Modeling Propagation Over Forested Terrain Using MWFDTD
Wireless InSite | tagged Radiowave Propagation -
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The moving window FDTD model is capable of modeling the effects of foliage on radio propagation over terrain. In this example, the MWFDTD calculations are compared to path loss measurements for a 230 MHz signal propagating from an open transmitter location to a receiver located in a grove of trees [1]. The measurements are available from the Institute for Telecommunication Sciences (ITS) website.
The transmission path includes two deciduous trees located directly between the transmitter and receiver in addition to the foliage surrounding the receiver. In the ITS report, the trees are described as being 150 m away from the transmitter and 20 m tall [1]. The distance between the transmitter and receiver is approximately 557 meters.
Importing Terrain Data into Wireless InSite
Terrain information for the area surrounding the transmitter and receiver was imported into InSite (Project>Import>Terrain) from Digital Terrain Elevation Data (DTED) information. The terrain in this region is relatively flat and gently slopes downward about three meters from the transmitter to the receiver location. The settings used to import the DTED terrain information are given in Figure 1.
Waveforms and Antenna
In Figure 2, Blackman Envelope with a carrier frequency of 230 MHz and a Pulse width of 0.2 m s was used to describe the waveform. A simple horizontally polarized isotropic antenna was defined for both the transmitter and receiver.
Transmitter Point
A single transmitter point was added to the project and located by setting the tx set's origin to longitude =105.0997 and latitude = 40.1553. The height of the transmitter point was 6.6 meters above the terrain by changing the z value of the point's control point.
Vertical Receiver Route
Figure 3 shows a receiving tower created using a receiver route. Initially, the route was placed in the project at an arbitrary location. The route was then located in the project by setting the set's origin longitude and latitude to the same location as the transmitter point. The vertical route was placed 557 meters east of the transmitter and defined to extend 25 meters from the terrain by editing the location of the set's control points.
Foliage Features
The ITS report describes the transmission path as open, grassy land. About 150 meters from the transmitter, there are two deciduous trees standing about 20 meters high. The receiver tower is surrounded by trees reaching a height of 18 m. InSite's foliage feature was used to model theses areas. Foliage features were created by selecting Project>New>Foliage and tracing out the area foot print of the foliage in the editor. The material properties of the foliage were entered as a complex permittivity of 1.0297 - j0.0382 [2].
Study Area and MWFDTD Settings
The Fit to features option for creating a study area was used to automatically generate a study area encompassing all of the active features in the project. The Moving window FDTD using 5 Cells per wavelength was designated as the propagation model. A picture of the complete project with the transmitters, receivers, and foliage features is shown in Figure 5.
Running the Calculation and Viewing the Results
The calculation can be initiated by clicking Project>Run>New . Calculation progress can be monitored in the Calculation log window. After the calculation is completed, the output is available for view and plotting in the Main project window. To compare the ITS path loss measurements to the calculation results, the path gain of the receivers was plotted. The ITS results were imported into the plot by right-clicking in the graph and selecting Import plot . The MWFDTD calculation shows good agreement with the ITS measurements.
