Antennas are the beginnings and ends of most wireless communications systems, and are growing in numbers as wireless RF/microwave technologies are increasingly employed in applications ranging from automotive radars to Internet of Things (IoT) sensors. But antennas add size and weight to a system, depending upon the frequency and wavelength, requiring newer antenna designs to consider multiple resonator arrangements for multiple-band frequency coverage with the equivalent of a single antenna.
With that goal in mind, researchers from the Society for the Applied Microwave Electronic Engineering and Research Center for Electromagnetics in Chennai, India have created an antenna design based on a shared-aperture-antenna (SAA) concept, in which multiple planar antennas are embedded into a single antenna structure. The multiple-frequency-band antenna is a possible lightweight solution for airborne and unmanned-aerial-vehicle (UAV) applications requiring lightweight antennas capable of operating over several different frequency bands.
The single antenna structure consists of an L-probe-fed, suspended-plate, horizontally polarized antenna for 900 MHz; an aperture-coupled, vertically polarized microstrip antenna for 4.2 GHz; a 2 × 2 microstrip patch array for X-band frequencies; a low sidelobe-level (SLL) corporate-fed, 8 × 4 microstrip planar array for synthetic aperture radar (SAR) at X-band; and a printed, single-arm, circularly polarized tilted-beam spiral antenna for C-band use, with all of these antennas integrated into a single aperture for simultaneous operation.
The number of antennas carried by an aircraft continues to increase, but that number is limited by the size and weight of each discrete antenna. The SAA concept allows all of the antennas to share a common compact space and operate simultaneously, without causing interference with the other antennas in the shared aperture space. The concept is not new, but requires careful selection of antenna types and polarization schemes to ensure mechanical and electrical compatibility.
For this particular SAA design, each antenna was designed independently and simulated with a three-dimensional (3D) electromagnetic (EM) simulation software program, ignoring mutual coupling effects between the different antennas. Following the design of each independent antenna, they are integrated into the SAA configuration and measurements are performed on the common structure.
The researchers found that the critical factor in the design of an SAA structure is the optimum placement of the individual antennas. The coupling between ports plays a key role in finding the optimum performance of the integrated antenna structure. Optimization can be performed with the aid of measurements of the reflection coefficients of the separate antennas in different placements. The L-band antenna was set in the middle of the antenna structure as a reference, and the other four antennas were moved one at a time within the SAA configuration to observe the effects on different antenna gain and reflection coefficient parameters.
Once optimum positions were found for each antenna, the placement of the SAA relative to the body of the aircraft upon which it was mounted was analyzed and optimized to better understand finite ground-plane effects on the SAA structure. Measurements of the prototype, five-antenna structure revealed that the design concept is a feasible solution for aircraft requiring multiple antennas for different applications, including radar and communications systems. The coupling coefficients between antenna elements were found to be better than 039 dB, for good isolation between the five closely spaced antennas with consistent, evenly spaced radiation patterns.
See “A Multiband, Multipolarization Shared-Aperture Antenna,” IEEE Antennas & Propagation Magazine, Vol. 59, No. 4, August, 2017, p. 26.