JED - February 2009 - 50

EW101 Communications EW – Part 21 Communication Jamming By Dave Adamy (continued) T 50 The Journal of Electronic Defense | February 2009 his month, we will consider various situations in which jamming is performed against hostile communications. The situations are chosen to illustrate the impact of the different propagation models. High-altitude Communication Jammer Consider the jamming situation shown in Figure 1. A fi xed-wing aircraft flying at 3,000 meters altitude jams a 250-MHz communication net with stations 5 km apart. All stations in the target net are transceivers with 2-meter-high whip antennas (2 dB gain). The output power from each transceiver’s transmitter is 1 Watt. The jamming aircraft is 50 km from the area over which the target net is operating. The jammer outputs 100 Watts into a 3-dBi antenna. What J/S is achieved? LOSS J = 32.4 + 20 log(dist) + 20 log(frequency) = 32.4 + 20 log(50) + 20 log(250) LOSS J = 32.4 + 34 + 48 = 114.4 dB The ERP of the target link transmitters is 30 dBm (i.e., 1 Watt) + 2 dBi = 32 dBm The ERP of the jammer is 50 dBm (i.e., 100 Watts) + 3 dBi = 53 dBm The J/S ratio (from the December 2008 EW 101) is: J/S = ERPJ – ERPS – LOSS J + LOSSS = 53 - 32 – 114.4 + 136 = 42.6 dB The fact that the airborne jammer link has line of sight loss allows it to generate very high J/S against the target net that has two-ray loss. Stand-in Jamming Now we consider a “stand-in” jammer operating against the same target net described in the above problem. This is a low-power jammer that is very close to the receiver. In this case, there might be a number of low-power jammers placed throughout the area in which the target net is operating. Each jammer has 5 Watts ERP from a 0.5-meter-high whip antenna. Figure 2 shows one such jammer located 500 meters from a receiver. We will consider this the typical jamming case, i.e., there are assumed to be stand-in jammers about 500 meters from each transceiver in the target net. What J/S is achieved? The desired signal link as described above operates with two-ray propagation. Its ERP is 32 dBm and its link loss is 136 dB. Figure 1: Significant J/S can be achieved from a high altitude airborne jammer. First we must determine the appropriate propagation models for each link. The target link Fresnel zone distance (from the August 2007 EW 101) is: FZ = (2 x 2 x 250)/24,000 = .0417 km = 41.7 meters This is far less than the 5-km transmission path, so tworay propagation is appropriate for the target link. The target link loss is thus: LOSSS = 120 + 40 log (dist) – 20 log (hT) – 20 log (hR) LOSSS = 120 + 40 log(5) – 20 log(2) – 20 Log (2) = 120 + 28 – 6 – 6 Figure 2: Stand-in jamming can provide high J/S with low jammer power. = 136 dB The jamming link Fresnel zone distance is: Now calculate the FZ for the jamming link: FZ = (3000 x 2 x 250)/24,000 = 62.5 km. FZ = (hT x hR x freq)/24,000 Because the Fresnel zone distance is greater than the jamming link transmission distance, line of sight propagation ap= (0.5 x 2 x 250)/24,000 plies. The loss in the jamming link is: = .01 km = 10 meters

JED - February 2009

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

JED - February 2009
Contents
The View From Here
Calendar
From the President
The Monitor
Washington Report
World Report
Charting the Course of Indian EW
What's Next in Data Recorders?
Technology Survey: RWR/ESM Systems
EW 101
2009 AOC Industry and Institute/University Member Guide
JED Sales Offices
Index of Advertisers
JED Quick Look
JED - February 2009 - JED - February 2009
JED - February 2009 - Cover2
JED - February 2009 - 3
JED - February 2009 - Contents
JED - February 2009 - 5
JED - February 2009 - The View From Here
JED - February 2009 - 7
JED - February 2009 - Calendar
JED - February 2009 - 9
JED - February 2009 - 10
JED - February 2009 - 11
JED - February 2009 - From the President
JED - February 2009 - 13
JED - February 2009 - 14
JED - February 2009 - The Monitor
JED - February 2009 - 16
JED - February 2009 - 17
JED - February 2009 - 18
JED - February 2009 - 19
JED - February 2009 - 20
JED - February 2009 - 21
JED - February 2009 - Washington Report
JED - February 2009 - 23
JED - February 2009 - World Report
JED - February 2009 - 25
JED - February 2009 - Charting the Course of Indian EW
JED - February 2009 - 27
JED - February 2009 - 28
JED - February 2009 - 29
JED - February 2009 - 30
JED - February 2009 - 31
JED - February 2009 - 32
JED - February 2009 - What's Next in Data Recorders?
JED - February 2009 - 34
JED - February 2009 - 35
JED - February 2009 - 36
JED - February 2009 - 37
JED - February 2009 - 38
JED - February 2009 - Technology Survey: RWR/ESM Systems
JED - February 2009 - 40
JED - February 2009 - 41
JED - February 2009 - 42
JED - February 2009 - 43
JED - February 2009 - 44
JED - February 2009 - 45
JED - February 2009 - 46
JED - February 2009 - 47
JED - February 2009 - 48
JED - February 2009 - 49
JED - February 2009 - EW 101
JED - February 2009 - 51
JED - February 2009 - 52
JED - February 2009 - 2009 AOC Industry and Institute/University Member Guide
JED - February 2009 - 54
JED - February 2009 - 55
JED - February 2009 - 56
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JED - February 2009 - 87
JED - February 2009 - 88
JED - February 2009 - Index of Advertisers
JED - February 2009 - JED Quick Look
JED - February 2009 - Cover3
JED - February 2009 - Cover4
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