Radio link analysis using SPLAT!

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SPLAT! is a cross-platform, open-source software that can be used to analyze a radio link between two locations and to generate coverage maps of RF transmitters. Coverage maps are calculated using Longley-Rice Irregular Terrain Model (ITM) algorithm. SPLAT! can predict RF coverage for any frequencies between 20 MHz and 20 GHz. It is thus useful for ham radio, broadcast radio, terrestrial television and wireless networks.

To use SPLAT!, you need to know some parameters of the transmitter. These are the exact location (coordinates), antenna height, transmission frequency, polarization and effective radiated power (ERP). SPLAT! can then calculate both path loss and received signal strength.

The procedure of installing SPLAT! is described in a previous article. The point-to-point analysis calculates some useful parameters like: azimuth and elevation of receiving antenna, distance to transmitter, mode of propagation, received signal strength and density. You must also supply a receiver parameters file to SPLAT!. This will contain the location, antenna height and some other terrain parameters. SPLAT! will generate a report and a graph if you have Gnuplot installed.

SPLAT! height profile graph
SPLAT! height profile graph

Input files

Let's mention that SPLAT! takes coordinates as degrees latitude West and degrees longitude North. So if we're talking about European locations, latitude must be negative. I'll use for demonstration purposes a Romanian television transmitter with the following parameters:
  • Location: 45° 25" 37.49 North / 25° 29" 06.0 East;
  • Antenna height: 94 meters (at 2490 meters altitude);
  • Frequency: 474 MHz and horizontal polarization;
  • ERP: 150 kW;
For this transmitter I must make two files with the same name:
tx1.qth
Site-1 ; write whatever name you want here
45 25 37.49
-25 29 06.0
94m
tx1.lrp
15.000 ; Earth Dielectric Constant (Relative permittivity)
0.005 ; Earth Conductivity (Siemens per meter)
301.000 ; Atmospheric Bending Constant (N-units)
474.000 ; Frequency in MHz (20 MHz to 20 GHz)
5 ; Radio Climate (5 = Continental Temperate)
0 ; Polarization (0 = Horizontal, 1 = Vertical)
0.50 ; Fraction of situations (50% of locations)
0.90 ; Fraction of time (90% of the time)
150000.0 ; Effective Radiated Power (ERP) in Watts (optional)
In the LRP file remember to adjust climate and terrain parameters if the case. The default will work for most situations. Adjust frequency, polarization and ERP. The statistical parameters play an important role (situations and time). For digital television in ATSC standard values 0.5 and 0.9 are recommended for situations, respectively time.

The same files are needed for the receiver site too. I guess it doesn't take into account ERP and other transmitter parameters. Here is an example:
home.qth
Home
44.791931
-27.483563
5m
home.lrp
15.000 ; Earth Dielectric Constant (Relative permittivity)
0.005 ; Earth Conductivity (Siemens per meter)
301.000 ; Atmospheric Bending Constant (N-units)
; Frequency in MHz (20 MHz to 20 GHz)
5 ; Radio Climate (5 = Continental Temperate)
0 ; Polarization (0 = Horizontal, 1 = Vertical)
0.50 ; Fraction of situations (50% of locations)
0.90 ; Fraction of time (90% of the time)
; Effective Radiated Power (ERP) in Watts (optional)
It's time to run SPLAT!. The simplest command is the following:
splat -t <path_to_transmitter_qth> -r <path_to_receiver_qth> -metric -m 1.333 -d <path_to_sdf_files>
If everything is OK, SPLAT! will load SDF files and you will see only lines like:
Loading "/path/to/sdf-sd/44:45:333:334.sdf" into page x... Done!
and no line like:
Region  "45:46:332:333" assumed as sea-level into page x... Done!
If you see a line like above it means the corresponding SDF file is missing. The result of running SPLAT! will be a text file named <transmitter>-to-<receiver>.txt that will contain useful information. Here's an overview of some parts of it:
Receiver site: Home
Distance to Site-1: 172.01 kilometers
Azimuth to Site-1: 294.95 degrees
Elevation angle to Site-1: +0.2659 degrees

Field strength at Home: 58.27 dBuV/meter
Signal power level at Home: -72.50 dBm
Signal power density at Home: -87.51 dBW per square meter
Voltage across a 50 ohm dipole at Home: 67.83 uV (36.63 dBuV)
Voltage across a 75 ohm dipole at Home: 83.07 uV (38.39 dBuV)
Mode of propagation: Line of Sight
Besides showing how to point the antenna (azimuth and elevation) it calculates signal level at receiver site. In my example that is -72.5 dBm. Because I'm talking about digital terrestrial television signal and knowing that most receivers have a minimum signal threshold of -80 dBm, this means I can receive the signal as long as there are no losses more than 7.5 dB (result of antenna gain + amplifier gain - cable and other losses should be not less than -7.5 dB).

You may be interested in a height profile map. This is easy to generate if you have gnuplot installed. You must add this argument to SPLAT!: -H <map_profile>.png. For example:
splat -t <path_to_transmitter_qth> -r <path_to_receiver_qth> -metric -m 1.333 -d <path_to_sdf_files> -H linkmap.png
The output image is the graph at top of this post. It contains Earth curvature, terrain profile, line-of-sight line and Fresnel zone.

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