In many RFID practical applications, it is required that reader can effectively read tags which are placed in radiation covering area randomly. In this paper, a passive UHF dual-dipole tag antenna with quasi-isotropic patterns is designed, which can reduce the sensibility of tag read-orientation in a long distance. Two dipoles with four-axis reflection symmetric structure are used, and the two arms of the dipole are bent to fill the space of the antenna. In this way, a quasi-isotropic tag is easier to be obtained. The test results show that the gain deviation of the proposed antenna was less than 3.25 dB, and the maximum reading range in different directions was from 6.9 m to 10.0 m, with better quasi-isotropic performance and reading range than other commercial tags.
Radio frequency identification (RFID) technology is a noncontact automatic identification technique that acquires relevant data of an object with radio frequency to identify it. Since the passive ultra-high frequency (UHF) technology is characterized by great identification speed, multitarget identification, and long identification distance, it has been widely used in logistics and supply management, product manufacture and installation, library management, transportation, and other fields. A UHF RFID system consists of two units, a reader and a tag. Since a tag is not always oriented to a reader in some practical applications, such as management for luggage in airport or garment retail, it is required that reader can read tags which are at random placed within radiation coverage. Ordinary tag is usually a passive single-dipole tag and has a null reading zone along its axis, which may cause missing a read.
Isotropic antenna is a hypothetical antenna that radiates energy equally in all directions of the space. In reality, an antenna whose gain deviation in any arbitrary direction is less than 6 dB can be considered as a quasi-isotropic antenna [
In this paper, a passive UHF dual-dipole tag antenna with quasi-isotropic patterns is designed. Two dipoles with four-axis reflection symmetric structure are adopted; namely, after being folded the tag antenna can completely overlap not only along the axis of 0°–180° or 90°–270°, but also along the axis of 45°–225° or 135°–315°. The two arms of the dipole are folded to fill the space of the antenna. If only the resonant frequency of this antenna is adjusted in an appropriate range, its gain deviation in any arbitrary direction in the space can be held to be about 3 dB and a quasi-isotropic tag is easier to be implemented. The test results show that the gain deviation of the tag designed in this paper could achieve less than 3.25 dB and the maximum reading range in different directions was up to 6.9–10.0 meters.
The rest of this paper is organized as follows. In Section
Ordinary dipole tag is generally thin and long and is a deformation of half-wave dipole. The antenna is fed from its center. Hence its electric current is distributed along the antenna as roughly sinusoidal pattern and is zero at both ends. If the axis of an ordinary dipole tag coincides with
To weaken the influence of null reading zone of an ordinary dipole and guarantee a large reading range, a kind of dual-dipole that consists of two mutually perpendicular dipoles can be designed. The two dipoles are axial symmetric to offset respective null reading zone of a single dipole. For a half-wave dipole, its half power beam width is about 80°, so two mutually perpendicular dipoles can cover a half power band width of about 320° in the whole space. And in the rest space of about 40°, the power radiated by the two dipoles will be added, so the gain along the axis of two antennas will greatly increase to offset null reading zone of a single dipole. If the dual-dipole tag antenna designed in this paper completely overlaps after being folded not only along the axis of 0°–180° or 90°–270°, but also along the axis of 45°–225° or 135°–315°; that is, the dual-dipole antenna is four-axis reflection symmetric, it will radiate energy uniformly in the space. Theoretically, gain deviation of a dual-dipole tag antenna with four-axis reflection symmetry is approximately 3 dB. Its schematic drawing is shown as Figure
Radiation patterns of tag antenna with four-axis reflection symmetry and non-four-axis reflection symmetry.
Four-axis reflection symmetry
Non-four-axis reflection symmetry
The structure of the dual-dipole tag antenna designed in this paper is shown in Figure
Structure of the proposed dual-dipole tag antenna. The substrate of the antenna is made of PTFE, with an area of
The proposed antenna was simulated using HFSS (High Frequency Simulation Structure) software of Ansys Company based on the finite element method. The line width
First, a linear dual-dipole tag antenna was simulated. Figure
Simulated gain values in three planes of the three tag antennas (Unit: dB).
Gain |
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Gain deviation |
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Linear antenna max. |
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3.64 |
Linear antenna min. |
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One-fold antenna max. |
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3.28 |
One-fold antenna min. |
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Proposed antenna max. |
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2.64 |
Proposed antenna min. |
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Structure of the linear dual-dipole tag antenna. The substrate of the antenna is made of PTFE, with an area of
Then the segment
Structure of the one-fold dual-dipole tag antenna. The substrate of the antenna is made of PTFE, with an area of
Simulated normalized radiation patterns of the three tag antennas.
In order to obtain the uniform radiation and reduce the area of tag antenna, the antenna was bent to fill the space between
Parameter values of optimized antenna.
