NOVEL CIRCULARLY POLARIZED LOOP COUPLED DIPOLE ANTENNA Andrey S. Andrenko Fujitsu Laboratories LTD. YRP R&D Center, 5-5, Hikari-no-Oka, Yokosuka, 239-0847, Japan, Email:
[email protected] ABSTRACT This paper presents the design principle of a novel circularly polarized printed antenna having simple unobstructed layout and single feeding port. At the same time, the proposed antenna demonstrates good circular polarization performance. The design and optimization of two antenna prototypes have been carried out for RFID applications in 953 MHz frequency band. Both simulation and measurement data on VSWR, circular polarization AR, and antenna gain are presented. 1.
INTRODUCTION
Latest developments of wireless communication technologies have recently brought about a renewed need for the design of circularly polarized (CP) antennas. In particular, fast-growing RFID applications require compact, low-cost CP antennas to be used in various RFID read-write (R/W) systems. Simplicity in antenna feeding network is a critical design requirement for RFID system applications. Conventional approach in designing CP planar printed dipole/monopole antennas uses two separate antenna elements fed by their own feeding networks usually containing phase shifters or 90-degree hybrids, which contributes to the system design complexity. There are also several known techniques in designing single-feed CP patch antennas [1]. This paper presents the design and operational principle of a novel CP loop coupled dipole antenna (LCDA). The objective of this work has been to design an RFID R/W antenna having simple unobstructed layout with a single feeding port yet demonstrating good CP performance. CST Microwave Studio has been used for full-wave EM simulation and optimization of antenna parameters towards improving the CP axial ratio (AR). 2.
r
loops would produce E-field El that is normal and
r
equal in amplitude to the dipole radiation field Ed with 90-degree phase difference between them at a design frequency of CP operation. The conditions are as follows: r r ⎧⎪ | Ed |=| El | ⎫⎪ (1) r ⎨ r o⎬ ⎪⎩∠Ed = ∠El + 90 ⎪⎭ These design requirements have been achieved by carefully selecting the shape of rectangular loops and their placement with respect to the dipole element so that the key design parameters are the distance between the dipole and loop strips, the distance from loop center line to the dipole end point, and the loop length-to-width ratio. These parameters have been numerically optimized to realize the best right-hand circularly polarized (RHCP) at RFID operation frequency of 953 MHz. EM simulations have shown that in producing the CP radiation the resonance frequency of dipole element should be slightly lower that of EM coupled loops. In such a case, CP with a minimal AR would be achieved at a frequency between those resonant frequencies of dipole and loops. LCDA shown in Fig. 1 produces RHCP E-field as observed along the z-axis. However, if one places coupled loops symmetrically with respect to the dipole element line LHCP radiation is easily obtained.
DESIGN PRINCIPLE OF LCDA
The proposed design is presented in Fig. 1 showing the top view of a planar antenna printed on one side of a glass substrate. The antenna consists of two parts: a 2 mm-wide strip dipole element fed at its center by a conventional 50-Ohm balanced network, and pair of EM coupled rectangular loops being assembled by 1 mmwide strips and placed in a dipole’s close vicinity as shown in Fig. 1. Glass thickness is t = 6 mm, and its material parameters are ε r = 6.75 and tan δ = 0.008. The main idea is that in a far-field zone EM coupled
_____________________________________________________ Proc. ‘EuCAP 2006’, Nice, France 6–10 November 2006 (ESA SP-626, October 2006)
Figure 1. Design layout of LCDA.
Figure 2. Simulation model of LCDA fed by a quarterwavelength balanced coupled line.
Figure 3. Peak surface current (in vector form) on dipole element and EM coupled loops at 40-degree phase instant.
Fig. 2 presents the simulation model of LCDA fed by a quarter-wavelength balanced coupled line. The antenna is printed on a finite size glass substrate. As has been confirmed by EM simulations the coupled feed line produces practically no effect on the radiation characteristics of LCDA. This antenna is to be used in RFID R/W applications in identifying various samples inside the glass show-case. The principle of producing circularly polarized radiation is shown in Fig. 3 and 4 in terms of current distribution on dipole element and EM coupled loops. The shape of rectangular loops is selected in order to have mostly longitudinal, i.e. y-directed, loop currents. As a result, these currents induced on rectangular loops produce strong y-directed E field in addition to x-directed E field radiated by dipole element. The position of rectangular loops, i.e. their distance to dipole along y-axis and the loops lateral shift in +/- x-axis direction, is optimized to provide 90-degree phase difference between Ex and Eyfield components. The Ex-field component radiated by dipole element leads the Ey-field component of rectangular loops by about quarter period that results in RHCP field radiated in +z-axis direction. In order to produce LHCP field in +z-axis direction each of two rectangular loops has to be placed on the other side of dipole element symmetrically with respect to the dipole centerline. Figs. 3 and 4 illustrate the 90-degrees phase shift between the currents on dipole element and EM coupled loops. E-fields calculated in a close proximity around LCDA reveal similar phase delay, which is required for CP operation. In this design, the size of loops is 95 mm X 15 mm so that the total loop length is 220 mm. The distance between the loops and dipole element is 7 mm while the loops are placed 33 mm from the center of the dipole in +/- x-axis direction. The dipole element length is 97.4 mm and the gap between the dipole strips at the center is 2.6 mm.
