Multi-layer Microstrip Antenna
Generally impedance bandwidth of patch antennas varies from 2-3% for single layer direct coupling or probe coupling and can be further enhanced by coupling the power from the strip line through an aperture (slot) in the ground plane and/or printing patch effectively on the foam material whose dielectric constant is nearly equal to that of air [. In such case the feed network is isolated from the radiating element by a ground plane, which prevents spurious radiation. The resonant or non-resonant slot couples energy from the stripline to the patch. If slot is resonant, it provides another resonance in addition to the patch resonance, resulting in an antenna with 10-15% impedance bandwidth. However, the resonant slot has strong backlobe, which substantially reduces the gain of the antenna. Non-resonant slot has better front to back ratio, which results in improvement in efficiency with narrow bandwidth. Hence multilayer microstrip antennas are difficult to optimize when maximum efficiency is required in limited allowable available space with weight as low as possible.
The proposed configuration is shown in Fig. 1, where patch and feed lines are printed on the same substrate and foam layer is sandwiched between the patch and the ground plane to improve radiation efficiency and bandwidth performance by reducing the effective dielectric constant of the combined sandwich layer. Terminating the patch with high impedance line can control spurious radiation from the feed line network and cross-polar performance is improved by employing proper phase cancellation technique.
Design of the antenna begins with proper selection of material, its dielectric constant and height of the substrate, which plays a crucial role when weight and gain has to be optimized simultaneously. Low dielectric constant, which enhances the radiation efficiency for the patch, can give rise to spurious radiation from the feed line network where generally high dielectric constant is generally preferred to confine the electromagnetic field. Foam material would be the best candidate for patch but it cannot be directly printed on foam material. A compromise is to be arrived so as to increase the dielectric constant by introducing a layer of RT Duroid with dielectric constant 2.2 and height 0.79 mm both for mechanical rigidity and electrical requirement of feed line. The single patch is optimized using IE3D CAD software in terms of bandwidth and cross polarization and it is terminated by 240
to reduce probable spurious radiation from a junction formed by patch and feed line. Optimized dimensions of single patch are shown in Fig. 2 with material properties in Table 1.