CHAPTER 2 Basics of Slot Antennas
2.2 High-Performance Techniques
In this section, we introduce a number of techniques that improve the performance along various dimensions: antenna size, impedance bandwidth, and so on.
A Novel Approach for Miniaturization of Slot Antennas [8]
By utilizing the virtual enforcement of the required boundary condition (BC) at the end of a slot antenna, the area occupied by the resonant antenna can be reduced. To achieve the required virtual BC, the two short circuits at the end of the resonant slot are replaced by reactive BC, including inductive or capacitive loadings. By means of these loads, we can reduce the size of the resonant slot antenna for a given resonant frequency without adding any condition on the impedance matching of the antenna.
Chapter 2 Basics of Slot Antennas
Fig. 2.8 Transmission line model of a slot antenna (a) half-wave slot antenna. (b) Inductively terminated slot antenna. (c) Two series inductive terminations
Fig. 2.9 The geometry of the antenna and its feed designed to operate at 300MHz
A Wide-Band Slot Antenna Design Employing a Fictitious Short Circuit Concept [9]
A resonant narrow slot antenna is equivalent to a magnetic dipole, and its first resonant
Chapter 2 Basics of Slot Antennas
frequency corresponding to the electrical length of λg / 2(λg / 2is the guided wavelength in the slot). If the slot antenna is fed near an edge by a microstrip line and the slot width is properly chosen at a frequency above the first slot resonance, then a fictitious short circuit near the microstrip feed may be created. The reason is that the tangential component of the electric field created by the microstrip line at a particular distance cancels out the electric field of the slot excited by the return current on the ground plane of the microstrip line. When appropriately fed, these two resonances can merge and result in an antenna with a much larger bandwidth or two separate bands of operation with similar radiation characteristics.
Fig. 2.10 Electric field distributions and three-dimensional geometry of a microstrip-fed slot antenna (a) Normal field distribution (b) Field distribution at a slightly higher frequency showing a fictitious short circuit along the slot causing the second resonance (c) Three-dimensional geometry
Bandwidth Enhancement and Size Reduction of Microstrip Slot Antennas [10]
The variation in the slot shape, from straight to L and T shapes, which generate
Chapter 2 Basics of Slot Antennas
additional resonances when coupled to the original resonances of the slot, further increases impedance bandwidths. By suitably selecting the design parameters, the impedance bandwidths of up to 60% 84%, and 80% are achieved for these straight, L and inverted T slots, respectively.
Fig. 2.11 Geometry of the straight, L and T shapes slot antennas
Band-Rejected Design of the Printed Open Slot Antenna for WLAN/WiMAX Operation [11]
First, the broadband characteristic of open slot antenna design with small size is implemented and measured. Next, we insert a single strip on the broadband antenna. The dispensable band is rejected by inserting a metal strip on the open slot of the broadband antenna. Due to the metal of strip connected to the ground plane, the major current distribution will be concentrated on the strip and will produce a band-rejected function. The length of the inserting strip, which corresponds to quarter-wavelength, determines the center frequency of rejected band.
Chapter 2 Basics of Slot Antennas
Fig. 2.12 Geometry of the band-rejected open slot antenna for WLAN bands
Fig. 2.13 Measured return loss for various lengths (L3) of the antenna
Microstrip-Fed Slot Antennas with Suppressed Harmonics [12]
As shown in Fig. 2.14, the rectangular slot type harmonic suppressed antenna is composed of the conventional rectangular slot antenna and a double spur-line in the rectangular slot connected with ground plane to achieve wide-band harmonic suppression.
Conductor line (Lc1) and the gap (Wc1) between conductor line and ground plane can be modeled as a shunt-series resonator (Ls-Cs) with wide-bandstop characteristic over the second
Chapter 2 Basics of Slot Antennas
and third harmonic frequencies, and its equivalent circuit can be inserted inside the slot radiator as shown in Fig. 2.13(d).
Fig. 2.14 (a) Top view, (b) side view, (c) limped-element equivalent circuit model for the rectangular slot type radiator, and (d) total equivalent circuit of the rectangular slot type harmonic suppressed antenna
Chapter 2 Basics of Slot Antennas
Fig. 2.15 Simulated return losses for (a) L and (b) W of the rectangular slot type HAS
Multiband Printed Monopole Slot Antenna for WWAN Operation in the Laptop Computer [3]
The antenna is formed by three slots operated at their quarter-wavelength modes and arranged in a compact planar configuration. A step-shaped microstrip feedline is used to excite the three monopole slots at their respective optimal feeding position, and two wide operating bands at about 900 and 1900 MHz are obtained for the antenna to cover all the five
Chapter 2 Basics of Slot Antennas
operating bands of GSM850/900/1800/1900/UMTS for WWAN operation.
