We propose a wide bandwidth antenna with a circular polarization for universal Ultra High Frequency (UHF) radio-frequency identification (RFID) reader applications. To achieve a wide 3 dB axial ratio (AR) bandwidth, three T-shaped microstrip lines are inserted into the ground plane. The measured impedance bandwidth of the proposed antenna is 480 MHz and extends from 660 to 1080 MHz, and the 3 dB AR bandwidth is 350 MHz and extends from 800 to 1155 MHz. The radiation pattern is a bidirectional pattern with a maximum antenna gain of 3.67 dBi. The overall size of the proposed antenna is 114 × 114 × 0.8 mm3.
Radio-frequency identification (RFID) technology is used in devices that transmit and receive information using radio frequency (RF) from electronic tags attached to objects in various applications. The RFID frequency bands are the HF band at 13.56 MHz, UHF band from 860 to 960 MHz, and ISM band at 2.4 GHz [
In the UHF band RFID system, the antenna plays an important role in the communications between the reader and tag. The characteristics of the antenna determine the recognition distance between the reader and the tag. To maximize the distance, the return loss characteristics of the antenna should be as small as possible in the operating frequency band, and the antenna should be designed with a circular polarization [
In this paper, we describe the design and fabrication of an antenna with circular polarization for an RFID reader in the UHF band. In terms of impedance matching, we used three microstrip lines to achieve an impedance of −10 dB across the wide application band. We also used a T-shaped microstrip line to induce a circular polarization characteristic.
The structure of the proposed antenna is shown in Figure
Structure and dimensions (in mm) of the proposed antenna:
Photograph of the fabricated antenna.
The design process used for the antenna consisted of three steps, which are illustrated in Figure
Three steps in the antenna design process.
Antenna 1
Antenna 2
Antenna 3 (proposed)
The reflection coefficients and axial ratio (AR) simulation characteristics of the three steps were analyzed using HFSS version 12, and the results are shown in Figure
Simulation analysis results of the three methods.
Reflection coefficient
Axial ratio
Antenna 1 is a basic antenna with a CPW L-shaped feed structure. It exhibited a good impedance matching characteristic, and a −10 dB reflection coefficient bandwidth was achieved over a wide bandwidth of 331 MHz from 739 to 1070 MHz. However, it did not achieve the desired 3 dB axial ratio bandwidth (ARBW). In Antenna 2, a T-shaped microstrip line was added to improve the 3 dB AR characteristics. The resulting antenna exhibited a suitably wide bandwidth of 345 MHz from 772 to 1117 MHz, although the impedance matching was poor. Therefore, in Antenna 3, the impedance was connected to the ground of the three microstrip lines. As a result, a −10 dB reflection coefficient bandwidth of 480 MHz from 714 to 1194 MHz was achieved, and the 3 dB ARBW was 380 MHz from 775 to 1155 MHz.
The 3 dB ARBW results for the values of
Detailed results of the proposed antenna for various T-shaped microstrip line lengths.
|
|
3 dB ARBW (MHz) |
---|---|---|
35 | 10 | 209 |
35 | 12 | 212 |
35 | 14 | 291 |
|
|
|
35 | 18 | 338 |
35 | 20 | 342 |
Three dB axial ratios of the proposed antenna with various
The impedance bandwidth of the manufactured antenna was measured using a Network Analyzer (Agilent Co.), and the results are shown in Figure
Simulated and measured reflection coefficient results of the manufactured antenna.
The simulated −10 dB reflection coefficient bandwidth of the proposed antenna extended from 714 to 1194 MHz (480 MHz), and the fractional bandwidth was 50.3%. The measured −10 dB reflection coefficient bandwidth of the manufactured antenna was 660–1080 MHz (420 MHz), and the fractional bandwidth was 48.27%.
The simulated and measured 3 dB ARBW results of the manufactured antenna are shown in Figure
Simulated and measured AR bandwidth results of the manufactured antenna.
The simulated 3 dB ARBW of the proposed antenna is 775–1155 MHz (380 MHz), and the measured 3 dB ARBW of the manufactured antenna is about 800–1150 MHz (350 MHz).
The radiation pattern of both simulated and measured values, for the
Radiation pattern of the manufactured antenna.
Simulated radiation pattern result
Measured radiation pattern result
The radiation pattern of the proposed antenna exhibited good bidirectional characteristics.
In addition, right-hand circular polarized radiation (RHCP) was radiated along the front side of the proposed antenna, and left-hand circular polarized radiation was radiated along the back side of the proposed antenna. The maximum gain of the proposed antenna (RHCP and LHCP) was concentrated along the +
The gain and radiation efficiency results of the proposed antenna are shown in Figure
Gain and radiation efficiency results of the manufactured antenna.
The gain analysis results fluctuated between 3.4 and 3.8 dBi from 750 to 1000 MHz. The measured maximum gain was 3.67 dBi at 750 MHz, and the simulated maximum gain of 3.8 dBi was observed at 1000 MHz. The proposed antenna observed a radiation efficiency of over 90% in impedance bandwidth both simulated and measured results.
The comprehensive results of the proposed antenna are listed in Table
Comprehensive results of the proposed antenna.
Simulated | Measured | |
---|---|---|
−10 dB reflection coefficient range [MHz] | 714–1194 | 660–1080 |
Impedance bandwidth [MHz] | 480 | 420 |
Fractional bandwidth [%] | 50.3 | 48.27 |
Resonant frequency [MHz] | 1000 | 900 |
Maximum gain [dBi] | 3.8 | 3.67 |
The proposed antenna is compared to other antennas with UHF band for RFID reader in Table
Comparison of the proposed antenna and different antenna.
Antennas | −10 dB |
3 dB |
Gain |
Dimensions |
---|---|---|---|---|
[ |
904–941/37 | 918–929/11 | 3.8 | 90 × 90 × 4.572 |
[ |
618–998/480 | 791–1123/332 | 3.4 | 120 × 120 × 0.8 |
[ |
860–930/70 | — | 3.7 | 110 × 110 × 5 |
[ |
891–928/37 | 907–915/8 | 5.85 | 54 × 54 × 1.6 |
[ |
820–880/60 |
864–887/19 |
1.6 | 90 × 90 × 1.6 |
[ |
902–928/26 | 900–936/36 | 1.35 | 105 × 90 × 1.6 |
Proposed antenna | 660–1080/420 | 775–1155/380 | 3.67 | 114 × 114 × 0.8 |
The proposed antenna exhibited circular polarized wideband characteristics. A T-shaped microstrip line induced the circular polarization characteristics and 3 dB ARBW. In addition, the wideband characteristics were matched due to the three microstrip lines. The overall size of the fabricated antenna was 114 × 114 × 0.8 mm3.
The measured impedance bandwidth (−10 dB reflection coefficient) results were 420 MHz from 660 to 1080 MHz, and the 3 dB ARBW results were 350 MHz from 800 to 1155 MHz. The analysis of the radiation pattern showed a bidirectional pattern and a maximum measured antenna gain of 3.67 dBi.
The results of a comprehensive analysis of both the measurements and simulation were in good agreement.
The authors declare that they have no conflicts of interest.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03931806).