Studies on Microwave Antennas: Passive Intermodulation Distortion in Antenna Structures and Design of Microstrip Antenna Elements

Passive intermodulation (PIM) measurements of antennas and the design of dual-polarised microstrip antenna elements are investigated in this thesis. The emphasis is on the development of a passive intermodulation source localisation method and on the studies of the PIM source behaviour under various impedance loading conditions. Passive intermodulation distortion can be harmful in wireless communication systems where the difference between the transmitted and the received power is large such as in GSM base stations. One of the most critical components is the antenna. As the antennas become more complex, the risk of getting excessive PIM distortion increases. Therefore, the antenna designer needs to have knowledge about the PIM phenomenon and proper tools to investigate various antenna structure configurations. The concept of PIM near-field measurement is introduced and the implementation of the equipment for the GSM 900 frequency band is presented. The performance of the scanner is analysed and demonstrated with measurements. The scanner is capable of locating PIM sources in antennas and other open structures down to −110 dBm when the transmitted power is 43 dBm. The limiting factors of the scanner sensitivity are considered and several sensitivity measurements are performed. An analysis of the impedance loading effects on PIM measurements is carried out. The conducted measurements support the theory that the loading impedance effects can be several decibels, when the source or load return loss is less than 20 dB, which is the case with many filters and antennas used in base stations. Dual-polarised antenna elements are needed in various array antenna applications. Two compact antenna element designs with moderate bandwidth and dual polarisation are introduced in the 12 GHz frequency band. The elements use a dual-resonant structure in order to achieve an impedance bandwidth of 16 % (L retn > 10 dB). The arrangement of the antenna feeds results in an isolation of 35 dB between the ports.