Compact CLL Antenna for Circular Polarization in RFID Readers
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Solution Overview
Problem
Current RFID reading devices in the UHF frequency range face challenges in achieving compact antenna designs that are robust against environmental disturbances and can operate effectively at various angular positions, while also covering the required frequency band and maintaining sufficient bandwidth and polarization.
Innovation Solution
The development of a compact, circularly polarized antenna using interconnected Capacitively Loaded Loop (CLL) individual antennas, where at least two CLL antennas are oriented differently and operated out of phase to achieve circular polarization, with adjustable capacitance values to tune the resonant frequency and adapt to environmental conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If conventional linearly polarized antennas are used to reduce antenna size, then the antenna base area can be smaller than λ/4, but the reading distance is limited to direct contact or a few centimeters
Solution Approach 1:
The antenna is divided into two separate CLL structures (first and second capacitively loaded loops) that are oriented perpendicular to each other. Each CLL is independently tuned and operated, allowing the system to achieve circular polarization while maintaining a compact footprint smaller than λ/4. The segmentation enables each loop to contribute to both horizontal and vertical polarization components.
Solution Approach 2:
The antenna system dynamically switches between different polarization states by independently controlling the excitation of the two perpendicular CLL structures. By adjusting the phase and amplitude of the signals fed to each loop, the system can generate circular polarization for extended reading distances or linear polarization for optimized performance in specific orientations, adapting to different operational requirements.
2Adaptability or versatility
If circularly polarized antennas are used to enable reading at various angular positions, then the antenna can supply energy in all angular positions, but the antenna size increases to at least λ/4×λ/4
Solution Approach 1:
The first and second CLL structures are nested within each other in a perpendicular configuration, with both loops sharing the same compact spatial envelope. This nested arrangement allows the antenna to achieve circular polarization capabilities typically requiring larger areas, while maintaining a base area smaller than λ/4×λ/4. The inner CLL is positioned within the spatial bounds of the outer CLL, maximizing space utilization.
3Volume of stationary object
If compact antenna designs are used to reduce device size, then the housing can be more compact, but the bandwidth and adaptability to environmental conditions are reduced
Solution Approach 1:
Each CLL structure incorporates adjustable capacitive elements that allow independent tuning of the resonance frequency. By varying the capacitance values in the first and second CLL structures, the system can be adapted to operate across different frequency bands within the UHF range (850-950 MHz). This parameter adjustment capability enables the compact antenna to maintain optimal performance across the required frequency band without increasing physical size.
4Reliability
If conventional antennas are used to achieve robustness against environmental disturbances, then the antenna can operate near metal surfaces, but the antenna size and complexity increase
Solution Approach 1:
The antenna system uses two identical or similar CLL structures oriented perpendicular to each other, where the second CLL is essentially a copy of the first but rotated 90 degrees. This copying approach provides redundancy and balance, improving robustness against environmental disturbances such as nearby metal surfaces or dielectric objects. The symmetrical configuration ensures that degradation in one polarization component is compensated by the other, maintaining overall system reliability without requiring complex adaptive structures.
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 enables a compact RFID reading device with improved robustness and adaptability, capable of operating across the UHF frequency band with circular polarization, reducing production costs and enhancing performance in diverse applications.
Implementation Method 1
The change in time of a transmission signal applied to the antenna generates a change in the magnetic field. This magnetic flux is absorbed and amplified by the CLL (resonance increase). The current induced in the two arms of the CLL creates a high voltage difference across the capacitive element. If the dimensions are correct, the capacitance stored in the electrical field is large enough to balance the inductance of the two coils formed by the antenna loop and the CLL. The behavior is reminiscent of an RLC resonant circuit.
Implementation Method 2
The change in time of a transmission signal applied to the antenna generates a change in the magnetic field. This magnetic flux is absorbed and amplified by the CLL (resonance increase). The current induced in the two arms of the CLL creates a high voltage difference across the capacitive element.
Implementation Method 3
circularly polarized antennas are preferably used in RFID reading devices in order to supply and read energy to transponders in all angular positions
Data Source
Figure 1~2
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Figure 5~6
AI summary
An antenna (100) for an RFID reading device is described, comprising at least one CLL single antenna (10). A CLL single antenna (10) is an antenna having an inner antenna loop (12) and a capacitively charged conductor loop (14) surrounding the inner antenna loop (12), into which a capacitor (16) is inserted. The antenna (100) includes several CLL single antennas (10a-d) oriented differently and a power supply circuit (26) to operate the CLL single antennas (10a-d) to generate a circular polarization with a mutual phase shift.