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Where does the signal go?
The science of predicting coverage is complemented by the art of displaying the predictions in a form that makes the most sense of what the predictions mean. A good coverage map will accurately predict coverage over a station's total coverage area. Whether it is a map of station coverage used by your sales force or the FCC map in your public file, knowing where your coverage goes is an essential ingredient in the successful operation of a broadcast facility today. For the station engineer, a most practical application is comparing your station's predicted signal strength with the actual measured values to determine if the station's antenna is operating properly.
All prediction methods begin with a reference standard so that propagation under different conditions can be evaluated. The free-space formula is based on the use of an isotropic radiator that transmits equally in all directions. The strength of the signal transmitted varies inversely with the square of the distance. The free-space formula for a theoretical loss-less antenna is expressed in dB attenuation.
dB (loss) = 32.44 + 20Log(F) + 20Log(d)
In this equation, F is the frequency in megahertz (MHz) and d is the distance between the transmitting antenna and the receiving antenna in kilometers (km). Note that this equation does not consider antenna gain or other losses due to terrain or atmospherics. For broadcast industry-related calculations this formula is usually expressed in dBu or logarithmically in terms of above 1 microvolt per meter. When antenna gain is considered the formula becomes:
E-Field (dBu) = 105 + Power (dBk) + Gain in dB (above isotropic)
Because many radio paths are not line of sight, the free-space formula must be altered to consider the impact of the real world.
Expanding free-space calculations
Waves that glance off the earth are weakened by absorption and to some degree are also reflected. Phase changes caused by reflection can be destructive or additive when the reflected rays mix with the primary rays and other reflections. The degree to which a wave is reflected is based on the frequency of the wave and the ground constants of conductivity and permittivity.
For FM frequencies and those generally above 30MHz, the skywave passes through the ionosphere and ends up in space, so the waves that propagate close to the earth are what we need to consider. These waves tend to diverge more than would be predicted by the inverse square law. Typically, the refractive index of the atmosphere decreases with the height of the radiated waves so that waves closer to the Earth's surface actually travel slower than higher altitude waves. This results in a bending, so that the waves travel farther than the line-of-sight. For the purposes of calculating coverage this bending can be represented as an effective earth radius (Er), which allows the waves to be presented as straight lines. The ratio of Er to the true earth radius can be defined as the constant k. For temperate climates, such as in most of the United States, this can be represented as 1.33 or the ¾ earth model. Most STL paths are plotted over ¾ graph paper, however in an effort to be more conservative, some engineers use true earth.
FM radio stations and TV stations commonly enjoy coverage beyond the radio horizon. The equation used to compute the distance to the radio horizon is:
distance (km) = 3.57 * square root (Ant height * k)
Coverage beyond the radio horizon is said to be defracted and the actual loss is determined by defraction methods including knife-edging and forward scatter.
|Snow and Ice||0.0dB|
|Table 1. The TSB-88 attenuation for various types of land cover.|
One factor that particularly affects a defracted signal is land cover. The TSB-88 standard attenuation table shows the attenuation for various types of land cover. These attenuations will vary depending on the frequency of the waves being propagated. The values for the VHF frequencies are shown in Table 1.
RF planners have noted that there are various iterations of the land cover tables to cover such variations as dense urban, urban and suburban areas.
Building penetration can be a key factor in knowing where your signal goes. For FM frequencies, a building can cause a drop in signal strength of from 4dB to 6dB, while for higher frequencies in the UHF range the attenuation can be as great as 30dB. Vegetation will impact the vertically polarized wave to a greater extent than the horizontal wave. Because vegetation usually varies depending on the time of year, predictive models should define the time of year used for the predictions.
Longley-Rice. In addition to the standard FCC method, the most commonly used prediction method today is Longley-Rice. This method has been used extensively by the FCC for calculating coverage and interference for the U.S. transition to digital TV (HDTV). The basis for the Longley-Rice method is Technical Note 101, published in two volumes by the National Bureau of Standards in the mid-1960s. Going well beyond the FCC curves, the Longley-Rice method considers atmospheric absorption, rain attenuation, sky-noise, terrain roughness, knife-edge refraction, diffraction, forward scatter and long term power fading. A Longley-Rice calculation of signal strength is usually displayed in pools of coverage defined on a map by differing colors. The terrain along a radio path is analyzed from the transmitter all the way to the intended receiving point, where the FCC method considers terrain only from two to 10 miles from the transmitter; a practice that often neglects high points beyond 10 miles that can increase signal strength on the up slope and decrease it on and past the down slope. Longley-Rice maps clearly show a reduction of the predicted signal caused by the lower terrain along rivers and streams.
