Coupling Device Impedance Matching Guided Wave Radar Probe
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Solution Overview
Problem
Guided Wave Radar (GWR) level transmitters face accuracy issues and reduced maximum level range due to parasitic reflections caused by impedance mismatches between coaxial cables, nozzles, and free-space environments, leading to a dead-zone near the top of tanks where measurements are unreliable.
Innovation Solution
The introduction of a coupling device with a subwavelength coaxial transmission line and a mode converter with metal fingers, optionally combined with a tapered dielectric coating, which provides improved impedance matching and minimizes ringing effects by confining electromagnetic energy, reducing signal loss and extending measurement range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a coaxial cable with 50-100 Ω impedance is used to connect the transceiver to the tank, then the signal can be transmitted effectively, but impedance mismatch with the free-space impedance (377 Ω) causes parasitic reflections that reduce measurement accuracy and create dead-zones
Solution Approach 1:
A mode converter structure is introduced as an intermediary component between the coaxial cable and the probe. This mode converter includes a coaxial section with specific impedance transformation characteristics that gradually transitions the signal from the 50-100 Ω coaxial cable impedance to match the 377 Ω free-space impedance, thereby reducing parasitic reflections while maintaining signal transmission reliability
Solution Approach 2:
The impedance parameter is gradually changed along the length of the mode converter structure. By varying the geometric parameters (such as the diameter of the outer conductor or the spacing between conductors) along the transmission path, the impedance is transformed from the low value at the coaxial cable end to the high value matching free-space impedance at the probe end, minimizing reflections
2Length of moving object
If the nozzle diameter is increased to accommodate larger probes, then the probe can be installed, but the impedance mismatch with free-space impedance increases causing larger dead-zones and reduced measurement accuracy near the tank top
Solution Approach 1:
The mode converter acts as an intermediary impedance transformation section between the nozzle and the probe. It compensates for the impedance effect of larger nozzle diameters by providing a gradual transition to free-space impedance, ensuring that even with larger nozzles required for probe installation, the measurement accuracy near the tank top is maintained
Solution Approach 2:
The geometric parameters of the mode converter are optimized to compensate for variations in nozzle diameter. By adjusting the dimensions and impedance profile of the mode converter, the system maintains proper impedance matching regardless of the specific nozzle size used for probe installation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enhances the accuracy of level measurements and increases the maximum level range by reducing unwanted echoes and dead-zones, allowing for more reliable product level detection near the tank's top.
Implementation Method 1
The introduction of a coupling device with a subwavelength coaxial transmission line and a mode converter with metal fingers, optionally combined with a tapered dielectric coating, which provides improved impedance matching
Implementation Method 2
an impedance adaptation region in the form of a tapered dielectric coating 11, having a length of about 2λ, such as 2λ ± 20%, to further improve the impedance adaptation towards to free-space impedance of about 377 Ω
Implementation Method 3
These parasitic reflections at the input and output of the nozzle reduce the remaining energy of the E-M wave to be used for level measurement of the product
Data Source
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Figure 3
AI summary
A coupling device (3, 5, 6) for impedance matching a probe of a guided wave radar (GWR) system (100). A feed-through (3) is for connecting to a coaxial cable or other transmission line connector (2) that includes an inner conductor which connects to an output of a transceiver (1) and an outer conductor that connects to an outer metal sleeve. A subwavelength coaxial transmission line (CTL) (5) having a length from λ/5 to λ/2 is coupled to the feed-through including an inner conductor connected to the inner conductor of the feed-through and an outer conductor connected to the outer metal sleeve. A mode converter (MC) (6) having a plurality of metal fingers (7) of length 2 λ ± twenty percent is connected to the outer conductor of the subwavelength CTL, where the MC includes a dielectric coating (58) on its inner conductor connected to the inner conductor of the subwavelength CTL.