JED - July 2009 - (Page 48)

EW 101 Communications EW – Part 26 Jamming Frequency Hopping Signals By Dave Adamy he major problem associated with the effective jamming of frequency hoppers is that the jammed system uses only one (randomly selected) channel at a time, while the jammer needs to deal with all of the channels from which the target transmitter can select. There are three general approaches to jamming frequency hoppers: • Barrage jamming • Partial band jamming • Follower jamming (continued) T Hopping Channels Jammed Channels Figure 1: A barrage jammer divides its power among all of the hopping channels. 48 The Journal of Electronic Defense | July 2009 Barrage Jamming A barrage jammer covers the entire frequency range over which the target system hops as shown in Figure 1. Thus, any channel chosen by the target transmitter/receiver will be jammed. This approach has the excellent advantage that the jammer need not receive the hopping signal; it eliminates the need for “look through.” Because lookthrough is difficult to achieve in remote jammers, barrage jamming may be the ideal approach. There are two major disadvantages to barrage jamming. One is “fratricide.” Barrage jamming also will jam any friendly communication (fixed frequency or hopping) that is operating in the same geographical area. The second disadvantage is that barrage jamming is notoriously inefficient. Because you need to jam all possible channels, the power per channel is determined from the formula: Power/channel = Total jammer power/number of hopping channels available The solution to both of these problems is to place the jammer near the enemy transmitter. Remember that J/S is the ratio of the received jamming signal strength to the received desired signal strength – both in the target receiver. The signal strength is reduced by the square or fourth power of the distance from the transmitter to the receiver (depending on the frequency and geometry – see the July-September 2009 “EW 101” columns). Therefore, as the range to the target receiver is reduced, the J/S is increased. If the range to the target receiver is significantly shorter than the range to friendly receivers, fratricide is significantly reduced. An example is shown in Figure 2. A one-Watt ERP, VHF transmitter is 10 km from its intended receiver. Both transmitter and receiver have whip antennas 2 meters above the ground. The signal hops over 1,000 channels. A barrage jammer with 1 Watt ERP is located 2 meters above the ground, 1 km from the target receiver. The propagation mode XMTR 1 watt ERP 2 meter high Whip ant. 10 Km Target RCVR 1 Km JMMR 2 meter high Whip ant. 25 Km Friendly RCVR 2 meter high Whip ant. 1 watt ERP 2 meter high Whip ant. Figure 2: A barrage jammer 1 km from the target receiver and 25 km from a friendly receiver provides excellent J/S while avoiding fratricide.

Table of Contents for the Digital Edition of JED - July 2009

JED - July 2009
Contents
The View From Here
Calendar
From the President
The Monitor
Washington Report
World Report
Advancing TWT Technology
Technology Survey: COMINT/DF Receivers
EW 101
AOC News
JED Sales Offices
Index of Advertisers
JED Quick Look

JED - July 2009

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