| Phased-Array Antennas |
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Phased-array antennas, or Electronically-Scanned Antennas (ESAs), are generally the most desirable form of antenna system from a capability point of view. Large antennas are needed to achieve high gain, but the beam associated with a large antenna must be scanned in order to be used over a large area of coverage. Unlike mechanically-scanned antennas, phased-array antennas are fixed in space and are scanned electronically be inducing either a phase shift or a time delay between adjacent antenna elements. This phase shift approach can be used to scan the beam very rapidly, and is not susceptible to the limitations of mechanical motors. Phased arrays are also highly desirable for conformal antennas, where the need for a large radome is eliminated. The main challenge limitation that is presently holding back the proliferation of phased arrays is cost. The reason is that the front-end electronics, such as a T/R module, is multiplied by the number of antenna elements, which can be rather large. There are also many technical challenges which make the implementation of phased arrays difficult from an engineering point of view. These include packaging, thermal issues, and prime power consumption. For example, at X-band the inter-element spacing is roughly 0.5". In a typical radar implementation, this would require a circulator, power amplifier (PA), low-noise amplifier (LNA), limiter and various switches to be implemented behind a 0.5x0.5" footprint, with the associated packaging, interconnect, and thermal issues. Historically, many approaches have been used to realized phased-arrays. Older analog ferrite phase-shifters can handle high power per element, but are subject to the calibration and drift issues associated with analog components. Switched lines have also been used to implement phase shifters. PIN diode and FET switches can handle a modest amount of power, but require high prime DC power consumption. Recent MEMS switches require almost no DC power consumption, but have other limitations such as reliability and poor power handling. Applied Radar has recently developed phased-array antennas using two new techniques: MMIC phase shifters and digital beamforming (DBF). |
| MMIC Arrays |
| MMIC phased shifters and attenuators are digitally-controlled analog elements with accurate, reliable performance. Applied Radar has recently developed a 256-element X-band phased-array using MMIC phase shifters and T/R modules. This type of design has many advantages, but the main limitation is the cost of the MMIC components required at every antenna element. |
| Digital Beamforming (DBF) |
| Digital beamforming has many advantages over analog arrays, such as multiple
simultaneous receive beams, dynamic reconfigurability of the various output
beams, dynamic null steering, and very low sidelobes through digital calibration.
Generally speaking, a DBF array requires a downconverter and A/D at every
"input channel." Due to the high cost of these components, DBF
is typically implemented at the subarray level for all but the smallest
of arrays. Hybrid arrays that employ low-cost analog phase shifters at the
element level and DBF at the subarray level allow the advantages of both
low cost and a digital array to be realized.
Applied Radar is an industry leader in the rapidly emerging field of digital beamforming. We recently delivered a 32-channel digital beamforming receiver to the US Air Force, and will soon be marketing this as a commercial product. We are also developing a transmit capability for this product.
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