Wideband Reconfigurable Antenna Designs
thesisposted on 05.08.2019 by Omid Manoochehri
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
We propose to study wideband reconfigurable lens antennas because these antennas are used in two main types of applications: (1) automotive radar systems and (2) direction finding radar systems. These antennas should be low profile, wideband and have capability to handle high RF power. Most the radar systems do not have enough space for their antennas therefore the main challenge for antenna designers is proposing small antenna. These antennas can be used also for satellite communications where all above requirements are not easily achieved. Automotive radar systems typically operate in the microwave region or mm-wave region where lens structures are good candidates to be used for reconfigurable antennas. A good example is the Luneburg lens antenna. A Luneburg lens is fed by microstrip or horn antennas. The main challenge of Luneburg antennas are their size and fabrication process. Some authors proposed semi-spherical dielectric lens fed by microstrip antennas to reduce the size but microstrip antenna could not handle high power. Others used one-layer dielectric instead of multi dielectric layers to have easier fabrication process, but the total efficiency is not sufficient. We have addressed some of these challenges in our preliminary results: low profile, wide band, capability to handle high RF power and minimum difficulty to fabricate. We designed and fabricated an ultra-wideband multibeam microwave lens antenna operating from 8 GHz to 18 GHz to cover X and Ku band for radar systems. The antenna consists of four excitation ports connected to a parallel plate waveguide filled with a cylindrical dielectric slab. We simplified the fabrication process by proposing a material with a uniform dielectric constant whose value is optimized to maximize gain performance while simplifying manufacturing process, instead of having an index of refraction that varies with the radial distance from the axis of the cylindrical lens. We addressed the capability to handle high RF power and reducing size by replacing the antenna feed with coaxial connector pins to reduce the overall size and increase the bandwidth compared to prior designs. Many direction finding systems typically have 8 to10 antennas placed along a circle and another one, the reference antenna, placed at its center. When a wave from an unknown direction is incident on the system, its phase and amplitude are measured by all antennas along the circle and compared with the reference antenna to estimate the direction of arrival of the signal. The reference antenna is omnidirectional and should have low ripple pattern in all directions to minimize the estimation error. Biconical antennas are good candidates as reference antennas, but the main challenge is designing wideband biconical antenna to receive both vertical and horizontal polarizations, since the polarization of the incident signal is unknown. To address the lack of knowledge about the polarization of the incident signal, some authors proposed the use of polarizers to rotate the polarization of the incident wave. However, the biconical antenna pattern has ripples in the azimuth plane at higher frequency and the main antenna beam drifts away from the horizontal plane and this could create amplitude estimation errors. We designed and fabricated a reference antenna consisting of an ultrawideband biconical antenna operating from 2 GHz to 18 GHz surrounded with strip polarizers to have the capability to receive both horizontal and vertical polarization and cover the S,C,X, Ku bands. This antenna uses 5 layers of strips and un-balanced cones with two different cone angles to create stable patterns. Several layers of polarizers are used to rotate the plane of polarization of the incident signals so that it becomes polarized at a slant angle of 45°. A direction finding system should be able to detect signals in the VHF and UHF bands and the main challenge is designing a physically small antenna. TEM horn antennas are good candidates and some solutions include using metamaterials and lumped loading elements to decrease the frequency operation. However, these (passive) methods are not sufficient to decrease the frequency operation to the VHF band. We also designed and fabricated TEM horn antennas operating from 20 MHz to 2.5 GHz. 8 TEM horn antennas can be placed on a circle with 1 m radius with 45° of angular separation. We proposed an active circuit instead of a passive matching circuit to compensate for the capacitive behavior of the antenna at low frequencies. A negative impedance capacitor circuit decreases the lowest frequency from 200 MHz to 20 MHz.