JED - June 2010 - (Page 44)
EW Against Modern Radars – Part 8
Side-Lobe Cancellation and Blanking
ide-lobe jamming is an important EW function because a single jammer, for example an EA-6B’s ALQ-99 pod, can jam a number of radars. This month, we discuss two electronic protect (EP) techniques to reduce the J/S produced by a sidelobe jammer. One works against CW side-lobe jamming and the other against pulsed side-lobe jamming.
CW Noise side-lobe jammer appears to Radar to be reduced signal In Main Lobe
The Journal of Electronic Defense | June 2010
Main Radar Antenna
Signal Received Stronger in auxiliary Antenna.
Figure 2: Inputs from auxiliary antennas are added to the output of the main antenna 180 degrees out of phase.
Figure 1: A coherent side-lobe canceller removes CW signals, which are stronger in the side lobes than in the main beam of the radar antenna.
As shown in Figure 1, the side-lobe canceller (SLC) requires an auxiliary antenna, which receives signals from the direction of the main radar antenna’s important side lobes. These are the side lobes close to the main beam. The auxiliary antenna has greater gain in the side-lobe direction than the side lobes of the main antenna beam. Thus, the radar can determine that the signal arrives from the side-lobe direction and can discriminate against it. This technique is also called “Coherent Side-Lobe Cancellation” (CSLC) because the (jamming) signal is reduced in the input to the radar’s receiver by coherently canceling it. As shown in Figure 2, the jamming signal from the auxiliary antenna is used to generate a copy, which is shifted by 180 electrical degrees. The process of making a phase shifted copy of a signal requires some sort of a phase-locked-loop circuit, and in order to have high quality phase control (i.e., very close to 180
degrees) this must have a narrow-loop bandwidth. Note that a wide-loop bandwidth allows fast response, but a high quality lock requires a narrow loop – hence a slower response. The narrow loop requires a continuous signal, for example a noisemodulated CW signal, such as used in a standoff noise jammer. It is important to understand that the closer the phase shifted signal is to exactly 180 degrees out of phase with the jamming signal, the greater the reduction of the jamming signal into the radar receiver will be. Each jamming signal that is cancelled requires a separate antenna and phase-shift circuit. Because there are two auxiliary antennas in Figure 2, this radar would be able to cancel two CW side-lobe jammers. It is interesting to note that the Fourier transform of a pulse signal (i.e., the pulse signal viewed in the frequency domain) has a large number of distinct spectral lines, as shown in Figure 3. The top part of the figure shows a pulse signal in the time domain (as it would be viewed on an oscilloscope) and the bottom part of the figure shows the same signal in the frequency domain (as it would be viewed on a spectrum analyzer). Note that the main lobe of the frequency response is 1/PW wide, where PW is the pulse width in the time domain. Also note that the spectral lines are separated by the pulse-
Table of Contents for the Digital Edition of JED - June 2010
JED - June 2010
Table of Contents
The View From Here
From the President
Technology Profile: RF Interconnect Solutions
AOC Election Guide
Index of Advertisers
JED Sales Offices
JED Quick Look
JED - June 2010