I'm attempting to use the M3 map and the FCC groundwave curves graphs to estimate field strengths of some stations, mostly at my location, but also at a few other places for a couple stations. I understand that the graphs are "calibrated" to 100 mV/m @ 1 km and I need to adjust based on the 1 km field of the station as specified in the FCC database. (For directional stations, I use the field in the direction I want to measure.) I'm wondering, though, if I'm doing the calculations correctly? I've noticed that not all the conductivity lines for all the frequencies are at the 100 mV/m level at 1 km. For example, on the graph for 1560 kHz, it shows a field of about 30.5 mV/m @ 1 km for a conductivity of 0.1 mS/m. I've been assuming I need to base my initial conversion factor on the 100 mV/m level, regardless of the actual ground conductivity.
Here's an example of how I'm doing the calculations. I've patched together the western portion of the large M3 map in GIMP, but there are some imperfect seams. I've already figured out my location (and drawn a crosshair on the map). I have two ways of estimating the transmitter site. One is I take a screenshot of it on google maps, rotate and scale it (after measuring the length and angle of a straight state border), overlay it on the M3 map at half opacity, then draw a crosshair at the bottom of the marker. The other way is I measure the state lines, calculate a scale conversion factor, measure from a tri-corner border or the coast, then up at a 90° angle to the transmitter site and place the crosshair. I then draw a straight line on the M3 map from the transmitter's crosshair to my location, then measure the path length, from the transmitter site, to the various changes in conductivity. With the help of the FCC search, I calculate the actual length (in km) of the paths.
As an example, I'll use a fictitious station on 1560 kHz with a specified field at 1 km of 1,000 mV/m. Since 1000 (actual field) / 100 (graph calibration) = 10, that's my initial conversion factor. For the first 20 km, the conductivity is 0.1 mS/m. At that point, the graph says 0.12 mV/m. Multiplying that by the initial conversion factor gives me a reading of 1.2 mV/m.
The next 80 km (to 100 km) is saltwater, with a conductivity of 5000 mS/m, which reads about 4.94 mV/m at 20 km on that graph. Dividing 1.2 (actual field) by 4.94 (what graph says) gets me 0.2429, my new conversion factor. At 100 km (20 + 80) the 5,000 mS/m graph says 0.86 mV/m. Multiply that by the conversion factor (0.243) gets me a field of about 0.2089 mV/m.
For the next (and final) 60km, the conductivity is 8 mS/m. At 100km for 8 mS/m, the graph says 0.026 mV/m. Again, dividing 0.2089 (actual field) by 0.026 (graph) gets me 8.03488, the final conversion factor. At 160 km, the 8 mS/m graph says 0.0076 mV/m. Multiply that by the conversion factor (8.03488) results in a calculated field of 0.061 mV/m @ 160 km, a signal that based on my previous experiences & calculations should be listenable (if somewhat weak) on a portable radio in a quiet environment.
Am I doing these calculations correctly, or do I need to make any modifications to my methods (including any that will streamline the process, short of spending any $ on a FIM)? Also, how do I make calculations when I'm "falling off the bottom edge of the graph" before I reach the end of that measurement segment, even though I'm still able to hear the signal in some cases? For example, this happened when attempting to calculate the field at my location for 700 KALL North Salt Lake City, a signal I've heard near noon with the PL-606 and Select-A-Tenna on several occasions here near El Cajon / La Mesa, CA. Also at approximately what distance do I switch between calculating based on the actual ground conductivity vs. the inverse distance formula, and/or calculate based on the non-directional RMS field of a directional station when I'm estimating the field fairly close to the transmitter site (for example, within a few km, or within a few wavelengths, or in a few cases between towers - closer to one than to another)?
Here's an example of how I'm doing the calculations. I've patched together the western portion of the large M3 map in GIMP, but there are some imperfect seams. I've already figured out my location (and drawn a crosshair on the map). I have two ways of estimating the transmitter site. One is I take a screenshot of it on google maps, rotate and scale it (after measuring the length and angle of a straight state border), overlay it on the M3 map at half opacity, then draw a crosshair at the bottom of the marker. The other way is I measure the state lines, calculate a scale conversion factor, measure from a tri-corner border or the coast, then up at a 90° angle to the transmitter site and place the crosshair. I then draw a straight line on the M3 map from the transmitter's crosshair to my location, then measure the path length, from the transmitter site, to the various changes in conductivity. With the help of the FCC search, I calculate the actual length (in km) of the paths.
As an example, I'll use a fictitious station on 1560 kHz with a specified field at 1 km of 1,000 mV/m. Since 1000 (actual field) / 100 (graph calibration) = 10, that's my initial conversion factor. For the first 20 km, the conductivity is 0.1 mS/m. At that point, the graph says 0.12 mV/m. Multiplying that by the initial conversion factor gives me a reading of 1.2 mV/m.
The next 80 km (to 100 km) is saltwater, with a conductivity of 5000 mS/m, which reads about 4.94 mV/m at 20 km on that graph. Dividing 1.2 (actual field) by 4.94 (what graph says) gets me 0.2429, my new conversion factor. At 100 km (20 + 80) the 5,000 mS/m graph says 0.86 mV/m. Multiply that by the conversion factor (0.243) gets me a field of about 0.2089 mV/m.
For the next (and final) 60km, the conductivity is 8 mS/m. At 100km for 8 mS/m, the graph says 0.026 mV/m. Again, dividing 0.2089 (actual field) by 0.026 (graph) gets me 8.03488, the final conversion factor. At 160 km, the 8 mS/m graph says 0.0076 mV/m. Multiply that by the conversion factor (8.03488) results in a calculated field of 0.061 mV/m @ 160 km, a signal that based on my previous experiences & calculations should be listenable (if somewhat weak) on a portable radio in a quiet environment.
Am I doing these calculations correctly, or do I need to make any modifications to my methods (including any that will streamline the process, short of spending any $ on a FIM)? Also, how do I make calculations when I'm "falling off the bottom edge of the graph" before I reach the end of that measurement segment, even though I'm still able to hear the signal in some cases? For example, this happened when attempting to calculate the field at my location for 700 KALL North Salt Lake City, a signal I've heard near noon with the PL-606 and Select-A-Tenna on several occasions here near El Cajon / La Mesa, CA. Also at approximately what distance do I switch between calculating based on the actual ground conductivity vs. the inverse distance formula, and/or calculate based on the non-directional RMS field of a directional station when I'm estimating the field fairly close to the transmitter site (for example, within a few km, or within a few wavelengths, or in a few cases between towers - closer to one than to another)?