The desirable features of antenna for Airborne SAR applications include shaped radiation pattern and wide bandwidth capability, good cross polarization isolation and high power capability. Shaped radiation across the track improves the target dynamic range and compensates the requirement of STC (Sensitivity Time Control) correction at receiver end. Wide bandwidth performance results in finer resolution. Planar array antenna is preferred as compared to reflector type antenna for less air drag.
Proper selection of radiating element for planar array dictates the requirement of large bandwidth, low cross polarization with high gain. Multilayer electromagnetically coupled printed antenna is selected, which overcomes the bandwidth limitation of the conventional Microstrip antenna. Aperture coupled antenna although offers the advantage of optimizing feeder network and patch independently, but the impedance matching is poor due to improper grounding when coaxial feeding below the ground is required for the array.
The complex excitation coefficients for the required shape are computed using null perturbation technique where roots of the polynomial are moved both in radial and angular direction in shaped region and only angular displacement for non-shaped region . The design and development of shaped radiation pattern within 35 degrees with only 8 elements with ripple level better than 0.5 dB is shown here. Eight elements in the linear array also make it possible to design the feeder network for dual polarization with corporate feeding within the limited available inter-element spacing, making the feed network less frequency sensitive. Two linear arrays (8 X 2) are combined at Microstrip feed network level.
Design and Simulations of Microstrip Array
Multilayer printed antenna is selected as the basic radiating element for large bandwidth. In this structure microstrip patch is effectively printed on the foam material whose dielectric constant is nearly equal to air to enhance bandwidth and also to minimize the surface wave propagation. Since patch cannot be printed on the foam material so patch is printed on the opposite face of dielectric sheet and this can be used as radome to protect the antenna against environment. E.M. coupled feeding was selected to avoid the soldering connection to patches and making it more reliable. RT Duroid material 5880 with thickness 0.79 mm is selected for the feeder network to meet the high power requirement. Upper patch dimensions are taken larger than lower patch and is fabricated on RT5880 with h = 0.254 mm and Rohacell foam is introduced with 5 mm height for bandwidth enhancement.
Linear and planar array design begins with the computation of excitation distribution to be given to each antenna element. Null Perturbation technique was applied to compute the complex excitation distribution with element spacing 41.66 mm.Orchard’s pattern synthesis technique is used here to show that one could obtain a set of excitation distribution resulting in same pattern in elevation plane. For each root that lies off the unit circle, it is possible to have alternate root by keeping the phase angle fixed while the magnitude is inverted.
Feed line network was designed for the distribution and it was optimized using ensemble E.M. simulator for required amplitude and phase with return loss better than –25 dB. Corporate feeding mechanism is preferred for broad bandwidth performance and the gap between the lines are kept minimum 1.2 mm to accommodate feeder network for both the polarization in the limited available inter-element spacing. The feed network was first simulated and optimized by EEsof circuit simulator by modeling asymmetrical coupled line to take into account the effect of the coupling between the lines. This results in less ripples in the shaped patterns because of better phase control of the order of 5 degrees. The same optimized layout was later revalidated by Momentum and minor adjustment in length was carried out for required phase matching. The performance of the array was simulated taking cynaede easter 1516 adhesive into account.
Simulated return loss performance of the antenna shows required bandwidth. The 2:1 VSWR bandwidth of the antenna is better than 250 MHz.