It is easy to understand when some principles are well understood.
- Line Of Sight (LOS) in wireless telecommunication
- The annihilation of an electromagnetic wave occurs when a phase shift is equal to 180°
- Mathematical properties of an ellipse
Annihilation of an electromagnetic wave when a phase shift of 180°occurs.
When an EM wave is transmitted, a reflection on an obstacle can result in a phase shift of precisely 180° to the direct wave. Both waves will be subtracted, and depending on the strongness of both signals, the signal can be strongly reduced in strength or even annihilated (equal strength).
Mathematical properties of an ellipse
When we know the properties of an ellipse, we can easily see what it has to do with the annihilation of a signal if an obstacle sits on a particular ellipse between the transmit and receive antennas.
Each point of the ellipse has the property that the sum of both distances to both focal points is a constant distance.
This means that if you put the transmit and receive antennas in each of the focal points, you can find ellipses in which each point will reflect the signal 180° compared to the direct signal. We will call the first ellipse that does this the first Fresnel zone. Other ellipses will exist that are multiples of half–wave distances. These will be the 2nd, 3rd, and 4th Fresnel zones.
To make the LOS communication work, obstacles should be avoided in the first Fresnel zone. For example, half-wave reflections that could substantially reduce the received signal strength should be avoided. All the obstacles that would create such a phase shift will be positioned on an ellipse.
Conclusion:
A Fresnel zone is an ellipse where transmitting and receiving antennas are placed in the focal points. And where the ellipse distances are multiple of a half wave compared to the direct Line of Sight. An obstacle in these zones will reduce or annihilate the received signal.
A line of sight is not enough to achieve a perfect transmission. A more expansive open zone around the beam has to be considered to avoid signal reduction in the receiver. We want to remind you that DAB+ is not subject to this phenomenon. DAB+ uses a low bit rate on multiple carriers, so practical reflections within the transmitter’s operational range will not reduce the received.