Fig. 2.16 (a) Geometry of the multiband printed monopole slot antenna for WWAN (wireless wide area network) operation in the laptop computer (b) Dimensions of the antenna
Fig. 2.17 Measured and simulated return loss for the proposed antenna
Chapter 3 Review of Multiband Antennas for Mobile Phones
Chapter 3 Review of Multiband Antennas for Mobile Phones
3.1 Literature Review
Modern mobile phone is required to operate multiband to satisfy various communication services. The demand of compact, light weight, reduced height, and multifunctional mobile phones is a challenge to antenna designers. Novel antenna designs are needed to meet the requirements of these emerging trends in mobile communication. In this chapter, we discuss some multiband antennas for mobile phone applications as follows.
The internal multi-resonant monopole antenna is designed for GSM900/DCS1800/US-PCS/S-DMB band and is shown in Fig 3.1. The multi-resonant operation of the antenna is achieved by utilizing a bent tuning stub, a shorting strip line, a U-shaped slit and the addition of a narrow resonant slit to the main radiator. The first resonant frequency is excited by the main radiator of the antenna, and the second resonant frequency is controlled by the length of the bent tuning stub. Finally, the third resonant frequency is generated by adding a narrow resonant slit to the perimeter of the main radiator. Hence, the resonant frequencies of the antenna can be independently controlled by tuning the length of the individual elements of the antenna. The dimensions of the main radiator are obtained using
Chapter 3 Review of Multiband Antennas for Mobile Phones
( )
0 4
f c
W L
= + (3.1)
where c is the speed of light in free space, f0 is the desired resonant frequency, W and L are the width and length of the main radiator.
Fig. 3.1 Geometry of the triple-band antenna (a) 3-D view, (b) fabricated internal monopole antenna, (c) unfolded structure of radiating elements
Chapter 3 Review of Multiband Antennas for Mobile Phones
Fig. 3.2 Surface current distributions on the radiating elements for triple-resonant antenna: (a) 930 MHz, (b) 1850 MHz, (c) 2630 MHz
Fig. 3.3 Comparison between measured and simulated results of triple-resonant antenna
Chapter 3 Review of Multiband Antennas for Mobile Phones
The compact multiband folded loop chip antenna is present in [16]. The loop antenna can be seen as a chip antenna to be surface-mountable on the system circuit board. However, unlike above discussions, the first resonant mode of the multiband loop antennas for mobile phone applications is a 0.5-wavelength resonant mode instead of quarter-wavelength resonant mode.
The antenna has a simple metal pattern comprising a folded loop strip and a tuning pad, and is suited for application in small-size mobile phones (ground plane length 60 mm only). The metal pattern is attached on the surfaces of a foam base, and the first three resonant modes (0.5-, 1.0-, and 1.5-wavelength modes) of the antenna can be excited with good impedance matching by simply adjusting proper dimensions of the tuning pad and its location along the folded loop strip.
Fig. 3.4 (a) Geometry of the proposed folded loop chip antenna (b) Detailed dimensions of the antenna unfolded into a planar structure
Chapter 3 Review of Multiband Antennas for Mobile Phones
Fig. 3.5 Measured and simulated return loss for the proposed antenna
The Hepta-band internal antenna is designed for personal communication application [15].
The antenna consists of a basic PIFA radiator, an L-patch located beneath the PIFA element and directly connected to the feed pin and an additional resonating strip for operation in WLAN band around 5.2 GHz.
Fig. 3.6 Geometry of the antenna
Chapter 3 Review of Multiband Antennas for Mobile Phones
Fig. 3.7 Comparison between the measured and simulated return losses
The printed monopole multiband slot antenna is present in [2]. The slot antenna consists of two slots of different lengths cut at the edge of the ground plane, which are excited by a 50-Ω microstrip feed line. Slot 1 has a longer length S1 and controls the excitation of the antenna’s lower band, while the slot 2 has a shorter length S2 and controls the excitation of the antenna’s lower band. The lengths are about one-quarter wavelength at 900 and 2100 MHz, respectively.
