The Fresnel effect is that set of interference phenomena that are always present in radio frequency transmissions. The use of high frequency also requires that the antennas are within optical range and that there are no obstacles of any kind interposed.
We define LOS (Line of Sight) or "line of sight" that direct line of sight without obstacles between two points. This last condition is easily verifiable and, in the case of particularly long distances, the use of binoculars is a valid help. The obstacles that can obscure LOS can be of various kinds:
- Characteristic elements of the area: mountains or hills;
- Palaces or other buildings;
- Plants or woods;
- Earth curvature: only at great distances
In a radio link it is not enough to consider the LOS alone, part of the radiated energy is around it. One can imagine this area as an ellipsoid or an airship whose axis is the LOS itself. This space is referred to as Fresnel area and should never be crossed by objects or items listed above. If a solid object such as a mount or a building falls within this zone, the signal can be deviated (by reflection) and / or attenuated by power (by absorption or by multiple paths of the signal). The Fresnel area assumes variable dimensions depending on the frequency and the signal path.
The image above may represent the typical example of a non-free Fresnel zone, although the LOS turns out to be so. Diffraction and reflection phenomena can deflect part of the original signal. Since these reflections are never in phase, the signal can be attenuated in power or completely canceled (typical in multipath phenomena). The presence of plants also attenuates the signal.
From these considerations it is clear that antennas with direct "optical visibility" do not theoretically guarantee the efficiency of a radio link. Practical tests "in the field" have however shown that it is sufficient to have 60% of the maximum radius of the Fresnel zone free from occlusions to have an efficient link and in particular, 60% for DSSS modulation and 80% for FHSS Frequency Hopping Spread Spectrum.
The Fresnel zone calculation is used to dimension the height position of antennas. In case of particular occlusions, it is advisable to opt for the change of position of the antenna in order to have at least 60% of the maximum radius of this free area. Let's see how to perform the calculation of the Fresnel zone at a given point.
R =17.3 *sqrt((d1*d2)/(f*(d1+d2)))
Where:
R: Radius of the Fresnel area expressed in meters
d1, d2: Distance of the obstacle from the antennas expressed in meters
f: Signal frequency expressed in Mhz
For example the Fresnel area in the middle of a radio link of 2 km, which transmits to 2.437GHz (802.11b channel 6) is:
R = 17.31 * sqrt ((1000 * 1000) / (2437 * 2000)) equal to 7.84 m