Intermodulation Distortion in Periodic Structures and Microstructures

Doktorsavhandling, 2025

The rapid growth of wireless communication increases the demand for higher data rates, low latency, and availability. This leads to a complex infrastructure where multiple systems coexist in a densely populated frequency spectrum, requiring multi-carrier systems with high power and sensitive receivers. However, the requirements of modern communication systems increase the risk of interference from weak spurious signals. The spurious signals result from nonlinear mixing of two or more signals with different frequencies. The mixing products are denoted as intermodulation distortion (IMD) in active devices and passive intermodulation (PIM) in passive ones. The demand for highly linear devices is increasing due to these new challenges. As a result, understanding nonlinear behavior is crucial to develop accurate nonlinear models that will help mitigate IMD and PIM.

This thesis analyzes and models nonlinear effects in different structures, where the first part regards IMD in periodic structures. The periodic structure employed in this thesis is a loaded-line phase shifter that is periodically loaded by varactor-diodes. Various design factors were studied, including periodicity, bias, input power, and unit cell configuration. A polynomial varactor model was validated experimentally and scaled for circuit simulations to optimize phase-shift/loss and linearity. Results showed that evenly distributing varactor capacitance improves phase shift/loss, with a trade-off between minimizing loss and IMD.

The second part of this thesis explores PIM generation in microstructures. Firstly, PIM was analyzed using a rectangular coaxial transmission line with a replaceable aluminum alloy (AlSi10Mg) center conductor fabricated via additive manufacturing (AM) and compared to a milled aluminum counterpart. A nonlinear distributed transmission line was developed. It was found that AM fabricated conductors exhibit higher PIM levels due to increased surface roughness.

Secondly, the generation of PIM in anodized antenna feeding networks for satellite communications was analyzed. It was found that anodization could both improve and degrade PIM performance compared to untreated feeding networks. Cracks formed in the anodized coating, which were hypothesized to act as nonlinear sources. A thinner anodized coating resulted in fewer cracks and the best PIM performance, while a thicker coating produced more cracks and led to the worst PIM performance.