RF Propagation Definitions

Radar propagation models allow you to predict the propagation and attenuation of radio frequency (RF) waves as signals travel through and interact with the environment.

This page provides a framework for you to understand RF propagation in the context of the Radar Toolbox.

Topics covered include:

How? — Propagation Geometry

Propagation models assume either a straight‑line or a curved refracted propagation path. Curved propagation paths are longer relative to the corresponding slant range, straight-line distance.

In the table below and all following tables, applicable path geometry and frequency are indicated for each term (see How? — Propagation Geometry for more information).

Propagation Geometry Category	Propagation Geometry	Definition	Path Geometry		Frequency
Propagation Geometry Category	Propagation Geometry	Definition	Straight-Line	Curved Path	Low (< 1 GHz)	High (> 1 GHz)
Path Geometry	Straight-Line / Line-of-Sight	Straight line, geometric line-of-sight path.	✔		★	★
Path Geometry	Curved Path	Curved propagation path due to refraction.		✔	★	★
Path Length	One-way path length	Propagated distance between two points (from the transmitter to the target /surface or from the target /surface to the receiver).	✔	✔	★	★
Path Length	Two-way path length	Propagated distance connecting three points (round trip from the transmitter to the target / surface and then back to the receiver for monostatic systems).	✔	✔	★	★

[★] Indicates the propagation geometry is frequency independent.

Propagation path length (propagation distance) is important:

Propagation losses are path length or distance-dependent.
Path length is the total arc length traversed by the signal along all of its propagation path segments.
- When the propagation path is approximated as a straight-line, the arc length is the Euclidean distance of each path segment, referred to as slant range.
- The distance traversed over a curved refracted path is longer than the corresponding geometric slant range.

A one-way path accounts for a single path segment between two points, typically from the transmitter to a target, from a receiver to a target, or between a transmitter and a receiver
A two-way path accounts for two path segments connecting three points. The first path segment is typically from the transmitter to the target and the second is from the target to the receiver.
- For a monostatic radar, it is common to assume that the second path segment is identical to the first, but that the signal propagates along this path in the opposite direction to return back to the radar, and both path segments have the same path length.

The figure below shows the curved refracted propagation path in blue and the corresponding line-of-sight path in black.

See Line-of-Sight and Radar Horizon for more information.

Target angle (height) and range errors are prominent at long range:

If you ignore the geometric impact of refraction on the propagation path, targets will appear at higher altitudes and closer ranges than their actual positions, especially over longer distances.

In the figure below, the black line shows the pointing geometry or line-of-sight of the radar, the blue curve is the actual refracted propagation path, and the orange line is the corresponding slant range distance from the radar to the target. Notable differences between the ideal free-space line-of-sight path and the actual refracted propagation path becomes apparent for distances greater than ~10 km. The angle between the black and blue paths is large at long range. You can see that there is a 0.15 km height error at a distance of 84 km. Similarly, the reported target range will be longer than the geometric slant range because radar range is derived from round trip time. You can see in the figure below that the actual curved propagation path (blue) is longer than the geometric slant range distance (orange), which results in range error.

See Modeling Target Position Errors Due to Refraction for more information.

Why? — Propagation Mechanisms

Propagation mechanisms that you need to model depend on operating environment, radar system specifications (especially frequency), and the level of fidelity that your application requires. Because operating‑environment terminology is context‑dependent and may be defined differently across applications, it is advantageous to distinguish propagation models based on propagation environment.

First-order propagation models assume an ideal free-space propagation environment. At short range or for lower-complexity applications, you can make simplifying assumptions, like a uniform atmosphere, and approximate the propagation path as a straight-line (line-of-site models). For long range and higher-fidelity applications, you need to account for propagation path bending that results from refraction in a layered atmosphere.

Within Radar Toolbox, propagation‑related functions can be subdivided into models that assume a straight‑line or a curved refracted propagation path (see RF Propagation Models). Therefore, propagation mechanisms listed in the tables below are categorized by path geometry (straight-line or curved path) as a proxy for propagation environment. Each mechanism's generally applicable frequency range (low or high) is also indicated.

Path Losses

This section provides an overview of path losses that occur as the RF signal traverses a medium. Losses are mainly a function of distance and frequency.

