Journal of Materials Science and Engineering 5 (2011) 354-361
Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems Ashraf Shamseldin Yahia, Nashwa Mohamed Shaalan, Mostafa Abdel Fatah El-Aasser and Mohamed Hussein Abdel Razik Electronics Group, Physics Department, Faculty of Science, Ain Shams University, Cairo 11566, Egypt Received: July 02, 2010 / Accepted: September 02, 2010 / Published: March 10, 2011. Abstract: Modeling and simulation of electromagnetic radiation of microstrip antennas, in radio frequency identification (RFID) systems, have been realized. The fields of RFID tags have been optimized for different frequencies and materials. Specific absorption rate (SAR) due to the use of these devices has also been investigated to get the lowest possible absorption dose to comply with the universal standards. The simulation shows that the electromagnetic radiation levels are less than the acceptable standard for various materials at two different operating frequencies which is safe and important for mobile system applications. Key words: Microstrip antennas, RFID systems, simulation of electromagnetic radiation, specific absorption rate (SAR).
1. Introduction Several publications in the scientific literature have raised concern about the individual and public health impact of adverse non-ionizing radiation (NIR) from electromagnetic field (EMF) exposure emanating from certain power, electrical and wireless devices commonly found in the home, workplace, school and community [1]. Guidelines for limiting electromagnetic fields exposure are published by ICNIRP [2] and IEEE [3]. Due to widely spread applications of using RFID systems, there is also a concern about the increased level of environmental emitted radiation. A typical RFID system consists of a tag and reader, application software, computing hardware, and middleware [4]. A RFID tag consists of an integrated circuit with memory, which is essentially a microprocessor chip. RFID tags can be active (with batteries), passive (without batteries) or semi-passive (stand-by mode). The tag has an identity (ID) that can be broadcast to a reader that is Corresponding author: Ashraf Shamseldin Yahia, professor, research fields: electronics, electromagnetics and information technology. E-mail:
[email protected].
operating on the same frequency and under the same tag protocol. This specific area of research includes tag antenna design simulation, and performance analysis. The antennas are the conduits for the communication of data between the tag and the reader. A RFID antenna has a reading range both sideways and in front of the antenna. Antenna design and placement play a significant part in determining the coverage zone, range, and accuracy of communication of a tag, because the tag antenna both draws energy from the reader’s signal to energize the tag and sends the data that are received by the reader. A RFID reader is a device that can read data from and write data to compatible RFID tags. Communication between tag and reader enables the location information of an item to be recorded and transferred to a server through a computer network, thus allowing the movement of the item to be tracked and traced. To ensure the compatibility of the communication, the tag and reader must work at the same specified working frequency and comply with specific regulations and protocols [4]. In high-performance aircraft, spacecraft, satellite,
Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems
and missile applications, where size, weight, cost, performance, ease of installation and aerodynamic profile are constraints, low-profile antennas may be required. Presently there are many other government and commercial applications, such as mobile radio, RFIDs and wireless communications that have similar specifications. To meet these requirements, microstrip antennas can be used [5]. These antennas are low profile, conformable to planar and nonplanar surfaces, simple and inexpensive to manufacture using modern printed-circuit technology. Mechanically robust when mounted on rigid surfaces, compatible monolithic microwave integrated circuits (MMICs) designs. When the particular patch shape and mode of the antenna are selected, they are very versatile in terms of resonant frequency, polarization, pattern, and impedance. In addition, by adding loads between the patch and the ground plane, such as pins and varactor diodes, adaptive elements with variable resonant frequency, impedance, polarization, and pattern can be designed [6]. In this paper, modeling and simulating the electromagnetic radiation of microstrip antennas, in RFID systems have been realized. The fields of RFID tags are optimized for different frequencies and materials to get the lowest possible absorption dose to comply with the universal standards.
2. Microstrip Antenna Overview A microstrip antenna, as shown in Fig. 1, consists of a very thin metallic strip (patch) placed at a small fraction of a wavelength λ0 above a ground plane. Usually, the height h is less than the wavelength λ0 (h 0.02 λ0). Both the microstrip feed line and the probe possess inherent asymmetries that generate higher order modes which produce cross-polarized radiation. To overcome
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Fig. 2
Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems
Types of feeding.
some of these problems, non-contacting aperture-coupling is used for feeding.