Note that the two slots are chosen to have a wide width of 5mm (w1 and w2), which is helpful for widening the bandwidths for the antenna’s lower and upper bands.
Chapter 3 Review of Multiband Antennas for Mobile Phones
Fig. 3.8 (a) Geometry of the proposed printed monopole slot antenna for mobile phone application (b) Side view of the geometry in (a) with a 1-mm-thick plastic housing (c) Detailed dimensions of the antenna.
Fig. 3.9 Measured and simulated return loss for the antenna; the 1-mm-thick plastic housing is included
Chapter 3 Review of Multiband Antennas for Mobile Phones
In conclusion, we list Table 3.I to compare the advantages and disadvantages of the above multiband antennas.
Advantages disadvantages
Monopole Low profile. It needs an enough large
ground plane.
PIFA Reduce antenna area. Narrow bandwidth and
larger thick due to shorting
Table 3.I Comparison of various multiband antennas
In my opinion, the advantages of the slot antenna outweigh its disadvantages from Table 3.I.
Hence, we select the slot antenna as our approach to multiband antenna.
Chapter 4 The Proposed Slot Antenna
Chapter 4 The Proposed Slot Antenna
4.1 Antenna Configuration and Operation
Fig. 4.1 shows the geometry of the proposed printed slot antenna for mobile phone applications. The antenna is printed on the top portion of the system circuit board of the mobile phone, whose dimensions are selected to be 60 mm in length and 35 mm in width. The selected dimensions are reasonable for general mobile phones, and in this study the circuit board is fabricated using a 0.8-mm-thick FR4 substrate of relative permittivity 4.4 and loss tangent 0.02. The printed metal on the FR4 substrate has a conductivity of 5.8×107S/m. A 50 Ω microstrip feedline is used in this antenna design to excite two L-shaped slots on the ground in order to generate two bands, respectively. The longer slot (L1+L2+L3) controls the excitation of the lower band centered at about 900MHz to cover the GSM operation (890-960MHz), while the shorter slot (L5+L6) controls the excitation of the upper band centered at about 2100MHz to cover the DCS/PCS/UMTS/WLAN/WiMAX operation (1710-2600MHz), which correspond to quarter-wavelengths at their resonant frequencies, respectively. Finally, the straight parasitic stripf (Lp), which is about half-wavelength of the WLAN band (5150-5350MHz), is electromagnetically coupled and excited by the shorter slot.
The detailed antenna design parameters are listed in Table 4.I.
Chapter 4 The Proposed Slot Antenna
Fig. 4.1 Geometry of the proposed printed slot antenna
Wg Lg Wf d1 d2
Chapter 4 The Proposed Slot Antenna
(a)
(b)
Fig. 4.2 Photographs of the fabricated antenna (a) top view (b) back view
Chapter 4 The Proposed Slot Antenna
4.2 Design Considerations and Procedure
To demonstrate the operation of the proposed antenna, Fig.4.3-4.6 performs the current distribution of each operation bands.
A. GSM (890-960MHZ) operation
Fig. 4.3 Surface current distribution at 900MHz
First of all, in order to generate the GSM band, a 50Ω microstrip feedline printed at the location of d1 = 20 mm away from the edge of the ground plane excites the longer slot (L1+L2+L3), which is about quarter-wavelength at 900MHz.
Chapter 4 The Proposed Slot Antenna
B. DCS/PCS/UMTS/WLAN/WiMAX (1710-2600MHz) operation
Fig. 4.4 Surface current distribution at 2100MHz
The shorter L-shaped slot (L5+L6) helps in generating additional resonances, which when coupled to the original resonances of the slot, further increases impedance bandwidth [9]. It shows that the two orthogonal arms of the slot act as separate and tightly coupled resonators and their mutual coupling displaces their resonances toward lower and higher end frequencies.
This phenomenon enhances the bandwidth significantly. By using this property, we can cover broad bandwidth (1710-2600MHz) with this simple structure.
C. WLAN (5150-5350MHz) operation
Chapter 4 The Proposed Slot Antenna
Fig. 4.5 Surface current distribution at 5200MHz
Fig. 4.6 Surface current distribution of the parasitic strip at 5200MHz
Chapter 4 The Proposed Slot Antenna
Finally, from Fig. 4.5 we can observe that the current distribution of the shorter slot at 5200MHz. In order to generate the WLAN band (5150-5350MHz) and mitigate the return loss degradation of other operating bands caused by the mutual coupling effect, we put a parasitic strip (Lp) on the top portion above the shorter slot, which is about half-wavelength corresponding to 5.2GHz. The slots of ground which is the driven element electromagnetically couple and excite the parasitic strip controlling the operating frequency [12]. Thus, the parasitic strip is placed above the slot to generate the operating band, which increases the design freedom and possible applications.
