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Text File | 1992-05-27 | 20KB | 324 lines

COMMENTS TO PATHCALC Jack Taylor, N7OO PATHCALC can be an invaluable tool for NodeOps or other amateurs interested in making preliminary RF path evaluations. Before running PATHCALC, be advised the only ways to get out of the program are to complete a menued sequence, or to push the "big red button"! Here is the initial menu: ╔═════════════════════════════════════════════════════════════════════════════╗ ║ *************** PATHCALC version 0.73 *************** ║ ║ ║ ║ ║ ║ 1 - Program Information and General Help ║ ║ 2 - Enter Terminal Point Coordinates ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ 7 - Enter General Information ║ ║ ║ ║ 9 - Exit to DOS ║ ║ ║ ║ ║ ║ ║ ║ Note: Above Items in Gray will be ║ ║ available in FUTURE releases. ║ ║ ║ ║ ║ ║ ║ ║ ║ ╚══════════════════════════<<<<< Select Number >>>>>══════════════════════════╝ Menu selection (1) provides program information and help. Menu selection (2) requests an input of site name, elevation and, latitude/ longitude information. For initial program test purposes for trying out the program, site coordinates will be required. Failing to have this information on hand, here's sample information that can be used. TERMINAL 1 Site name = LMN Site Elevation = 9100 Latitude = 32N26.5 Longitude = 11W47 TERMINAL 2 Site name = DEWEY Site Elevation = 7800 Latitude = 34N42 Longitude = 112W07 After this input in verified, an expanded menu is provided: ╔═════════════════════════════════════════════════════════════════════════════╗ ║ *************** PATHCALC version 0.73 *************** ║ ║ ║ ║ ║ ║ 1 - Program Information and General Help ║ ║ 2 - Enter Terminal Point Coordinates ║ ║ 3 - Calculate Path Length and Headings ║ ║ 4 - Calculate Radio Link Parameters ║ ║ 5 - Calculate Environmental Path Parameters ║ ║ ║ ║ 7 - Enter General Information ║ ║ 8 - Print Out Detailed Report ║ ║ 9 - Exit to DOS ║ ║ ║ ║ ║ ║ ║ ║ Note: Above Items in Gray will be ║ ║ available in FUTURE releases. ║ ║ ║ ║ ║ ║ ║ ║ ║ ╚══════════════════════════<<<<< Select Number >>>>>══════════════════════════╝ Selecting item (3) provides information pertinent to the two sites: ╔═════════════════════════════════════════════════════════════════════════════╗ ║ *************** PATHCALC version 0.73 *************** ║ ║ ║ ║ ║ ║ >>>>> PATH GEOMETRY <<<<< ║ ║ ║ ║ Terminal Point 1 Terminal Point 2 ║ ║ ║ ║ LMN DEWEY ║ ║ 9100 Feet AMSL 7800 Feet AMSL ║ ║ Latitude 32N26.5 Latitude 34N42 ║ ║ Longitude 110W47 Longitude 112W7 ║ ║ ║ ║ Bearing to TP2 > 334.2 degrees Bearing to TP1 > 153.4 degrees ║ ║ Elevation to TP2 > -1.3 degrees Elevation to TP1 > -1.2 degrees ║ ║ ║ ║ Airline Distance between TP1 and TP2 > 174.0 Miles ║ ║ ║ ║ NOTE: > Bearings are with respect to TRUE North ║ ║ > Elevation angle is based upon K=1 Earth ║ ║ ║ ║ ║ ╚══════════════════════<<<<<Hit Any Key To Continue>>>>>══════════════════════╝ The above site "data window" is a good one to include (as are the following) into a file for future reference. For instance the sample file could be titled: LMN-DWY.DAT. The procedure for extracting this info from PATHCALC was to use a shareware program called PRN2FILE.COM. PRN2FILE redirects material written on the screen from a parallel printer port into a path and filename (which you specify). PCTOOLS can be used to edit out the tabs and line-feeds which seem to be a characteristic of the PRN2FILE screen transfer program. At this time, we have the path distance in statute miles, the path "takeoff" angles in either positive or negative degrees and the bearings to each site relative to true north. The takeoff angles are handy in determining useful- ness of the path. For instance if one or both sites were located in a saddle and negative take-off angles are indicated, it could mean there is harmful blockage obstructing the path. A real example is the case of the LMN site which is located on a ridge with a higher elevation to it's immediate west, thus rendering linking to a distant site in that direction marginal. Selection (4) gives radio path calculations of which the minimum info to input is the path's frequency. This and other inputs are accomplished by use ofthe UP and DOWN arrows, space key (to initialize the entry) and ENTER: ╔═════════════════════════════════════════════════════════════════════════════╗ ║ *************** PATHCALC version 0.73 *************** ║ ║ ║ ║ RF Frequency (MHz) > 145.010 ║ ║ Transmitter Output Power (Watts) > 10.0 ║ ║ Transmitter System Losses (dB) > 3.0 ║ ║ Transmitter Antenna Gain (dBi) > 8.0 ║ ║ Path Obstruction Loss (dB) > 0.0 ║ ║ Path Absorption Loss (dB) > 0.0 ║ ║ Receiver Antenna Gain (dBi) > 8.0 ║ ║ Receiver Antenna Noise Temperature (K) > 40 ║ ║ Receiver System Losses (dB) > 2.0 ║ ║ Receiver Noise Bandwidth (kHz) > 15.0 ║ ║ Receiver Noise Figure (dB) > 2.0 ║ ║ Desired Threshold SNR (dB) > 10.0 ║ ║ Calculated RF Path Loss (dB) > 124.6 ║ ║ Calc Apparent Receiver Noise Floor (dBm) > -130.0 ║ ║ Calculated Level of Received Signal (dBm) > -73.6 ║ ║ Calculated SNR of Received Signal (dB) > +56.4 ║ ║ Calculated Signal Fade Margin (dB) > +46.4 ║ ║ ║ ║ Exit to Main Program > ║ ╚═════════════════════════════════════════════════════════════════════════════╝ RF frequency, transmitter power, antenna gain and transmitter system losses are values which should be "knowns". Losses due to path obstructions and absorption may not be so obvious. According to NBS TechNote 101, below 1 GHz radio path absorption due to moisture does not exceed 2 dB per 1000 kilometers. This converts to approximately .003 dB per mile, an insignificant value except perhaps on long paths in climatic zones with high moisture content. Therefore, for most path calculations in the VHF/UHF range, "path absorption loss" can be defaulted to zero. Path Obstruction Loss are those losses due to obstructions which impede the signal along the path. This is something very difficult to arrive at without actually making signal level measurements from end-to-end. Normally this entry is left at the default value of zero. Receiver antenna noise figure of 40 degrees Kelven is an adequate default for all but deep space paths. Receiver Noise Bandwidth refers to the last IF bandwidth which for FM voice and narrow channel data is nominally the default value of 15 KHz. The Desired Receiver Threshold Signal-to-noise-ratio refers to the minimum amount of signal above the noise level that's required to decode the signals without error. In most cases the default of 10 dB is adequate. The Calculated Amount of Path Loss is the free space attenuation of the line- of-sight path, in dB. Perhaps the most significant data point on the above display is that of "Calculated Signal Fade Margin", in dB. RF paths are engineered in terms of "percentage of reliability". The amount of fade margin available over an RF path impacts the percentage of reliability as follows: RELIABILITY IN PERCENT FADE MARGIN IN DB ====================================================== 90 ------------------ 10 96 ------------------ 15 99 ------------------ 20 99.5 ---------------- 24 99.9 ---------------- 30 99.95 --------------- 34 99.98 --------------- 38 99.99 --------------- 40 Normally the circuit users desire a path that is 100 percent reliable. For this reason a 40 dB or better fade margin is required. However for lessor amounts of fade margin, the relationship to reliability is as indicated above. Here it should be noted we are refering to the reliability of the path itself, not that of the equipment. It should also be noted, a strong received signal level at harsh electronic environment locations, such as found at today's typical mountain top site, helps to overcome local interfering signals. FRESENEL ZONE BOUNDRY --------------------- ╔═════════════════════════════════════════════════════════════════════════════╗ ║ *************** PATHCALC version 0.73 *************** ║ ║ ║ ║ Enter Earth Radius K factor (1.00 - 1.80) > 1.25 ║ ║ ║ ║ TERMINAL WAY WAY WAY WAY WAY WAY WAY ║ ║ POINT POINT POINT POINT POINT POINT POINT POINT ║ ║ 1 1 2 3 4 5 6 7 ║ ║ 32N26.5 32N35.5 32N44.6 32N53.6 33N2.71 33N11.7 33N20.8 33N29.8 ║ ║ 110W46.9 110W52.2 110W57.4 111W2.66 111W7.92 111W13.2 111W18.4 111W23.8 ║ ║ +9100' +8009' +7061' +6257' +5596' +5079' +4705' +4475' ║ ║ 3' 623' 849' 998' 1104' 1177' 1223' 1245' ║ ║ ║ ║ WAY WAY WAY WAY WAY WAY WAY TERMINAL ║ ║ POINT POINT POINT POINT POINT POINT POINT POINT ║ ║ 8 9 10 11 12 13 14 2 ║ ║ 33N38.8 33N47.9 33N56.9 34N5.95 34N14.9 34N23.9 34N32.9 34N42.0 ║ ║ 111W29.1 111W34.4 111W39.8 111W45.2 111W50.6 111W56.0 112W1.53 112W6.99 ║ ║ +4389' +4445' +4646' +4990' +5477' +6108' +6882' +7800' ║ ║ 1245' 1223' 1177' 1104' 998' 849' 623' 