3. Analysis and Modeling There are many methods for analyzing for microstrip antennas. The most popular models are the transmission-line, cavity, and full wave (which include primarily integral equations, Moment Method) [10, 13]. The transmission-line model is the easiest of all, which gives good physical insight, but is less accurate and is more difficult to model coupling [11]. Compared to the transmission-line model, the cavity model is more accurate but at the same time more complex, and it also gives a good physical insight. 3.1 Transmission Line Model The transmission line model represents the microstrip antenna by two slots, separated by a low-impedance Zc transmission line of length L [11]. Because the dimensions of the patch are finite along the length and width, the fields at the edges of the patch undergo fringing. The amount of fringing is a function of the dimensions of the patch and the height of the substrate. For the principal E-plane (xy-plane) fringing is a function of the ratio of the length of the patch L to the height h of the substrate (L/h) and the dielectric constant εr of the substrate. Since for microstrip antennas L/h >> 1, fringing is reduced; however, it must be taken into account because it influences the resonant frequency of the antenna. For a microstrip line shown in Fig. 1, most of the electric field lines reside in the substrate and parts of some lines exist in air. As
W/h >> 1 and εr >> 1, the electric field lines concentrate mostly in the substrate where W is the patch width. Fringing in this case makes the microstrip line look wider electrically compared to its physical dimensions. Since some of the waves travel in the substrate and some in air, an effective dielectric constant εr is introduced to account for fringing and the wave propagation in the line. For the principal E-plane (xy-plane), Fig. 3b where the dimensions of the patch along its length has been extended on each end by a distance Δ L, which is a function of the effective dielectric constant ε reff and the width-to-height ratio (W/h). For a line with air above the substrate, the effective dielectric constant has values in the range of 1 < εeff > 1), the value of εeff will be closer to the value of the actual
Fig. 3 Physical and effective length of microstrip patch [9].
Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems
dielectric constant εr of the substrate. The effective dielectric constant is also a function of frequency. As the frequency of operation increases, most of the electric field lines concentrate in the substrate. Therefore, the microstrip line behaves more like a homogeneous line of one dielectric (only the substrate), and the effective dielectric constant approaches the value of the dielectric constant of the substrate. For low frequencies, the effective dielectric constant is essentially constant. At intermediate frequencies its values begin to monotonically increase and eventually approach the values of the dielectric constant of the substrate. The initial values (at low frequencies) of the effective dielectric constant are referred to as the static values, and they are given by [12, 13, 16]: ε reff
ε +1 ε −1 ⎡ h = r + r ⎢ 1 + 12 2 2 ⎣ W
such that
1 ⎤− 2 ⎥ ⎦
(1)
W / h >>1
3.2 Cavity Model Microstrip antennas resemble dielectric-loaded cavities, and they exhibit higher order resonances. The normalized fields within the dielectric substrate (between the patch and the ground plane) can be found more accurately by treating that region as a cavity bounded by electric conductors (above and below it) and by magnetic walls (to simulate an open circuit) along the perimeter of the patch. This is an approximate model, which in principle leads to a reactive input impedance (of zero or infinite value of resonance), and it does not radiate any power. However, assuming that the actual fields are approximate to those generated by such a model, the computed pattern, input admittance, and resonant frequencies compared well with measurements [11]. To shed some insight into the cavity model, let us attempt to present a physical interpretation in to the formation of the fields within the cavity and radiation through its side walls. When the microstrip patch is energized, a charge distribution is established on the
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upper and lower surfaces of the patch, as well as on the surface of the ground plane, as shown in Fig. 4. The charge distribution is controlled by two mechanisms an attractive and a repulsive mechanism [15]. The attracttive mechanism is between the corresponding opposite charges on the bottom side of the patch and the ground plane, which tends to maintain the charge concentration on the bottom of the patch. The repulsive mechanism is between like charges on the bottom surface of the patch, which tends to push some charges from the bottom of the patch, around its edges, to its top surface. The movement of these charges creates corresponding current densities Jb and Jt, at the bottom and top surfaces of the patch, respectively (Fig. 4). Since for most practical microstrips the height-to-width ratio is very small, the attractive mechanism dominates and most of the charge concentration and current flow remain underneath the patch. A small amount of current flows around the edges of the patch to its top surface. However, this current flow decreases as the height-to-width ratio decreases. In the limit, the current flow to the top would be zero, which ideally would not create any tangential magnetic field components to the edges of the patch. This would allow the four side walls to be modeled as perfect magnetic conducting surfaces which ideally would not disturb the magnetic field and, in turn, the electric field distributions beneath the patch. Since in practice there is a finite height-to-width ratio, although small, the tangential magnetic fields at the edges would not be exactly zero. However, since they will be small, a good approximation to the cavity model is to treat the side walls as perfectly mag netic conducting. This model produces good normalized electric and magnetic field distributions (modes) beneath the patch [13].