Figure 4. The same as in Fig. 3 but at 130-degree phase instant. 3.
RADIATION CHARACTERISTICS OF LCDA
Next, the radiation characteristics of the proposed LCDA have been calculated and analyzed. Fig. 5 shows the calculated 3D pattern of RHCP antenna gain of LCDA printed on finite-size glass substrate. Maximum RHCP gain of 1.57 dB is produced along +z-axis. The value of LCDA gain is similar to that of conventional dipole or loop antenna. Since the read range of the proposed LCDA in RFID R/W applications is less then 2 meters, the obtained RHCP gain is enough for successful R/W operation. Simulation data have been confirmed by measurements. The total E-field antenna gain pattern calculated at 953 MHz in x-z plane is presented in Fig. 7. Compared to RHCP gain, the total field gain pattern looks more “omnidirectional” as the total gain ranges between -0.08 and 2.23 dB. Fig. 6 shows the calculated 3D pattern of CP AR at 953 MHz, i.e. the CP AR as function of Theta and Phi in a spherical coordinate system.
4.
Figure 5. Calculated 3D pattern of RHCP antenna gain at 953 MHz.
MICROSTRIP BALUN INTEGRATED LCDA
This section presents the design of another type of LCDA. As has been noted in the previous section, the dipole element of a CP LCDA has to be fed by a balanced source. In actual implementation of RFID R/W systems the most common feed network between the processing unit and an antenna is an unbalanced coaxial cable. Hence there is a need to include an unbalancedto-balanced transition into the antenna layout. Microstrip balun has been selected to provide matching between a coaxial connector and balanced coupled line feeding the LCDA [2]. In this design, an antenna layout is printed on both sides of a dielectric board characterized by the following parameters: thickness is t = 0.74 mm, ε r = 3.3, tan δ = 0.003. Dipole strips and each of EM coupled rectangular loops are printed on the opposite sides of the substrate as shown in Fig. 8. Distance between dipole strips and EM coupled loops is 4 mm. Actual layout of a microstrip balun is also shown here. Balun-integrated LCDA has been simulated and optimized in terms of both VSWR and RHCP AR. EM simulations have shown that the best impedance match between a 50-Ohm connector and LCDA is achieved when the balun is somewhat longer then quarterwavelength. This antenna has been designed as a RFID R/W element for simplifying access control of tagequipped document files.
Figure 6. 3D CP AR pattern at 953 MHz. As a result of antenna parameters optimization, minimum CP axial ratio of 0.02 dB is obtained. However, the AR data presented in Fig. 6 reveal the fact that the best CP radiation is produced in the sectors around Theta=+30,-30 degree in y-z plane.
Figure 8. Front view of a balun-integrated LCDA prototype, left, and back view of the same antenna, right.
Figure 7. Total E-field antenna gain pattern at 953 MHz in x-z plane.
Fig. 9 presents measured and simulated VSWR of the balun-integrated LCDA. One can see good agreement between measured and simulated data. Very wide bandwidth in terms of impedance matching is observed. This result can be explained by the fact that the central resonant frequencies of the dipole and EM coupled loops are different so as to produce the best RHCP radiation at a frequency between them. Compared to VSWR bandwidth shown in Fig. 7, CP AR bandwidth is narrowed and remains less then 3 dB within the
frequency band 945 to 970 MHz, which is still a good result taking into account the narrow-band AR characteristics of the most commonly used CP antennas.
ranges between 2 and 4 dB. Somewhat worsened CP performance as compared to the LCDA shown in the previous section is due to the effect of microstrip balun on the total field radiated by the dipole and EM coupled loops.
VSWR
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Frequency [Hz]
Figure 9. VSWR of balun-integrated LCDA. Calculated and measured total antenna gain and CP AR at 953 MHz versus Theta in the plane normal to the antenna substrate and normal to the dipole element of LCDA are presented in Figs. 10 and 11, respectively. Note that in the measurement setup vertical z-axis from which the angle Theta is measured goes along the substrate normal to the dipole of LCDA so that in the EM simulations the coordinate system has been transformed accordingly. Here, Theta=+90,-90 degree denotes the direction normal to antenna substrate.
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Figure 10. Antenna gain of balun-integrated LCDA.
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Figure 11. CP AR of balun-integrated LCDA. As shown in Fig. 11, within the operational angular sector of 50
