47 C.F.R. Subpart P—Standards for Computing Public Coast Station VHF Coverage
Title 47 - Telecommunication
This subpart specifies receiver antenna terminal requirements in terms of power, and relates the power available at the receiver antenna terminals to transmitter power and antenna height and gain. It also sets forth the co-channel interference protection that VHF public coast station geographic area licensees must provide to incumbents and to other VHF public coast station geographic area licensees. [64 FR 26887, May 18, 1999] (a) The requirements for reception by a marine VHF shipboard receiver are satisfied if the field strength from the coast station, calculated in accordance with §80.771 is at least +17 dBu above one microvolt. (b) These field strengths, voltages and powers at the receiver input are equivalent: (1) −132 dBW (decibels referred to 1 watt). (2) 1.8 microvolts across 50 ohms. (3) +17 dBu (decibels referred to 1 microvolt per meter). (4) 7 microvolts per meter. Applications for maritime frequencies in the 156–162 MHz band must include a map showing the proposed service area contour. The service area contour must be computed in accordance with the following procedures. (a) In the preparation of profile graphs and in determining the location and height above sea level of the antenna site, the elevations or contour intervals must be taken from U.S. Geological Survey topographic quadrangle maps, U.S. Army Corps of Engineers maps or Tennessee Valley Authority maps, whichever is the latest, for all areas for which maps are available. If such maps are not published for the area in question, the next best topographic information must be used. The maps used must include the principal area to be served. U.S. Geological Survey topographic quadrangle maps may be obtained from the Eastern Distribution Branch, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202, for maps of areas east of the Mississippi River, including Minnesota, Puerto Rico, and the Virgin Islands, and from the Western Distribution Branch, U.S. Geological Survey, Federal Center, Denver CO 80225, for maps of areas west of the Mississippi River, including Alaska, Hawaii, Louisiana, Guam and American Samoa. Sectional aeronautical charts are available from the Distribution Division, National Ocean Service, Riverdale, MD 20840. (b) In lieu of maps, the average terrain elevation may be computer generated, using elevations from a 30 second point or better topographic data file such as those available for the U.S. Geological Survey's National Geographic Information Center or the National Oceanic and Atmospheric Administration's National Geophysical Data Center. In case of dispute maps will be used to determine the correct value. (a)(1) Draw radials from the antenna site for each 45 degrees of azimuth starting with true north. Any such radial which extends entirely over land from the antenna site to the point of +17 dBu field strength need not be drawn. (2) If the distance from the antenna site to the point of +17 dBu field strength between any of the 45 degrees radials would be less than the distances calculated along these radials, an additional radial between such adjacent radials must be plotted and calculations made in each case. Each additional radial must be that radial along which it appears by inspection that transmission loss would be greatest. (b) Draw a circle of 16 km (10 statute mile) radius using the antenna site as the center. Divide each radial into 320 meter (0.2 statute mile) increments inside the circumference to the 3.2 km (2 statute mile) point. (c) Calculate the height above sea level of each 320 meter (0.2 statute mile) division by interpolating the contour intervals of the map, and record the value. (d) Average the values by adding them and dividing by the number of readings along each radial. (e) Calculate the height above average terrain by averaging the values calculated for each radial. [51 FR 31213, Sept. 2, 1986, as amended at 58 FR 44953, Aug. 25, 1993] The following graphs must be employed where conversion from one to the other of the indicated types of units is required. (a) Graph 1. To convert effective radiated power in watts to dBk or to dBW, find the power in watts on the horizontal axis. Move vertically along the line representing the power to the diagonal line. Move horizontally from the diagonal to the right side to read dBW and to the left to read dBk. (b) Graph 2. To convert microvolts across 50 ohms to received power in dBW, find the signal in microvolts on the horizontal axis. Move vertically to the diagonal line, then move right horizontally to read dBW. (c) Graph 3. To convert received power in dBW to field intensity in dBu find the received power in dBW on the horizontal axis. Move vertically to the diagonal line, then move right horizontally to read dBu. The effective height of the antenna is the vertical distance between the center of the radiating system above the mean sea level and the average terrain elevation. Effective radiated power is used in computing the service area contour. The effective radiated power is derived from the transmitter output power, loss in the transmission system including duplexers, cavities, circulators, switches and filters, and the gain relative to a half-wave dipole of the antenna system. The propagation graph, §80.767 Graph 1, must be used in computing the service area contour. The graph provides data for field strengths in dBu for an effective radiated power of 1 kW, over sea water, fresh water or land (smooth earth); transmitting antena heights of 4,800, 3,200, 1,600, 800, 400, 200, and 100 feet; based on a receiving antenna height of 9 meters (30 feet), for the 156–162 MHz band. The use of this is described in this section. (a) Calculate the effective radiated power of the coast station, Ps in dB referred to 1 kW (dBk), as follows:
