EM Design for New Ultra Wide Band Technologies: Eleven Feed Baluns, Bowtie Antennas and Gap Waveguides

University dissertation from Chalmers University of Technology

Abstract: UWB (ultra wideband), depending on the application requirement, is usually referred to more than 500 MHz bandwidth for radio/wireless applications. Here in this thesis it is also used to denote extremely wideband antennas and components of several octaves bandwidth, particularly for high frequency applications. One of the critical parts of an UWB antenna is its feeding circuits, which should deliver the signal from the UWB antenna to LNAs with as small losses as possible. This is in particular important in future radio telescopes for SKA (Square Kilometer Array) and VLBI2010 (a Very Long Baseline Interferometry project), to avoid severe degradations of their low noise performance. The first part of this thesis is primarily associated with designing UWB baluns for the decade bandwidth Eleven feed, which is a candidate for use in large reflector antennas for SKA and VLBI2010 telescopes. The second topic of the thesis addresses a four-port dual polarized UWB bowtie antennas, an alternative simpler candidate for commercial UWB sensor and radar applications in the unlicensed 3-10 GHz frequency band. The third part deals with UWB waveguide components and antennas using the novel gap waveguide technology, mainly intended for millimeter and submillimeter wave applications such as radio links in V-, E-, and W-band covering 50-110 GHz. The gap waveguide is an UWB technology, because it inherently has a broader bandwidth than normal hollow waveguides and can be used over more than octave bandwidth. In addition, the gap waveguide is less dispersive than a normal hollow waveguide. The decade bandwidth Eleven antenna can be considered to have either four differential ports or eight single-ended ports, and different ways of combining the eight ports are needed for different purposes and applications. The most feasible combination is that the antenna is fed by four baluns and two additional power dividers, to get dual-polarized directive beam suitable for feeding a reflector antenna. A balun is a device that transforms a balanced two-wire line to an unbalanced coaxial or microstrip line. The first part of the thesis presents a new passive balun solution for the Eleven antenna and a way to integrate four of these baluns together with the antenna in such a way that the four differential ports transform to four single-ended ports. It is also important to verify and evaluate the radiation efficiency of a multiport antenna before being integrated in the system. This thesis addresses how to measure the radiation efficiency of a multiport antenna excluding the losses in the feeding network used for the measurement, particularly when the impedance match between the antenna and the feeding network is not so perfect. The new dual-polarized self-grounded bowtie antenna has many advantages. One of the characteristics of this antenna is its flexibility for different applications: a multi-port antenna in MIMO systems, a directional dual-polarized (linear or circular) antenna in radar or sensor systems. The four-armed geometry of the antenna has been optimized in the design for the best impedance matching to 50 ohm coax over 1.5 – 3 GHz, an octave bandwidth, which is desired for a micro base transceiver station antenna. The gap waveguide makes it possible to realize low loss circuits at millimeter and sub-millimeter waves, without the need of metal contact between joining metal pieces, a difficulty present when using normal hollow waveguides. The present work includes a study of the resemblance between the groove gap waveguide and the standard hollow rectangular waveguide, and between the ridge gap waveguide and the normal hollow ridge waveguide. The dispersion diagrams and characteristic impedances have been compared. These results are very useful for designing, simulating and measuring gap waveguide components of different kind, because they show under which conditions and accuracies standard waveguide interfaces can be used. The new design of microstrip-ridge gap waveguide by using via holes in the PCBs, makes a solution for high frequency circuits. The present thesis also includes the study about pin sensitivity, losses, and a comparison of this type of gap waveguide with the performance of normal microstrip line and inverted microstrip lines. A WR-15 to microstrip-ridge gap waveguide transition has also designed. The results are verified with measurements on microstrip-ridge gap waveguides with WR15 transitions at both ends. Finally, the thesis presents an UWB microstrip power divider that was designed and packaged using gap waveguide technology, and a ridge gap waveguide ring hybrid. The power divider has a bandwidth of 2–14 GHz, and it is intended to be used in the feed network for the Eleven feed.