Terrain databases. It is also important to note that the digital terrain elevation database used for Longley-Rice predictions, or for that matter any other prediction model, should be as accurate as possible. For most analysis work the FCC continues to use a stripped down 30 arc-second version of the 3 arc-second USGS terrain elevation database that was originally digitized from 1:250,000 topographic maps. There are known discrepancies in this database where hill peaks can be off as much as 15 seconds from where the peaks are actually located. (Were the maps wrinkled on the edges when digitized?)
There are vastly improved terrain elevation databases available today, including the Shuttle Radar Topography Mission (SRTM) database taken from space in February 2000 by the shuttle Endeavor. This database has the added advantage in that it contains buildings seen from space. The National Elevation Datum (NED) is a satellite corrected database that uses numerous modern techniques to ensure accuracy. While the SRTM database is available in the 3 arc-second resolution the NED database is available at the 30 meter resolution, which provides a data point at an interval of about every 100 feet. While for large scale mapping of a total coverage area one of the older, less accurate, databases will still be functional, for analysis of specific paths such as in microwave applications, you should always use the best database available.
Many advances in technology and methodology have made real-world prediction more accurate. Whether you are predicting coverage to determine if your antenna is operating properly or producing a map your sales people can use, propagation prediction is a science and an art worthy of being understood by all broadcast engineers and managers.
Vernier is president of V-Soft Communications, Cedar Falls, IA.
A listing of software and services for RF engineering
Many companies offer software packages in addition to those listed, and most also provide consultant services.
AM Query and FM Query are database search engines
FAA Query and TWR Query search the FAA and FCC tower databases
FM Search and FM Study perform FM spacing studies
AM Study, Night Limit, DA Design, ATU Design, Phasor, Diplex and Smith provide various tools to calculate and design AM systems
Pop Count shows the population within a contour
Site Check finds AM, FM and TV authorizations at a particular site or within a specified radius
ACS Map creates contour maps
Reradiation calculates the potential of nearby structures
REALcoverage.com is a Web-based mapping service that provides coverage maps and RF planning studies for AM, FM and TV.
Custom Mapping creates a custom map to provided specifications
Flag Service monitors stations or markets to notify the user of any changes
FM Explorer helps to plan FM allocations
AM and FM studies determine allocation possibilities, coverage areas and interference
Dataflex is a flexible data retrieval service to search based on specified criteria
Comstudy 2.2 runs on Windows and integrates coverage, interference and allocation studies for AM, FM, TV, point-to-point, point-to-multipoint and land mobile services. Features include differential studies, drag-and-drop FCC contour drawing, field strength calculation, 3D viewer of a field or terrain matrix, FM allocations for NCE and translator facilities, AM daytime/nighttime and user-defined ground conductivity studies, AM field calculation of ground and skywave and high-speed Longley-Rice calculation.
RF Investigator V3 provides spacing and contour analyses to present information in graphical and tabular formats; it supports multiple monitors.
RF Detective is a free FM spacing study tool, and databases can be purchased on a month-by-month basis
RF Profiler Light shows the terrain between two points. The free version can be registered to add additional capabilities.
PL-Server produces propagation files to display in RF Investigator DB-Builder is a personal database builder program to create custom databases from the CDBS
AM-Pro 2.0 performs accurate day and nighttime AM allocation studies. The program contains an interactive pattern editor allowing the user to change multiple tower parameters and to instantly view the allocation impact.
FM Commander produces interactive FM spacing and contour-to-contour studies and maps. The program contains a large tool set for completing allocation work including a quick draw path analysis tool.
Probe 3.0 is a multipurpose FM, TV and communications band propagation prediction and mapping program.
InterDLG creates FCC standard coverage and protected vs. interference contour maps RF Haz predicts RF hazard compliance
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