4. Antenna and Propagation Modeling

4.1. splat

splat is a Surface Path Length And Terrain analysis application which can perform path loss calculations as well as generate coverage maps. Primarily intended for VHF/UHF, it can help plan repeater coverage or plan emergency communications strategies.

4.1.1. Installation and setup

Installing splat is straightforward:

	su -c 'yum install splat'

4.1.1.1. Obtaining Terrain Files

Before it can be useful, splat requires files that describe the terrain around the station to be modelled. First, determine the latitude and longitude of the station. Then download the nine terrain files centered on that latitude and longitude from http://e0srp01u.ecs.nasa.gov/srtm/version2/SRTM3/.

Unzip the nine files and convert them from hgt files to sdf with the srtm2sdf utility. For example:

	  srtm2sdf N41W082.hgt

Do this for each of the nine files. Those files can now be placed in a directory where you wish to store terrain files, or they can be placed in the directory where you wish to work with splat

If you will be modelling stations over a wide geographic area, you may wish to download and convert additional files. splat will select those files it requires for a particular calculation.

4.1.1.2. Obtaining cartographic boundary files

splat will work with just the terrain files. However, for path loss maps, the resulting maps can be more useful if they are marked with political boundaries and names of towns and cities. For the United States, county outlines can be downloaded from http://www.census.gov/geo/www/cob/co2000.html#ascii and 'census designated areas' from http://www.census.gov/geo/www/cob/pl2000.html#ascii.

For each of these, there are two files, an xxyy_d00.dat and xxyy_d00a.dat, where xx is 'co' for county and 'pl' for place, and yy is a state number. A file of place names can be generated from the 'a' file with the citydecoder utility. For example:

	  citydecoder pl37 >cities.dat

The cities.dat file is simply a list of names followed by latitude and longitude. You may edit the file with a text editor to insert additional places which will be marked on the map with a red dot.

4.1.2. Using SPLAT!

splat can perform calculations for a particular path, or generate a map showing path loss or signal strength over a region. In any case splat needs at least one file identifying the transmitter location. For a specific path, it needs an identical file for the receiver. If you would like signal strength calculations, you will need another file with more details about the transmitter.

4.1.2.1. The QTH file

You tell splat about a particular station (transmitter or receiver) with a qth file. This file has four lines:

The name of the station
The latitude of the station
The longitude of the station
The antenna height above ground

Here is an example qth file:

The qth file should be named for the station. The name of the file in the above example would be W8KEA-4.qth.

By default, splat uses British units; heights are in feet, distances are in miles. However, invoking splat with the -metric switch will cause it to use metric units.

4.1.2.2. The LRP file

If you would like splat to calculate signal strengths, it needs to know a little more about the transmitter. You provide this information in a file whose name matches that of the qth file but has an extension of lrp.

The lrp file has 9 lines:

Earth Dielectric Constant. If you do not have measured data available, the splat man page has a table that can help you estimate a value.
Earth Conductivity
Atmospheric Bending Constant
Frequency
Radio Climate. This is a code describing the terrain. See the table in the man page
Polarization
Fraction of situations. This and the following line reflect how the Longley-Rice calculations are to be carried out. In the example below, splat will calculate the maximum path loss experienced 50% of the time in 50% of the situations.
Fraction of time
Effective radiated power - power out less feedline loss times antenna gain

	  15.000  ; Earth Dielectric Constant (Relative permittivity)
	  0.005   ; Earth Conductivity (Siemens per meter)
	  301.000 ; Atmospheric Bending Constant (N-Units)
	  145.090 ; Frequency in MHz (20 MHz to 20 GHz)
	  5       ; Radio Climate
	  1       ; Polarization (0 = Horizontal, 1 = Vertical)
	  0.50    ; Fraction of situations
	  0.50    ; Fraction of time
	  126.00  ; ERP

You may leave out the last line in which case splat will calculate only path loss.

4.1.2.3. Making a map of coverage

Coverage Map with constrained distance

Figure 15. Coverage map with constrained distance

Coverage Map with unconstrained distance

Figure 16. Coverage map with unconstrained distance

Signal strength legend at bottom of map

Figure 17. Signal Strength Legend