflected from these surfaces. The result of such interference is impossible to predict.

What can we do?

  1. When choosing the installation location of the subscriber antenna, it is necessary to take measurements in all places suitable for its installation. Each measurement must be done at several points to understand the distribution of highs and lows of the resulting signal. By the way, they may not be, or they may be mild.
  2. It is necessary to choose a place to install a subscriber antenna so that there are no obstacles in the path of the main signal going in a straight line from the transmitting antenna to the receiving antenna, and there are no reflective surfaces around this path.
  3. When installing an antenna, move the antenna down some distance (a few tens of centimeters – half-meter – meter) and measure the signal level. Then you should move the antenna up some distance and measure the signal level there. There are cases that, depending on the terrain parameters, the interference will be such that the signal will differ by 20-30 dB at these points from each other!