Mode-Free Radiating Electromagnetic Bandgap Materials and Their Applications to Microstrip Phased Arrays
In addition to radiation, microstrip phased arrays excite substrate waves. These substrate waves are not desired since they keep energy, in the substrate, that is supposed to be radiated. Thus, array elements couple to each other through the substrate waves, and eventually radiation performance of the array degrades. Depending on the elements' excitation phase, or along certain radiation directions, almost all of the power at the array input couples to the substrate waves and radiating array fails to function as a radiator, instead it behaves as a guiding structure; this phenomenon is known as scan blindness, and it must be solved for an array to function properly.
Electromagnetic bandgap (EBG) materials are engineered composite materials and capable to forbid wave propagation at frequencies within their bandgap. This thesis is focused on the integration of EBG materials and microstrip phased arrays. To be more specific, a multifunctional structure, which can be considered as a bandgap material, an antenna element, or a phased array consisted of the elements of the proposed structure, has been designed, fabricated, and measured. Numerical eigenmode and two port transmission methods are used for the bandgap investigation. Radiation properties of the structure has been improved using an impedance matching network. Scan analyses of both infinite and finite arrays have been studied. A prototype of 3 elements linear phased array of the proposed multifunctional structure has been fabricated and its active element pattern has been measured.
The proposed multifunctional bandgap-antenna structure suppresses both bound and radiating substrate waves. Therefore, it provides complete elimination of scan blindness along all directions. In addition, unit element of the multifunctional structure has similar radiation characteristics, such as radiation pattern, gain, and polarization, as a conventional microstrip antenna; whereas, its scan behavior is the key achievement over the conventional microstrip arrays. Different from conventional arrays, this structure does not support any mode propagating along the lateral directions in the array structure; therefore, power entering to the array structure will radiate to free space.