Principles of Modern Radar - Volume 2 1891121537

12.5 Antenna-Based EP 55512.5.1 Low SidelobesLow antenna sidelobes provide effective EP against both noise and deceptive sidelobeinterference. Low sidelobes in the receive antenna pattern force sidelobe jammers to havehigher power to achieve a desired JNR or JSR. This is evident through inspection ofEquations 12.9, 12.13, and 12.14 for the noise and coherent jammers, where a reductionin G rj corresponds directly to a reduction in JSR. Low sidelobes in the transmit antennapattern, G tj , force repeater and transponder jammers as well as receive-only ES systemsto have increased sensitivity to detect and characterize the radar transmission, as describedin Section 12.3.7.One of the limiting factors in repeater and transponder operation is the T/R isolation.The jammer T/R isolation requirement is directly impacted by both transmit and receiveradar sidelobes. For example, if the radar reduces its transmit sidelobes by 5 dB and receivesidelobes by 10 dB, the jammer T/R isolation requirement increases by 15 dB.Theoretically it is possible to achieve very low sidelobe levels through appropriateweighting functions across the radar aperture. In practice, however, limitations in design,fabrication, calibration, and component stability make realization of such levels elusive.Solid-state phased array antennas, for example, have multiple potential contributors to theantenna sidelobe level, including amplitude and phase errors, T/R module attrition, andcorrelated errors between groups of elements in the array. Reducing antenna sidelobes mustbe weighed against any accompanying reduction in main lobe directivity and widening ofthe main beam. Often it is preferable to employ a uniform aperture weighting on transmitto achieve the maximum ERP while relying on the receive patterns to provide the sidelobereduction benefits.12.5.2 Sector BlankingSector blanking offers some EP benefit to search radars that are subjected to EA interferencefrom known, specific angular regions. The radar avoids receiving or both transmittingand receiving in the defined angular sector. Although such a mode is tantamount to theradar conceding defeat in the blanked sectors, there may be some benefit as well. Onebenefit is that the radar might be able to prevent a sidelobe transponder or repeater fromdetecting and locking on to the radar frequency or synchronizing to the antenna scan patternby depriving the jammer of a main lobe illumination [3,10]. Sector blanking mightbe part of the operational tactics, techniques, and procedures (TTPs) that are developedfor the radar.12.5.3 Sidelobe BlankingSidelobe blanking helps eliminate sidelobe false targets that are strong enough to overcomethe antenna sidelobe pattern attenuation [6,10]. The concept is depicted in Figure 12-9. Theradar employs one or more broad-beam, auxiliary antennas in addition to the main antenna.The auxiliary antenna has higher gain than the main antenna in the region of the mainantenna sidelobes and lower gain than the main antenna in the region of the main antennamain lobe. The auxiliary antenna is connected to an independent radar receiver channelthat is essentially identical to that used for the main antenna. A comparison is made ofthe main and auxiliary channel detected signal amplitudes. If the auxiliary channel signalexceeds the main channel signal by some margin, the signal in the main channel is blanked;