Fig. 4 Charge distribution and current density creation on microstrip patch.
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Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems
4. Design and Simulation As a resonant cavity, there are many possible modes (like waveguides), thus a patch antenna is multimode and may have many resonant frequencies. The fundamental and dominant mode is TM100 (a half wave change along the x-axis and no changes along the other two axes). The radiation comes from the fringing fields at the two open ends, which is equivalent to two slot antennas separated by a distance L. The far-field can be expressed as [15]:
E = E 0 sinc ( βω sin θ sin φ ) cos(
βL 2
sin θ cos φ )
)(θˆ cos φ − φˆ cos θ sin φ )
W > λ0 ⎩8W λ0 , The larger the width, the larger the directivity. The
typical impedance at the edge of a resonant rectangular patch ranges from 100 to 400, the radiation impedance of a patch at the edge can be approximated as: 2
frequency fr are known, then the optimized width for an efficient radiator is [15]:
W = (2)
where β is the free space wave number. The first factor
2
εr ⎛ L ⎞ (5) ⎜ ⎟ (Ω ) ε r−1 ⎝ W ⎠ If the substrate parameters εr, h and the operational z a ≈ 90
1 2 f r μ 0ε 0
2 ε r +1
(6)
where the effective (relative) permittivity is given by
factor for the two-element slots separated by L in the x
Eq. (1). Because of the fringing effects, Δ L is given by: ΔL = [.412 d ( ε reff +0.3)(W / h + 0.264 )]
direction. For both components, the peak is at θ = 0,
(7)
is the pattern factor for a uniform line source of width W in the y direction and the second factor is the array
which corresponds to the z direction. It has a broadside unidirectional pattern. The radiation patterns in the two principal planes are:
(b) H-plane (φ = 90◦):
(3)
These results have neglected the substrate effects and slot width, but are good enough as estimates. Usually, antennas are characterized by their radiation pattern in free space. This characterization is performed by measurement of the radiated electric field, received or transmitted by the stand alone antenna. When the antenna is put on a structure, the radiation pattern is perturbed by the electromagnetic interaction with the surrounding structure [17]. The typical radiation patterns in the E- and H-planes are shown in Fig. 5; if the ground plane is finite, leaky radiation towards the lower half space will occur. With the radiated field, asymptotically, the directivity of the microstrip antenna can be expressed as:
The effective length of the patch is now: Leff = L + 2ΔL
(8)
5. Results and Discussion
βL (a) E-plane (φ = 0◦): E = θˆ E 0 cos( sin θ ) 2 βω E = −φˆ E 0 sinc ( sin θ ) cos θ 2
/[( ε reff −0.258 )(W / h + 0.8)]
We have modeled and simulated the electromagnetic radiation of microstrip antennas, in RFID systems at two frequencies 1800 MHz and 2.45 GHz. The frequency 1800 MHz and 2.45 GHz are well known for GSM systems and Bluetooth respectively. The fields of RFID tags (microstrip antennas) have been optimized for these two frequencies and for wide range of material dielectric constant (Table 1). The specific absorption rate (SAR) has also been investigated for these RFID tag devices in order to get the lowest possible absorption dose complying with FCC standards. The FCC (USA) sets a SAR limit of 1.6 W/kg measured in a 1 g mass of tissue, taking into consideration that this standard is the toughest all over the world. Fig. 6 shows the distribution of the electric field intensity over a patch antenna at 2.45 GHz. Fig. 7 shows the electric field intensity for different materials, dielectric constants, at 1800 MHz and 2.45 GHz.
Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems
Fig. 5
359
Typical radiation pattern of E-plane and H-plane.