Parameters |
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Values (mm) | 54.0 | 15.0 | 18.0 | 50.0 | 1.0 | 9.8 | 8.8 |
The above simulated results show that the quasi-isotropic performance of tag antenna is easier to be obtained as long as the dual-dipole tag antenna is four-axis reflection symmetric. As the dipole arms are bent to fill the antenna space gradually, the quasi-isotropic performance of tag antenna becomes better since the gain deviation of the antenna radiation pattern reduces gradually.
Impedance match of the antenna can be achieved mainly through the adjustment of sizes of feed loop if the four arms of dual-dipole are basically constant. The width of feed loop was fixed to be 1 mm, and by adjusting its inner radius and outer radius, appropriate impedance can be obtained to match the impedance of chip. The Monza 4D chip from Impinj was used, with an impedance of 11-j143 Ω at 915 MHz [
Simulated impedance of the proposed antenna with different sizes of the feed loop.
Simulated reflection coefficient of the proposed antenna with different sizes of the feed loop.
For comparison, three representative commercial quasi-isotropic tags, Invengo TF8023, Impinj H47, and UPM frog 3D, were also simulated. The three tags are all designed with Monza 4 of Impinj. Figure
Dimensions of the three commercial tag antennas.
Tags | Invengo |
Impinj |
UPM | |||
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Parameters |
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Values (mm) | 65 | 70 | 54 | 44 | 54 | 50 |
Simulated gain values of the three commercial tag antennas in three planes (Unit: dB).
Gain |
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Gain deviation |
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Invengo TF8023 max. |
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1.97 | 4.02 |
Invengo TF8023 min. |
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0.87 | |
Impinj H47 max. |
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1.59 | 4.39 |
Impinj H47 min. |
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0.44 | |
UPM frog 3D max. |
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2.61 | 5.12 |
UPM frog 3D min. |
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−0.25 |
Structure of the three commercial tag antennas. The area of substrate is
Invengo TF8023
Impinj H47
UPM frog 3D
The simulation results show that the tag antenna with four-axis reflection symmetric structure, due to its uniform far-field radiation, has less gain deviation than a tag antenna with non-four-axis reflection symmetric structure.
The photograph of the fabricated dual-dipole tag designed in this paper is shown in Figure
Photograph of the fabricated tag.
To verify the performance of the tag, its reading orientation, reading range, and antenna impedance were tested. For comparison, Invengo TF8023, Impinj H47, and UPM frog 3D were also tested. Speedway Revolution Reader of Impinj, with operation frequency of 902 MHz–928 MHz, output power of 10 dBm–30 dBm, and adjustment step of 0.25 dBm, was used to test reading orientation and reading range. CSL-RHCP of CSL Company was used as a reader antenna, with gain of 6 dBi.
The reading range and the reading orientation of the tags were measured in an anechoic chamber as illustrated in Figure
Measurement environment of the tag.
According to the parameter test method of performance of EPC UHF RFID tag [
Minimum power required for the test of the four kinds of tag antennas in three planes.
Power |
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Gain deviation |
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Proposed antenna max. |
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3.25 |
Proposed antenna min. |
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Invengo TF8023 max. |
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5.25 |
Invengo TF8023 min. |
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Impinj H47 max. |
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5.75 |
Impinj H47 min. |
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UPM frog 3D max. |
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6.50 |
UPM frog 3D min. |
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Measured normalized radiation patterns of the four kinds of tag antennas.
The test results indicate that the tag antenna designed in this paper, due to its four-axis reflection symmetry, has better quasi-isotropic performance than a tag antenna with non-four-axis reflection symmetric structure.
MS4624B vector network analyzer of Anritsu Company was adopted to measure the impedance of the tag antenna. The dual-dipole antenna designed in this paper can be considered as two dipole antennas with single port. Dipole antenna is a kind of balanced antenna and cannot be directly measured with a vector network analyzer. According to analysis of Tikhov et al., indirect measurement based on the method of images is feasible [
Measurement setup for the proposed antenna impedance.
Measured impedance of the proposed antenna.
Using a complex impedance reflection coefficient calculation formula, the measured power reflection coefficient curve was converted with the method described by Mo and Qin [
Measured reflection coefficient of the proposed antenna.
In this paper, a passive UHF dual-dipole tag antenna with quasi-isotropic patterns is designed, which can be read reliably from a long distance in different directions. Owing to the four-axis reflection symmetric structure and the bent arms, its gain deviation in any arbitrary direction is approximately 3 dB and it could be reliably read within a range of more than 6 meters in different directions. Three representative commercial dual-dipole quasi-isotropic tag antennas were compared in this paper. The results show that the proposed four-axis reflection symmetric tag antenna is with better quasi-isotropic performance and reading range than other commercial tags. Therefore, to effectively reduce null reading zone of ordinary single dipole and realize a larger reading range, four-axis reflection symmetric structure should be used for dual-dipole tag antenna with quasi-isotropic patterns.
The authors do not have any conflict of interests with the content of the paper.
This work was supported by the National Key Technology R&D Program of China (2009BADB9B09) and the National Water Pollution Control and Management Technology Major Projects (2008ZX07420-004).