Stepped Septum TEM Cell Impedance Matching
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Solution Overview
Problem
Conventional TEM test cells face challenges in maintaining a low voltage standing wave ratio (VSWR) while ensuring a uniform electromagnetic field, leading to limited bandwidth due to difficulties in impedance matching between the septum and port connectors.
Innovation Solution
A TEM mode test cell design featuring a stepped septum with a central portion and end portions having a stepped contour, with predetermined dimensions and slopes to minimize VSWR, coupled with a stepped transmission line transformer for optimal impedance matching and bandwidth extension.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional septum design is used in a TEM test cell, then the structure is simple, but the voltage standing wave ratio (VSWR) is high and bandwidth is limited
Solution Approach 1:
The septum is divided into multiple segments with different cross-sectional dimensions along its length. Each segment has a specific impedance transformation function, creating a stepped impedance transformer that progressively matches the impedance from the connector to the center of the cell, thereby reducing VSWR and extending bandwidth.
Solution Approach 2:
The cross-sectional dimensions of the septum are varied at different positions to create distinct impedance zones. By changing the physical parameters (width, height) of the septum segments, the characteristic impedance is transformed step-by-step to achieve better impedance matching across a broader frequency range.
2Reliability
If the septum cross-sectional dimension is reduced at the connector end, then impedance matching improves, but the electromagnetic field uniformity may be affected
Solution Approach 1:
The septum is segmented into multiple zones with gradually changing dimensions. This segmentation allows the impedance to be transformed in discrete steps rather than abruptly, maintaining field uniformity in the measurement region while achieving good impedance matching at the connector through the progressive impedance transformation.
Solution Approach 2:
Different portions of the septum have different cross-sectional dimensions optimized for their specific functions: the connector-end segments are narrower for impedance matching, while the central measurement region maintains appropriate dimensions for field uniformity. Each local region is optimized for its specific requirement.
Data Source
AI summary
A TEM mode cell having an internal TEM element and at least one port on the cell comprising a stepped transmission line transformer coupling said element to the port. The cell comprises a hollow metallic housing having an enlarged center with opposite open ends, and a pair of end sections closing off said center open ends and tapering to a distal end of reduced cross sectional area. A connector at the distal end of each end section, and a stepped septum mounted within the housing and insulated therefrom, the septum coupled to and extending between the connectors. The septum having a central portion joining a pair of end portions each having a stepped contour reducing in size toward its connector. Each end portion has at least three steps in contour, with transition slopes between the steps being greatest leading to said central portion and the least leading to its connector.