The Science of Line-of-sight propagation and Obstacles

Line-of-sight propagation relies on a clear path between the transmitter and receiver, but obstacles can disrupt this connection.

Buildings, hills, and mountains can all interfere with the signal, causing it to bend or be blocked.

Trees and foliage can also absorb or scatter the signal, reducing its strength and quality.

The height of the transmitter and receiver can make a big difference in the effectiveness of line-of-sight propagation.

A fresh viewpoint: Nauen Transmitter Station

A radio path study is essential to determine the quality of the link between endpoints. This study is typically conducted by specialists who use various resources to map the path between endpoints.

To accurately map the path, specialists use a topographic map to identify potential obstructions. They also consider the Fresnel Zone, which defines the critical keep-out area for an unobstructed line-of-sight path.

Maintaining at least 60% of the first Fresnel zone radius free of obstructions is a best practice to avoid fading of the received signal. Signal losses can be as much as 6 dB or more when obstacles intrude into the first Fresnel zone.

For more insights, see: Signal Strength in Telecommunications

Conducting a radio path study early on can save resources later. It's also a good idea to contact the local building department to determine if any new high-rise buildings or towers are being planned for the area within the path.

A radio path study typically includes visual depictions of the path and identifies potential obstructions. It's a crucial step in designing a reliable communication system.

Physical elements like mountains, buildings, and metallic structures can cause some of the propagating wavefront to be reflected, potentially increasing or decreasing the received signal level depending on the relative phase and amplitude of the wavefronts.

These reflected wavefronts can arrive at the receiving antenna at the same time as the direct wavefront, making them a significant factor in determining the quality of the signal.

Obstacles like mountains and hills can cause significant signal loss if they block the direct line-of-sight path. In such cases, diffraction becomes the dominant propagation mechanism, and the signal is heavily affected by the shape of the obstacle and the path geometry.

Consider reading: Mobile Phone Signal

For obstacles with smooth, rounded surfaces, the signal can be totally obliterated. However, if the obstacle has a sharp, knife-edge type of profile, a greater portion of the propagating wavefront will be diffracted around or over the obstacle.

To maintain an unobstructed line-of-sight path, a minimum volume of space normal to the direct LOS path must be kept free of obstructions. This required keep-out area is defined via the concept of Fresnel zones.

A Fresnel zone can be imagined as the area of a circle centered on and perpendicular to a given point located on the direct LOS path. The radius of the circle is related to the position of the point along the LOS path, with its maximum at the midpoint and its minimum at the endpoints.

The first (inner-most) Fresnel zone defines the critical keep-out area for an unobstructed LOS. Obstructions that intrude into this zone can decrease or fade the received signal level, with signal losses as much as 6 dB or more at the point where the obstruction becomes tangent to the LOS path.

Line-of-sight propagation relies on a clear path between the transmitter and receiver, but several factors can impair this propagation.

Tree branches, heavy rain, or snow can disrupt radio transmissions by causing diffraction effects. The first Fresnel zone should be free of obstructions for the best propagation.

Raising antennas further from the ground can reduce loss caused by reflected radiation from the surface of the surrounding ground or salt water, known as height gain.

The curvature of the Earth must be taken into account when calculating line-of-sight paths from maps, especially when a direct visual fix cannot be made.

Here's a list of factors that can impair line-of-sight propagation:

Line-of-sight (LoS) propagation is a characteristic of electromagnetic radiation where two stations can transmit and receive data signals only when they're in direct view of each other with no obstacles in between. This type of propagation is common in satellite and microwave transmission.

The Earth's curvature affects LoS propagation, causing high-frequency waves to eventually become tangential to the surface and shoot into space. As a result, a station positioned beyond the distance where the signal just touches the surface cannot receive the transmission.

For LoS propagation to work, a receiver must be positioned within the circular region surrounding the transmitter, with a radius equal to the signal's tangential distance. The higher the transmitter is, the further out the LoS propagation distance will be.

Radio signals and light waves are manifestations of electromagnetic (EM) radiation, differing chiefly in frequency. In LoS propagation, EM energy radiates from an antenna and propagates through space as a sequence of ever-expanding, spherical wavefronts.

