# ITU-R Recommendation P.833-10 (September 2021) — Attenuation in vegetation **Citation:** International Telecommunication Union. *Recommendation ITU-R P.833-10: Attenuation in vegetation*. Geneva: ITU Radiocommunication Sector, September 2021. Question ITU-R 202/3. Approval history: 1992, 1994, 1999, 2001, 2003, 2005, 2007, 2012, 2013, 2016, 2021. 33 pages. **Series:** P (Radiowave propagation). **Archive:** ITU Publications, `itu.int/pub/R-REC-P.833`. ## Scope and applicability ![[2021_ITU-R_P833-10_Vegetation_p01_scope_section2.png]] Page 1 of the Recommendation (PDF page 3) states the scope verbatim: > "This Recommendation presents several models to enable the reader to evaluate the effect of vegetation on radiowave signals. Models are presented that are applicable to a variety of vegetation types for various path geometries suitable for calculating the attenuation of signals passing through vegetation. The Recommendation also contains measured data of vegetation fade dynamics and delay spread characteristics." The `recommends` clause gives the frequency range and the applicable geometries: > "that the content of Annex 1 be used for evaluating attenuation through vegetation with various models addressing a frequency range from 30 MHz to 100 GHz." Section 1 *Introduction* states: > "Attenuation in vegetation can be important in some circumstances, for both terrestrial and Earth-space systems. However, the wide range of conditions and types of foliage makes it difficult to develop a generalized prediction procedure." Section 2 *Obstruction by woodland*, §2.1 *Terrestrial path with one terminal in woodland*: > "For a terrestrial radio path where one terminal is located within woodland or similar extensive vegetation, the additional loss due to vegetation can be characterized on the basis of two parameters: the specific attenuation rate (dB/m) due primarily to scattering of energy out of the radio path, as would be measured over a very short path; the maximum total additional attenuation due to vegetation in a radio path (dB) as limited by the effect of other mechanisms including surface-wave propagation over the top of the vegetation medium and forward scatter within it." ## Equation 1 — excess attenuation in woodland ![[2021_ITU-R_P833-10_Vegetation_p02_eq1_figure1.png]] Page 2 (PDF page 4) gives the core equation for excess vegetation attenuation when the transmitter is outside the woodland and the receiver is at a distance $d$ within the woodland: $A_{ev} = A_m \left[ 1 - \exp\left(- \frac{d \gamma}{A_m}\right) \right] \quad \text{(Eq. 1)}$ where: - $d$ = length of path within woodland (m) - $\gamma$ = specific attenuation for very short vegetative paths (dB/m) - $A_m$ = maximum attenuation for one terminal within a specific type and depth of vegetation (dB) The Recommendation explicitly notes: > "$A_m$ is equivalent to the clutter loss often quoted for a terminal obstructed by some form of ground cover or clutter." Figure 1 on the same page shows the geometry: the transmitter is outside the woodland, the receiver is inside, at a distance $d$ through the foliage. ## Figure 2 — specific attenuation due to woodland, frequency dependence ![[2021_ITU-R_P833-10_Vegetation_p03_figure2_specific_attenuation.png]] Page 3 (PDF page 5) shows Figure 2, the graph of specific attenuation $\gamma$ (dB/m) due to woodland as a function of frequency from approximately 30 MHz to 100 GHz, separately for vertical and horizontal polarisation. Typical values: - At 100 MHz: γ ≈ 0.04 dB/m (horizontal), 0.08 dB/m (vertical) - At 1 GHz: γ ≈ 0.15 dB/m - At 10 GHz: γ ≈ 0.5 dB/m The figure notes that "Below about 1 GHz there is a tendency for vertically polarized signals to experience higher attenuation than horizontally, this being thought due to scattering from tree-trunks." Table 1 on the same page gives measured parameters from a Russian campaign in mixed coniferous-deciduous forest near St. Petersburg, for paths of a few hundred metres to 7 km through trees of mean height 16 m, at 105.9 MHz, 466.475 MHz, 949.0 MHz, 1852.2 MHz, and 2117.5 MHz. ## Figure 3 — representative radio path geometry in woodland ![[2021_ITU-R_P833-10_Vegetation_p05_figure3_path_geometry.png]] Page 5 (PDF page 7) shows Figure 3, the representative geometry for the vegetation-attenuation models. The transmitter Tx is elevated at radio path elevation θ, and the receiver Rx is at antenna height $h_a$ over the ground inside the woodland. Average tree height is $h_v$, vegetation path length is $d$, and the distance from the antenna to the roadside woodland is $d_r$. All of the key parameters ($d$, $h_a$, $h_v$, $d_r$) are defined in metres and represent a geometry where the receiver is at or below canopy level. The equation that follows (Eq. 3 in the Recommendation) is calibrated to pine woodland measurements from Austria: $L(\text{dB}) = A f^B d^C (\theta + E)^G$ with empirical parameters fit to measurements where the terminal is near ground level inside or adjacent to the forest. ## Section 3 — single vegetative obstruction ![[2021_ITU-R_P833-10_Vegetation_p07_section3_single_obstruction.png]] Page 7 (PDF page 9) introduces Section 3, *Single vegetative obstruction*, which covers paths where a finite block of vegetation lies between the two terminals. §3.1 covers frequencies at or below 1 GHz and notes that Equation 1 should not be used for single isolated trees or small groups of trees, because the formula is calibrated for extensive woodland. §3.2 covers frequencies above 1 GHz with a more detailed diffraction-plus-through-scattering model. The geometry throughout Section 3 assumes the vegetation block is between the terminals. It does not cover paths where the terminals are separated by Earth curvature with no vegetation in the radio path. ## What the Recommendation does NOT cover The Recommendation is titled *Attenuation in vegetation* and the models inside it are all about additional path loss contributed by foliage. Three things the Recommendation is explicitly not for: 1. **Earth-curvature diffraction loss.** The Scope says the Recommendation covers "attenuation of signals passing through vegetation". Loss caused by a signal having to bend around the geometric horizon of a sphere is a different mechanism, covered separately in ITU-R P.526 *Propagation by diffraction*. P.833 and P.526 are complementary, not substitutes. 2. **Paths where the receiver is far above canopy height.** Every figure in the Recommendation (Figures 1, 3, 4, 7, 8) shows the receiver at or below the vegetation top. The models are calibrated to measurements where one or both terminals are within or immediately adjacent to the woodland. The Recommendation does not give parameters for path geometries where the receiver is at thousands of metres altitude above all ground-level vegetation. 3. **Free-space or clutter-free paths.** §2 equation 1 gives *additional* loss $A_{ev}$ on top of free-space loss, only for the portion of the path that actually passes through the vegetation medium. A path that does not pass through vegetation at all has $A_{ev} = 0$ by construction. ## See also - [[Knickebein_Propagation_Null]] - [[DC_Dan]] - [[GRWAVE_P368_BotB]]