JED - March 2015 - (Page 50)

EW 101 Radio Propagation over Water Radar Decoys - Part 9 By Dave Adamy I The Journal of Electronic Defense | March 2015 50 n several earlier columns, we have discussed the "link ERP PROPAGATION MODEL GR GT equation" for various circumstances. Typically, this equaTransmit Receive tion gives the signal strength Antenna Antenna received by a receiving system PR PT height height as a function of the configuration of XMTR RCVR the one-way transmission link and the surrounding circumstances. Figure 1 shows the basic link, which is characterized by the equation: Figure 1: A one-way link moves information from a transmitter to a receiver using one of several PR = P T + GT - L + GR propagation modes characterized by a propagation model. Where: PR is the received power at Figure 1: A one-way link moves information from a transmitter to a receiver using the output of the receiving is applicable, loss increases with the fourth one of several propagation modes characterized by a propagation model. power of range antenna in dBm or dBW but is reduced by the product of the squares of the heights of the transmitting and receiving antennas. Note that these loss P T is the transmitter power in the same units models are discussed in several earlier "EW 101" columns, most GT is the transmitting antenna gain in dB recently in the July to September 2007 "EW 101" columns. L is the link loss (in dB) GR is the receiving antenna gain Effective radiated power (ERP) is the product (or the sum in FCC OVER-WATER LOSS GRAPH dB) of the transmitter output and the gain of the transmitting Figure 2 shows a widely used chart that was developed by antenna in the direction of the receiver. Thus, the above equathe US Federal Communication Commission (FCC). It comes from tion can be written: the FCC regulations for part 80 - STATIONS IN THE MARITIME PR = ERP - L + GR SERVICE. The following discussion builds on an approach presented in an internet article entitled "Radio Propagation Over The loss term is a function of frequency, the surrounding Water - moderated discussion areas." This column (and at least environment, the link distance, and the heights of the transone more "EW 101") will convert the information from this FCC mitting and receiving antennas above the local terrain. For chart into the formats used in our previous propagation model each of the loss terms discussed, we have defined the propagadiscussions, and will develop a family of graphs for loss (in dB) tion loss between isotropic antennas. Each of the loss terms as a function of: range (in km), transmit antenna height (in developed to date has been for loss over land. Now, we will meters) and frequency (in MHz). discuss losses for propagation over water. Figure 3 shows how to use the FCC graph. Start at the disThe two primary propagation loss models we have covered tance - in this case 50 statute miles from the shore transmitin previous columns are shown in Table 1. Both of these forter to the ship. Move up to the data line for the height of the mulas assume loss in dB, distance in km and frequency in MHz. shore transmit antenna - in this case 1,600 feet. Move left to the ordinate of the graph, Table 1: Propagation loss previously discussed which shows the signal denPropagation Model Formula for loss per km Slope of loss curve vs. range sity in dBu at the 30-footLine of Sight L = 32.44 + 20 log(d) + 20 log F Range2 high receiving antenna on the Two Ray L = 120 + 40 log(d) -20 log hT -20 log hR Range 4 ship. This is shown to be 47 dBu. Also highlighted on this If line of sight loss is applicable, the loss increases as a figure is the signal density if the receiver is 60 km from the function of the square of range and frequency. If two-ray loss

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Charting the Future for DIRCM
How Far Can We Take GaN Technology?
Book Reviews
EW 101
AOC News
2015 AOC Industry Member Guide
Index of Advertisers
JED Quick Look

JED - March 2015

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