where, Pt=Transmitter output power in dB referred to 1 kW: Transmitter output power in watts is converted to dBk by Pt=10 [log10 (Power in watts)]−30. Also see §80.761 Graph 1 for a conversion graph. G=Antenna gain in dB referred to a standard half-wave dipole, in the direction of each plotted radial, and L=Line losses between the transmitter and the antenna, in dB. Notes: 1. To determine field strengths where the distance is known, for effective radiated powers other than 1kW (0 dBk): Enter the graph from the “statute miles” scale at the known distance, read up to intersection with the curve for the antenna height, read left to the “dBu for 1 kW radiated” scale and note the referenced field strength (Fe). The value of the actual field strength (F) in dBu will be F=Fe+Ps where Ps is the effective radiated power calculated above. 2. To determine distance, where the actual field strength is specified, for effective radiated powers other than 0 dBk: The value of the field referenced strength will be Fe=F−Ps in dBu. Enter the graph, from the “dBu for 1 kW radiated” scale at the corrected value of Fe, read right to intersection with the antenna height, read down to “statute miles” scale. (b) Determine the antenna height. For antenna heights between the heights for which this graph is drawn, use linear interpolation; assume linear height-gain for antennas higher than 4,800 feet. (c) For receiver antenna heights lower than 9 meters (30 feet), assume that the field strength is the same as at 9 meters (30 feet). (d) Assume that propagation over fresh water or over land is the same as that over sea water. Where the transmission path is obstructed the received signal must be adjusted to include shadow loss. Attenuation due to shadowing must be taken from §80.769 Graph 1, as follows: (a) Inspect the map(s) to determine if a hill(s) obstructs an imaginary line of sight (dashed line on illustrative profiles of §80.769 Graph 1 from the average terrain elevation at the coast station antenna to the water level at the ship location. If average terrain elevation exceeds the actual ground elevation at the antenna site, the latter elevation must be used as the average terrain elevation. (b) If a hill appears to obstruct the radio path, plot the antenna site elevation, the obstruction elevation and the height of the ship station on rectangular coordinate paper using elevation above mean sea level as the vertical scale and distance in statute miles as the horizontal scale. Then draw a straight line between the antenna and the ship. (c) If a hill obstructs the imaginary line of sight, determine its height (H) above the imaginary line and its distance (D) from either the coast or ship station, whichever is nearer, as illustrated by examples “A” and “B” on Graph 1. (d) Read the shadow loss from this Graph 1 and subtract that loss from the computed received signal. (e) Where more than one hill obstructs the transmission path, determine the height and position of a single equivalent hill, as illustrated by example “C” on this graph. Read the shadow loss from this graph for the equivalent hill. Compute the +17 dBu contour as follows: (a) Determine the effective antenna height above mean sea level according to the procedures in §§80.757–80.761. (b) Determine the effective radiated power according to §80,765. Determine for each radial the distance from the antenna site to the +17 dBu point of field strength using procedures of §§80.765 and 80.767. (c) Plot on a suitable map each point of +17 dBu field strength for all radials and draw the contour by connecting the adjacent points by a smooth curve. (a) Where a VHF public coast station geographic area licensee shares a frequency with an incumbent VHF public coast station licensee, the ratio of desired to undesired signal strengths must be at least 12 dB within the service area of the station. (b) Where a VHF public coast station geographic area licensee shares a frequency with an incumbent private land mobile radio licensee, the VHF public coast station geographic area licensee must provide at least 10 dB protection to the PLMR incumbent's predicted 38 dBu signal level contour. The PLMR incumbent's predicted 38 dBu signal level contour is calculated using the F(50, 50) field strength chart for Channels 7–13 in §73.699 (Fig. 10a) of this chapter, with a 9 dB correction factor for antenna height differential, and is based on the licensee's authorized effective radiated power and antenna height-above-average-terrain. (c) VHF public coast station geographic area licensees are prohibited from exceeding a field strength of +5 dBu (decibels referenced to 1 microvolt per meter) at their service area boundaries, unless all the affected VHF public coast station geographic area licensees agree to the higher field strength. [63 FR 40065, July 27, 1998, as amended at 64 FR 26887, May 18, 1999]
Title 47: Telecommunication
PART 80—STATIONS IN THE MARITIME SERVICES
Subpart P—Standards for Computing Public Coast Station VHF Coverage
§ 80.751 Scope.
§ 80.753 Signal strength requirements at the service area contour.
§ 80.755 Applicability.
§ 80.757 Topographical data.
§ 80.759 Average terrain elevation.
§ 80.761 Conversion graphs.
§ 80.763 Effective antenna height.
§ 80.765 Effective radiated power.
§ 80.767 Propagation curve.
§ 80.769 Shadow loss.
§ 80.771 Method of computing coverage.
§ 80.773 Co-channel interference protection.