Propagation Loss Category	Propagation Loss	Definition	Path Geometry		Frequency
Propagation Loss Category	Propagation Loss	Definition	Straight-Line	Curved Path	Low (< 1 GHz)	High (> 1 GHz)
Basic Propagation Loss	Free-Space Path Loss	Loss along the path due to geometric spreading.	★	★	✔	✔
Atmospheric Attenuation	Atmospheric Gas Attenuation	Loss along the path due to atmospheric gas absorption.	★	✔		✔
Atmospheric Attenuation	Fog and Cloud Attenuation	Loss along the path due to water vapor absorption.	★	★		✔
Weather Attenuation	Rain Attenuation	Loss along the path due to rain.	★	★		✔
Weather Attenuation	Snow Attenuation	Loss along the path due to snow.	★	★		✔

[★] Indicates the propagation loss typically depends on the propagated distance (path length) – the path geometry is not considered (see RF Propagation Models for information on specific models).

Path loss is proportional to propagation distance (path length) and frequency:

Loss along the propagation path is proportional to the total distance traversed between the radar transmitter and receiver and depends on frequency.
Emitted RF signals propagate outward from the source in all directions, resulting in an expanding spherical wave front, similar to optical signals. As the propagation path length increases, signal intensity or power decreases rapidly due to geometric spreading.
Ideal free‑space propagation models that assume transmission in a vacuum provide a reference against which additional losses from atmospheric attenuation and surface reflections can be quantified.

The figure below shows the range-dependent impact of propagation on detection.

See Radar Link Budget Analysis for more information.

Straight-line, short range propagation models assume a uniform atmosphere with average gas parameters that correspond to the radar antenna height. For long range applications, gas attenuation is modeled in a layered atmosphere with a curved refracted path. The frequency dependence of atmospheric attenuation makes high frequency radar systems poorly suited for long range surveillance applications.

The figure below shows atmospheric attenuation from gas as a function of frequency.

See Modeling the Propagation of Radar Signals for more information.

Propagation Effects

This section provides an overview of propagation mechanisms that go beyond simple losses and includes surface / target interactions, atmospheric effects, and the impact of obstacles.

Propagation Effect Category	Propagation Effect	Definition	Path Geometry		Frequency
Propagation Effect Category	Propagation Effect	Definition	Straight-Line	Curved Path	Low (< 1 GHz)	High (> 1 GHz)
Atmospheric Effects	Atmospheric Refraction	Signal bending due to gradients in the refractive index of air, or refraction-induced path curvature.		✔	★	★
Atmospheric Effects	Atmospheric Lensing	Signal focusing or defocusing due to localized large-scale refractive variations.		✔	★	★
Surface / Target Effects	Reflection	Signal attenuation due to single-bounce reflection and scattering off the surface / target (returned signal strength depends on material properties and local geometry).	✔	✔	✔	✔
	Multipath Interference	Interference between direct single-bounce path and ground‑reflected rays due to multi-bounce reflections from surfaces and targets (returned signal strength depends on material properties and local geometry).	✔	✔	✔	✔
	Diffraction	Bending and spreading around objects (including the Earth surface) that enables signal propagation into shadow regions.	✔	✔
	Occlusion	Signal attenuation due to blocking caused by terrain or targets.	✔	✔	★	★

[★] Indicates the propagation effect is largely considered to be frequency independent for Typical Radar Frequencies.

Refraction effects:

RF signals refract due to the decreasing index of refraction of air with altitude, which bends the propagation path downward.
Refraction impacts RF propagation path geometry and length and therefore affects measured target positions (see Angle (Height) and Range Error From Refraction).
Refraction is largely frequency independent and does not directly result in a loss.
- Curved refracted propagation paths are longer relative to the corresponding slant range, geometric distance, and therefore incur more atmospheric attenuation.
- Refraction-induced lensing can result in additional path losses or gains, if present.
- Anomalous refraction-induced propagation effects, including ducting and subrefraction, are beyond the scope of this page.

Multipath effects:

Interference between direct single-bounce path and ground‑reflected rays due to multi-bounce reflections can disproportionately impact the received signal and is strongly frequency dependent.
- Multipath reflections that combine coherently cause lobing in elevation coverage and fluctuations in detection range when the difference between the direct single-bounce path and bounce path length is small (see Multipath Fading).
- Multipath reflections can cause apparent ghost targets when the difference between the direct single-bounce path and bounce path length is large enough to be resolved (see Multipath Ghosts).
The figure below shows a two-bounce multipath geometry, with the direct single-bounce path in black and multipath reflections in orange.

The figure below shows lobes caused by multipath fading that are evident in a Blake chart (see Propagation Losses and Power-Level Models for more information on blakechart).

See Modeling the Propagation of Radar Signals for more information.

Where? — Surface / Target Interfaces

The reflection and scattering of RF signals depends on material properties, frequency, and geometric configuration.