3' ║ ║ ║ ║ >The above Way Points are spaced at 11.6 mile intervals ║ ╚══════════════════════<<<<<Hit Any Key To Continue>>>>>══════════════════════╝ The above window serves three purposes. A straightline on a map between two sites may not depict the actual path between them due to the map being a flat projection whereas the earth is curved. PATHCALC provides 14 points evenly spaced between the two sites that are calculated using great circle bearings. When the latitude/longitude of each point is plotted on the map and connecting lines drawn between them, the true path will be shown. Secondly, the centerline of the path is indicated in feet above sea level at each of the latitude and longitude "way points" along the route. Thirdly, the window provides the radius of the first Fresnel zone at each of 16 points. For those not familiar with the significance of the first Fresnel zone, antenna reference books should be consulted. For our purposes it is sufficient to describe the first Fresnel zone as an imaginary cone extending from one antenna to the midpoint in the path, where it joins a similar cone from the opposite antenna. The radius of the area described by this cone will vary with frequency and distance between the sites. The first Fresnel zone is significant since it roughly includes the area along the path that should be free from obstacles. Nearly 75 percent of the signal strength is received from propagation of the direct wavefront between the two sites. Approximately 25 percent of the remaining signal level is received within the boundry of the first Fresnel zone. As a general rule, optimum Fresnel zone clearance occurs when at least 0.6 of the first Fresnel zone radius, as plotted over a true earth profile, is clear. Less than a 0.6 first Fresnel zone clearance may cause obstacle losses, depending upon the degree of obstruction. In the above window, the Fresnel zone radius at TP 1 is 3 feet and expands out to 1245 feet at way points 7 and 8, then reduces back to 3 feet at TP 2. In order of priority then, a clear path between the two site antennas is desired. Secondly is the requirement for first Fresenel zone clearance of 0.6 or better, from obstructions. Normally the obstruction, if any, will be that of the ground terrain somewhere along the path. However obstructions can occur from bluffs, sides of mountains or canyon walls. In addition to path losses, obstructions of various sorts can cause multipath distortion. This is the case where reflections of the transmitted signal arrives out of phase with the main wave front. Multipath distortion adversely affects higher data rates and is more prevalent on non line-of-site paths. TOPOGRAPHIC MAPS Topographic maps are used to evaluate the terrain between two adjacent sites. An index and catalog of topographic maps, by state, is available free upon request from: U.S. Geological Survey Box 25286, Federal Center Denver, Colorado 80225 The USGS has available a variety of maps detailing different information. The ones of interest are the Standard Quadrangle maps. These cover systematically subdivided areas of latitude and longitude, and are published at various scales depending on the size of the area mapped. Standard Quadrangle formats range from 7.5 x 7.5 minutes covering geographic areas of 49 to 71 square miles, up to 1 x 2 degrees covering areas of 4580 to 8690 square miles. The 7.5 and 15 minute, and the 1 x 2 degree series show elevation and contour levels in feet. The 7.5 x 15 and 30 x 60 minute series list elevation and contours in meters. The amount of detail shown on a map is proportionate to the scale of the map. The larger the map scale, the more detail that is shown. For example, individual houses are shown on 1:24,000 scale 7.5 minute maps. On 1:100,000 scale 30 x 60 minute maps only landmark buildings are shown. Topographic maps are also available for individual states. These are convenient for plotting state-wide system links. 1 x 2 degree topographic maps cover a distance of approximately 100 miles and can give an excellent overview of longer radio paths. Map costs can be minimized by doing initial plotting on 1 x 2 degree topographics, then ordering 7.5 minute maps for area coverage of paths where more detailed analysis is required. Hopefully the above has demystified the art of radio path calculations. 73 de Jack, N7OO@NJ7P.AZ.USA