4.3 Experimental Results and Discussions
Fig. 4.7 shows the measured and simulated return loss of the proposed antenna. The simulated results are obtained from Ansoft simulation software High Frequency Structure Simulator (HFSS), and good agreement between the simulation and measurement is observed.
In simulations, we used an ideal discrete port to excite the antenna. However, to test the prototype antennas, a coaxial cable is required to connect the antenna to the network analyzer.
The feed cable being very close to the antenna while testing may affect the measurement results. Influence of the feed cable is more pronounced on the antenna radiation patterns as compared to the return loss. The proposed antenna generates the three bands centered at about 900, 2100 and 5200 MHz to cover the GSM/DCS/PCS/UMTS/WiMAX/WLAN bands, as shown in Fig.4.7.
Chapter 4 The Proposed Slot Antenna
Fig. 4.7 Simulated and measured return loss for the proposed antenna
The proposed slot antenna is enclosed by a 1-mm-thick plastic housing, which is shown in Fig. 4.8. The material to simulate plastic cover for mobile phone is acrylonitrile butadiene styrene (ABS) with relative permittivity 3.5 and loss tangent 0.026. The simulated result shown in Fig. 4.9 indicates that the resonant frequencies of proposed antenna with a plastic housing are slightly lower compared with free space situation. The reason is because the effective permittivity is higher with a plastic housing. Hence, we can fine tune this antenna to satisfy a practical case easily.
Chapter 4 The Proposed Slot Antenna
Fig. 4.8 Geometry of the proposed slot antenna with a 1-mm-thick plastic housing (Unit: mm)
Fig. 4.9 Simulated return loss of the proposed antenna w/o plastic housing
Chapter 4 The Proposed Slot Antenna
When we consider the slot antenna of the mobile communication device for practical applications, the performances of the antenna will be significantly affected by the coupling effect of nearby electronic components. To study the effect of the nearby electronic components, we put a conducting box (PEC) of size 33×30×6 mm3 on the ground as a battery to simulate this case. The simulated result in Fig. 4.10 indicates that the performance of the proposed antenna is almost not affected. Thus, it becomes possible that the nearby electronic components can be placed in close proximity to the proposed antenna, with small effects on the performances of the antenna. This property indicates that the proposed slot antenna is EM compatible with the nearby electronic components.
Fig. 4.10 A conducting box (PEC) of size 33×30×6 mm3 on the ground is used to simulate the battery case
Chapter 4 The Proposed Slot Antenna
Fig. 4.11 Simulated return loss of the proposed antenna with and without a conducting box of size 33×30×6 mm3 placed on the ground as a battery
The radiation patterns were measured in a 7×3.2×3m3 anechoic chamber at National Taiwan University of Science and Technology. The measurement was performed by an Agilent E8362B network analyzer along with the NSI 2000 far-field measurement software.
Fig. 4.12 illustrates the simulated and measured radiation patterns. The agreement between the simulations and measurements is fairly good in most of the results. The measured peak gains at 900 MHz, 1800 MHz, 2050 MHz, 2450 MHz, 2600 MHz, and 5200 MHz are listed in Table 4.II.
Chapter 4 The Proposed Slot Antenna
frequency Plane X-Y X-Z Y-Z
900 MHz 0.7 1.0 -0.2
1800 MHz 2.2 2.1 1.2
2050 MHz 1.9 0.1 1.6
2450 MHz 1.6 0.3 -0.4
2600 MHz 1.8 0.4 0
5200 MHz 1.1 -1.0 0.4
Table 4.II Peak Gain (Unit: dBi)
(a) 900 MHz
Chapter 4 The Proposed Slot Antenna
(b) 1800 MHz
(c) 2050 MHz
Chapter 4 The Proposed Slot Antenna
(d) 2450 MHz
(e) 2600 MHz
Chapter 4 The Proposed Slot Antenna
(f) 5200 MHz
Fig. 4.12 Simulated and measured radiation patterns
Chapter 4 The Proposed Slot Antenna
4.4 Summary
Antenna size Ground size bands
[14] 2008 36*15.8*6=3413mm3 65*36=2340mm2 4
[15] 2007 30*15*4=1800 mm3 45*100=4500 mm2 7
Slot [2] 2007 A microstrip feefline excites the two slots to generate multiband.