Surface / Target Characteristic Category	Surface / Target Characteristic	Definition	Path Geometry		Frequency
Surface / Target Characteristic Category	Surface / Target Characteristic	Definition	Straight-Line	Curved Path	Low (< 1 GHz)	High (> 1 GHz)
Surface / Target Properties	Material Composition	Dielectric constant / relative permittivity controls signal absorption and reflection.	★	★	✔	✔
Surface / Target Properties	Surface Texture	Surface roughness controls signal scattering.	★	★	✔	✔
Scattering Geometry	Incident Angle	Angle of the incoming signal at the surface, modified by Earth's curvature and/or local topography.	✔	✔	★	★
Scattering Geometry	Scattering Angle	Angle of the outgoing signal reflected from surface, modified by Earth's curvature and/or local topography.	✔	✔	★	★

[★] Indicates the surface / target characteristic is independent of path geometry or frequency.

Surface / target reflectivity:

Surface / target dielectric constant controls the proportion of the signal that is reflected.
Surface roughness (large and small-scale) controls the scattering behaviour.
Normalized Radar Cross Section (NRCS) parametrizes the geometric and frequency dependent reflection and scattering behavior of materials.
- Targets / surfaces with larger NRCS reflect more strongly and therefore have larger Radar Cross Section (RCS), which is a measure of the amount of energy returned in the direction of the receiver.
- rcsSignature and brcsSignature model target RCS.
Noise-like echos that are returned from surfaces (clutter) are addressed in Radar Surface Clutter Simulation

NRCS as a function of grazing angle is plotted in the figure below for a wooded and an urban surface. The urban surface is brighter and would therefore produce stronger returns, assuming everything else is held constant including radar power and distance. Note that the large-scale curvature of the Earth surface modifies the incoming geometry at long distances (see Grazing Angle).

See Radar Surface Clutter Simulation for more information.

What? — Propagation Phenomena

This section summarizes basic radar concepts and phenomena that arise due to various propagation mechanisms. For more information on each phenomenon listed in the table, see RF Propagation Concepts.

Phenomenon	Definition	Path Geometry		Frequency
Phenomenon	Definition	Straight-Line	Curved	Low (< 1 GHz)	High (> 1 GHz)
Line-of-Sight	Unobstructed straight-line, slant range geometric path between the radar and target.	✔		★	★
Radar Horizon	Farthest distance from a radar to a point on the Earth surface at which the radar beam remains unobstructed, factoring in Earth curvature and the effects of atmospheric refraction on the path geometry.		✔	★	★
Multipath Ghosts	Apparent erroneous target detections caused by multipath reflections – occurs when the difference between the direct single-bounce path and bounce path length is large enough to be resolved.	✔	✔	✔	✔
Multipath Fading	Lobing in elevation coverage and fluctuations in detection range caused by multipath reflections that combine coherently – occurs when the difference between the direct single-bounce path and bounce path length is small.	✔	✔	✔	✔
Angle (Height) and Range Error From Refraction	Ignoring refracted path curvature causes targets to appear at higher altitudes and closer ranges than their actual positions.	✔		★	★

[★] Indicates the phenomenon is largely considered to be frequency independent for Typical Radar Frequencies.

More About

expand all

Typical Radar Frequencies

Low frequency radar channels are below ~1 GHz and include HF (3-30 MHz), VHF (30-300 MHz), and UHF (300-1000MHz). [1].

High frequency radar channels are above ~1 GHz and include L-band (1-2 GHz), S-band (2-4 GHz), C-band (4-8 GHz), X-band (8-12 GHz), and Ku/K/Ka bands (12-40 GHz) [1].

Millimeter-wave radars operate between around 30 to 100 GHz and include V/W bands (40-110 GHz).

Information presented in this page is relevant for typical radar frequencies and is not geared toward mm-wave propagation. For example, refraction effects become increasingly frequency dependent at mm-wave frequencies, especially above 60 GHz. However, most commercially available mm-wave radars that operate above 60 GHz are designed for ranges < 250 m, and refraction is typically ignored.

References

[1] "IEEE Standard Letter Designations for Radar-Frequency Bands." IEEE Std 521-2019 (Revision of IEEE Std 521-2002) , vol., no., pp.1-15, 14 Feb. 2020, doi: 10.1109/IEEESTD.2020.8999849.

[2] Richards, M. A., & Melvin, W. L. (Eds.). Principles of Modern Radar: Basic Principles (2nd ed.). SciTech Publishing, 2023. ISBN 978‑1839533815.

[3] J. S. Seybold, Introduction to RF Propagation. Hoboken, NJ: John Wiley & Sons, 2005.