JED - April 2015 - (Page 52)

EW 101 Radio Propagation Over Water, continued Radar Decoys - Part 10 By Dave Adamy L 100 90 80 Free S 70 60 dBu for 1 KW Radiated The Journal of Electronic Defense | April 2015 52 Distance at which attenuation Slope of Transmit ast month, we looked at a graph from the increases above line of sight attenuation in antenna dB vs. distance Federal Communication Commission (FCC) height beyond line of that deals with radio propagation over water. Stat. mi. Naut. mi. km (in feet) sight range We correlated the free space line on this chart with the formula we have been using in 100 1 0.87 1.6 55 earlier columns dealing with radio propaga200 2.2 1.9 3.5 59 tion, and made a reasonably-credible correlation. Now, 400 4.5 3.9 7.2 64 we will deal with the other lines on the chart, which 800 8 7 12.8 67 cover propagation paths closer to the water. 1600 14 12.2 22.4 69 Figure 1 shows the FCC graph with the lines for various shore transmitter antenna heights. You will note that each 3200 22.5 19.6 36 70 of the lines in the highlighted area of the graph breaks 4800 29.5 25.7 47.2 66 away from the line-of-sight curve at a different distance Table 1: Distance at which propagation model changes from line of sight to the and that it has a different slope as a function of distance. higher loss of propagation over water and the attenuation slope beyond line of Table 1 shows the distance at which each of these curves sight range. leaves the line of sight line and its slope vs. frequency. Remember that conversion to dBu is by the formula: radiated power (ERP). dBu is dB above 1 µv/m, and the signal E (dBu) = 20 log10(E in µv/m) strength (in dBm) for any point (i.e., range and shore antenna height) on the graph can be determined from the formula: The FCC graph gives the received signal energy in dBu for a P = -77 + 20 log(E) - 20 log(F) signal transmitted from a shore transmitter with 1 kW effective 50 10 0' 40 40 20 0' 16 80 0' 0' 00 ' 4 32 800 00 ' ' pace 30 20 10 0 -10 2 3 4 5 7 10 15 20 30 40 50 70 100 150 Statute Miles Figure 1: The highlighted area of the FCC graph shows the propagation curves for various transmit antenna heights. Figure 1: The highlighted area of the FCC Graph shows the propagation curves for various transmit antenna heights.

Table of Contents for the Digital Edition of JED - April 2015

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Honoring the Prowler
Technology Survey: Low Noise Amplifiers
Preview: EW Europe 2015
EW 101
Index of Advertisers
JED Quick Look

JED - April 2015

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