Slot (This work) idth can be achieved
ted by the
Chapter 5 Conclusion
Chapter 5 Conclusion
A printed slot antenna for internal multiband mobile phone antenna has been proposed and demonstrated. The antenna has a simple structure and is easy to be printed on the top portion of the system circuit board of the mobile phone. In addition, although the antenna shows a simple structure and compact volume, it can generate three bands covering the GSM/DCS/PCS/UMTS/WiMAX/WLAN operation, and good radiation characteristics over the operating bands have been obtained. The present antenna does not contain the effect of the user’s head and hand on the antenna impedance and pattern characteristics. The effect of the operator head and hand on the proposed antenna is planned to be discussed in the future. In addition, the proposed antenna is EM compatible with the nearby components. Hence, the proposed printed slot antenna is suitable for application in the modern thin mobile phones as an internal antenna.
References
References
[1] K. L. Wong, Planar Antennas for Wireless Communications. New York: Wiley, 2003.
[2] Chun-I Lin and Kin-Lu Wong, “Printed Monopole Slot Antenna for Internal Multiband Mobile Phone Antenna,” IEEE Trans. Antennas Propag., vol. 53, pp. 3690–3697, Dec.
2007.
[3] Kin-Lu Wong and Li-Chun Lee, “Multiband printed monopole slot antenna for WWAN operation in the laptop computer,” IEEE Trans. Antennas Propag., vol. 57, pp. 324–330, Feb. 2009.
[4] A. Ishimaru, Electromagnetic wave propagation, radiation, and scattering, Prentice-Hall, 1991.
[5] John D. Kraus and Ronald J. Marhefka, Antenna for all application, 3nd ed. McGraw-Hill, 2003.
[6] C. A. Balanis, Antenna theory, 3nd ed. Wiley, 2005.
[7] K. C. Gupta, P. Garg, I. Bahl, and P. Bhartia, Microstrip lines and slotlines, 2nd ed.
Norwood, MA: Artec House, 1996.
[8] R. Azadegan and K. Saraband, “A novel approach for miniaturization of slot antennas,”
IEEE Trans. Antennas Propag., vol. 51, pp. 421–429, Mar. 2003.
[9] N. Behdad and K.Sarabandi, “A wide-band slot antenna design employing a fictitious short circuit concept,” IEEE Trans. Antennas Propag., vol. 53, pp. 475-782, Jan. 2005.
[10] Saeed I. Latif, Lotfollah Shafai, and Satish Kumar Sharma, “Bandwidth enhancement
References
and size reduction of microstrip slot antennas,” IEEE Trans. Antennas Propag., vol. 53, pp. 994-1003, Mar. 2005.
[11] Wen-Shan Chen and Kuang-Yuan Ku, “Band-rejected design of the printed open slot antenna for WLAN/WiMAX operation,” IEEE Trans. Antennas Propag., vol. 56, pp.
1163-1169, Apr. 2008.
[12] Hyungrak Kim and Young Joong Yoon, “Microstrip-fed slot antennas with suppressed harmonics,” IEEE Trans. Antennas Propag., vol. 53, pp. 2809-2817, Sep. 2005.
[13] Jen-Yea Jan and Liang-Chih Tseng, “Small planar monopole antenna with a shorted parasitic inverted-L wire for wireless communications in the 2.4-, 5.2-, and 5.8-GHz bands,” IEEE Trans. Antennas Propag., vol. 52, pp. 1903-1905, Jul. 2004.
[14] Seokjin Hong, Wonseob Kim, Hoon Park, Sungtek Kahng, and Jaehoon Choi, “Design of an internal multiresonant monopole antenna for GSM900/DCS1800/US-PCS/S-DMB operation,” IEEE Trans. Antennas Propag., vol. 56, pp. 1437-1443, May. 2008.
[15] Rashid Ahmad Bhatti and Seong Ook Park, “Hepta-band internal antenna for personal communication handsets,” IEEE Trans. Antennas Propag., vol. 55, pp. 3398-3403, Dec.
[15] Rashid Ahmad Bhatti and Seong Ook Park, “Hepta-band internal antenna for personal communication handsets,” IEEE Trans. Antennas Propag., vol. 55, pp. 3398-3403, Dec.