Table 1
εr
Material
Dielectric constants of the used materials in simulation. 2.2
4.4
6
10.2
12.9
Rogress RT/duroid 5880 (tm)
FR4
Rogress TMM6 (tm)
Rogress RT/duroid 6010 (tm)
Galium aresinde
Fig. 6 Distribution of the electric field intensity over a patch antenna at 2.45 GHz.
The electric field increases with increasing the dielectric constant and it is higher for higher frequencies. Fig. 8 shows the distribution of SAR over a patch antenna at 2.45 GHz. Fig. 9 shows SAR for different materials, dielectric constants, at 1800 MHz and 2.45 GHz. SAR increases with the dielectric constant and it is larger for higher frequencies. The simulation shows that SAR is less than the Federal Communication Commission (FCC) standards for all used materials and both frequencies. This is beneficial and safe for mobile system applications at 1800 MHz (dashed curve), 2.45 GHz (continuous curve) (dashed
Fig. 7 The electric field intensity for different materials.
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Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems
Fig. 8 Distribution of SAR over a patch antenna at 2.45 GHz.
References [1]
[2]
[3]
[4]
Fig. 9
SAR for different materials at 1800 MHz.
curve) and 2.45 GHz (continuous curve).
[5] [6]
6. Conclusions The simulated model of the radiation pattern for an RFID tag/microstrip antenna shows an increase in the electric field intensity with material dielectric constant at a certain used frequency. The operating frequency has a little effect on the field intensity. The specific absorption rate (SAR) also increases with the material ε r and shows lower values for lower operating frequencies. Accordingly, lower ε r materials are preferable for RFID tag designs.
[7]
[8] [9]
S.J. Genuis, Fielding a current idea: exploring the public health impact of electromagnetic radiation, Public Health (2008) 113-124. Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz), Health Physics (1998) pp. 494-522 . IEEE Standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300 GHz. J. Miettinen, V. Pekkanen, K. Kaija, P. Mansikkamaki, J. Mantysaloa, M. Mantysalo, J. Niittynen, J. Pekkanen, T. Saviauk, R. Ronkka, Inkjet printed System-in-Package design and manufacturing, Microelectronics Journal 39 (2008) 1740-1750. D.M. Pozar, Microstrip Antennas, IEEE (1992), pp. 79-81. M.P. Purchine, J.T. Aberle, A tunable l-band circular microstrip patch antenna, Microwave Journal (1994) 80-88. T. Shanmuganantham, S. Raghavan, Design of a compact broadband microstrip patch antenna with probe feeding for wireless applications, Int. J. Electron. Commun. (AEÜ) 63 (2009) 653-659. K.L. Wong, Compact and Broadband Microstrip Antennas, New York, Wiley, 2002. S. Raghavan, T. Shanmuganantham, M.S.K. Kumar, Reconfigurable patch antenna with switchable L-shaped slots for circular polarization diversity, Microwave Optical Technology Letters, New York, Wiley, Vol. 50, 2008.
Simulation of Electromagnetic Radiation Patterns of Microstrip Antennas in RFID Systems [10] J.R. James, P.S. Hall, Handbook of Microstrip Antennas, Institution of Engineering and Technology, 1989. [11] C.A. Balanis, Antenna Theory: Analysis and Design, John Wiley & Sons, Inc., 2005, pp. 811-812. [12] C.A. Balanis, Advanced Engineering Electromagnetic, John Wiley & Sons, Inc., 1989. [13] W.H. Hayt, Engineering Electromagnetics, Mc Graw Hill, 2005. [14] W.F. Richards, Y.T. Lo, D.D. Harrison, An improved theory for microstrip antennas and applications, IEEE Trans. Antennas Propagat. 29 (1981) 38-46.
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[15] Y. Huang, K. Boyle, Antennas from Theory to Practice, 1st ed., John Wiley & Sons, Inc., 2008, pp.184-190. [16] S. Park, C. Kim, Y. Jung, H. Lee, D. Cho, M. Lee, Gain enhancement of a microstrip patch antenna using a circularly periodic EBG structure and air layer, Int. J. Electron. Commun. (AEÜ) 64 (2010) 607-613. [17] N. Zerbib, M. Barek Fares, T. Koleck, F. Millot, Some numerical models to compute electromagnet antenna-structure interactions, C.R. Physique 6 (2005) 647-653.
