Conformal Antenna Arrays

In airborne applications (i.e. aircraft, missiles) antennas may increase the vehicle’s radar cross section (RCS) and interfere with the aerodynamics. Hence, it is desirable for the antenna structure to be conformal (see Figure 19). Further, conformal antennas are also desirable for the realization of body-wearable antennas.

Fig19
Fig. 19 Conformal antenna illustration.

SHAAS address defense and commercial requirements to combine different antennas that satisfy multiple functions like radar, communications, identification friend or foe (IFF) and GPS into one.4, 55, 56 In some applications like marine cruisers, legacy platforms may have more than 100 separate antennas on board.62 SHAAS solves this problem with a common aperture sharing multiple features (see Figure 20).

The multiple functionalities can be accessed either simultaneously or in a time-sharing mode. The challenges for antenna design are to reduce in-band and out-of-band coupling between operational bands, minimize interaction with the mounting platform, miniaturize the electronics and arrange inter-element grids to avoid grating lobes while mitigating scanning losses.4, 9, 63 Zhang et al.64 describe a dual-band shared aperture antenna utilizing the concept of structure re-use. SHAAS provide a solution for antenna systems to be greener (more efficient), more compact and lower cost.

Fig20
Fig. 20 Shared aperture concept on a moving platform.
 

Radar Antennas

Figure 21 shows the evolution of radar antenna since Hertz conceptualized the first spark plug experiment using a loop antenna. Later, Yagi and Uda (1920) introduced Yagi – Uda antennas, followed by the horn antenna in the 1930s, antenna arrays in the 1940s, parabolic reflectors in the late 1940s and early 1950s, microstrip patch antennas in the 1970s and PIFAs in the 1980s.

Later, mechanically scanned arrays (MSAs) were developed with fixed beam antennas mounted on servo rotor mechanisms. The inertia of a servo system limits the rotation rate, or scan rate, however. To overcome this, passive electronically scanned antenna arrays (PESAs) were developed that rotate, or scan, the beam electronically with phase shifters behind each antenna.

In a PESA, the amplitude distribution across the antenna aperture is fixed due to a fixed power division network at the back-end distributing rf power received from single high-power transmitter (HPT).4 Failure of the HPT represents a single-point-of-failure for the system. Active electronically scanned antenna arrays (AESAs) overcome this problem.

The AESA comprises individual transmitters and receivers, in a single housing, behind each element called a transmit-receive module (TRM).4, 6 A key feature of the AESA is graceful degradation, i.e., the radar operates with limited functionality even in the case of failure of few TRMs (typically less than 10 to 15 percent). The overall design requirements of an AESA may be summarized by the following parameters:

Functional Requirements

  • Spatial Scan Volume
  • Instantaneous/Operational bandwidth
  • Beamwidth
  • Peak and average sidelobe level
  • Antenna gain
  • Polarization
  • Peak power and average power output
  • Beam switching capability
  • Prime power requirement

Physical Requirements

  • Size, weight, transportation and mobility
  • Production, maintenance and reliability

Environmental Requirements

  • Shock and vibration capability
  • Operational temperature range
  • Humidity, salt, fog and fungus

 

Fig21
Fig 21 Antenna evolution.

 

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