Baluns, Hybrids and Power Dividers for Ultra- Wideband Antennas

Licentiatavhandling, 2012

UWB (ultra wideband), depending on the application requirement, is usually referred to 2:1 bandwidth for radio/wireless applications. Here in this thesis it is also used to denote extremely wideband antenna and components at several octave bandwidth, particularly for high frequency applications, such as the 1 – 10 GHz antenna system needed for the Square Kilometer Array (SKA) radio telescope and the 2 – 14 GHz antenna system for the VLBI2010 (Very Long Base line Interferometer) project. One of the critical parts of an UWB antenna is its feed circuits, because this network must deliver the signal from the UWB antenna to LNAs without introducing losses that severely degrade performance in low noise systems like SKA and VLBI2010. The first part of this thesis is primarily associated with designing UWB baluns for the decade bandwidth Eleven feed. The second part of the thesis addresses UWB components using the novel gap waveguide technology. The gap waveguide is an UWB technology, because it inherently is more broadband than normal hollow waveguides and can be used over more than octave bandwidth. Also, the gap waveguide is less dispersive than 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 when the antenna is fed by four baluns; the balun being 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 gap waveguide makes it possible to realize low loss circuits at millimeter and sub-millimeter waves, without having the problems of metal contact between joining metal pieces 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 when designing, simulating and measuring gap waveguide components of different kind, because they show under which conditions and accuracies standard waveguide interfaces can be used. 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.