The direct path between the transmitting and receiving antennas is often referred to as the line-of-sight (LOS) path of propagation. However, a number of factors can impact this path, including the presence of obstacles.

To maintain an unobstructed LoS path, a minimum volume of space normal to the direct LOS path must be kept free of obstructions. This required keep-out area is defined via the concept of Fresnel zones.

The first (inner-most) Fresnel zone defines the critical keep-out area for an unobstructed LoS. A best practice is to maintain at least 60% of the first Fresnel zone radius free of obstructions to avoid fading of the received signal.

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Here's a breakdown of the Fresnel zones:

The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth. The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range.

The maximum service range of a station is not equal to the line of sight vacuum distance due to the refractive effects of atmospheric layers. A factor k is used in the equation, where k > 1 means a longer service range and k < 1 means a shorter service range.

A radio path study is essential to determine the quality of the link between endpoints. This study involves mapping the path between endpoints to determine the best path, Fresnel Zone obstructions, and the need for ancillary equipment.

Take a look at this: Alternate Line Service

Propagation impairments can be a real challenge for line-of-sight propagation. Tree branches, heavy rain or snow, and even the curvature of the Earth can all disrupt radio transmissions.

The presence of objects not in the direct line-of-sight can cause diffraction effects that disrupt radio transmissions. For the best propagation, a volume known as the first Fresnel zone should be free of obstructions.

Reflections from the surface of the ground or salt water can also affect the signal, either canceling it out or enhancing it. Raising antennas further from the ground can reduce this effect, known as height gain.

Here are some specific signal loss values due to reflection or diffraction:

Signal loss due to reflection and diffraction can be a significant issue in radio transmissions. It's essential to understand how these impairments affect your signal.

For a path of propagation where the first Fresnel zone is clear of obstructions, signal loss due to reflections or diffraction is negligible. This is the ideal scenario, and it's what we strive for in our installations.

Heavy rain or snow can cause significant signal loss, and so can tree branches. These obstructions can create diffraction effects that disrupt radio transmissions. The presence of objects not in the direct line-of-sight can cause problems.

If more than 60% of the first Fresnel zone radius is obstructed, but the direct LOS is not obstructed, signal loss due to reflections can be up to 6 dB. This is a significant loss, and it's something we need to consider when designing our systems.

A single knife-edge type of obstruction can cause even more signal loss. If it occludes the direct LOS, but less than 60% of the first Fresnel zone radius above the LOS, signal loss due to diffraction can be up to 14 dB. This is a substantial loss, and it's something we need to take into account.

For a path of propagation where a single knife-edge type of obstruction occludes the direct LOS and the entire first Fresnel zone, signal loss due to diffraction can be greater than 20 dB. This is a critical loss, and it's something we need to mitigate in our designs.

Here's a summary of signal loss due to reflection and diffraction:

A direct line-of-sight path is crucial for radio transmissions, but it's not always possible due to various obstacles.

Tree branches, heavy rain, or snow can easily disrupt low-powered microwave transmitters.

The presence of objects not in the direct line-of-sight can cause diffraction effects that disrupt radio transmissions.

The first Fresnel zone is a critical area that should be free of obstructions for the best propagation.

Raising antennas further from the ground can reduce signal loss, known as height gain.

Reflected radiation from the ground or salt water can either cancel out or enhance the direct signal.

To minimize the effect of reflected radiation, it's essential to maintain at least 60% of the first Fresnel zone radius free of obstructions.

Obstacles in the first Fresnel zone can lead to signal losses of up to 6 dB.

The curvature of the Earth must be considered when calculating line-of-sight paths from maps.

A single knife-edge type of obstruction can cause signal loss due to diffraction of up to 14 dB or more.

Here's a summary of signal loss due to reflection or diffraction:

The Earth's atmosphere also affects LOS propagation, causing signals to bend towards the Earth. This bending is due to the declining pressure of the atmosphere with height, also known as vertical pressure variation.

The effect of the atmosphere on LOS propagation can be significant, increasing the effective Earth radius by a factor of around 4/3.