In this work, an analytical model for the study of passive intermodulation (PIM) on large reflector antennas is presented. Passive intermodulation, in the scattered field, arises when the scatterer is nonlinear, or when it presents junctions connecting linear materials. Its presence causes a degradation of some antenna parameters and, especially, the cross-polar level, which may rise by several tens of dB. It would then be useful to develop a technique to predict its influence, in order to take appropriate steps during antenna design. A heuristic model for the junction problem has previously been derived and validated with measurements in a time-domain physical optics (TD-PO) framework. These results are applied here to a TD-PO analysis of reflector antennas, in particular, for a satellite-communication antenna and for a radio-astronomy antenna.

Passive intermodulation on large reflector antennas

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.

Studies on microwave antennas: passive intermodulation distortion in antenna structures and design of microstrip antenna elements

Zones with nonlinear behavior on an object cause intermodulated components in the scattered electromagnetic field. This problem is relevant both for satellite communications and for radio-astronomical applications. Recently, a heuristic model for the passive intermodulation analysis has been derived in a time domain physical optics framework. The heuristic approach proposed requires the determination of few coefficients. Here, a genetic algorithm is introduced for the computation of the optimum values for these coefficients.

Genetic algorithms for the determination of the nonlinearity model coefficients in passive intermodulation scattering

A novel field-circuit hybrid numerical method is proposed to analyze the passive intermodulation (PIM) caused by nonlinear contacts. In this hybrid numerical method, the electromagnetic field part is simulated by a self-developed time-domain finite integration theory. Meanwhile, the circuit part is modeled by a number of nonlinear lumped elements, which can introduce the nonlinear feature of nonlinear contacts. After solving a multiscale problem, the field part and the circuit part can couple with each other self-consistently via the generalized Ampere's equation. To verify the accuracy of the proposed method, a practical example is discussed. The results show that the frequencies of each PIM order can match analytical results very well. Moreover, the PIM levels calculated by the proposed numerical method have a very good agreement with the measured results.

Numerical Analysis of Passive Intermodulation Arisen From Nonlinear Contacts in HPMW Devices

Passive intermodulation (PIM) arising from metallic contact junctions on the reflector antenna may cause the degradation of a satellite communication system. A time-domain method such as time-domain physical optics (TD-PO) can solve the nonlinear problems of PIM; however, the error accumulation is unacceptable in the analysis of electrically large objects. The rigorous full-wave frequency-domain method of electrically large objects is not suitable because it only can be applied at single frequency. An equivalent circuit model coupled with full-wave frequency-domain method is proposed to solve the PIM problems of electrically large objects. The responses of the nonlinearities at metal–insulator–metal (MIM) contacts are analyzed by the circuit method; the electromagnetic field distribution is evaluated by the rigorous full-wave frequency-domain method. Finally, the analysis of PIM on a reflector antenna illuminated by a practical horn is performed to exemplify the usefulness of the method presented in this paper.

Evaluation of Passive Intermodulation Using Full-Wave Frequency-Domain Method With Nonlinear Circuit Model

Passive intermodulation (PIM) is a phenomenon which often arises at junctions between different materials. This may be a major issue in tightly packed antenna farms as those typically present on communication satellites. Here, an heuristic nonlinear extension to the time-domain physical optics (TD-PO) is proposed, to take into account electromagnetic scattering at intermodulated frequencies when such a junction is illuminated by two impinging electromagnetic fields at different frequencies. Simulation results are compared with measures to validate the model.

C. Pelosi

Papers

A time-domain physical optics heuristic approach to passive intermodulation scattering