EM Simulation and Design of Frequency Reconfigurable Planar Antenna A. S. Andrenko1 Abstract − This paper presents the EM analysis and design of a frequency reconfigurable antenna for the mobile handset and wireless terminal applications. The proposed antenna has a simple planar layout and utilizes RF switches to provide the multiband frequency reconfigurable capabilities. Two designs are presented and their multimode antenna performances within the 0.8 to 2.45 GHz band are analyzed in terms of current density distributions and radiation characteristics.
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INTRODUCTION
During the last decade, various wireless services have become standard in the use of wireless communication devices, such as smartphones, laptops, and tablet PCs. Multiradio wireless modules supporting the operation of WiFi, GPS, 3G and 4GLTE communications have been integrated into the majority of mass-market wireless devices. From the antenna design perspective, the choice of multiband versus frequency reconfigurable antennas comes down to the comparison of their advantages and disadvantages in compact wireless terminals. In the smartphone front-end applications, multiband antennas operate with poor out-of-band rejection and require many RF filters for multiradio operation. Frequency reconfigurable antennas, on the other hand, provide more compact layout and because of their specific bandpass performance reduce the need of multiple filters and frontend duplexers [1]. Several RF switch integrated reconfigurable antenna designs have been reported. 5-band PIFA with capacitive RFMEMS switches for frequency tuning has been presented in [2]. Common problems associated with the use of MEMS switches are their high cost and high activating voltage. Two-port PIFA-monopole antenna combination with multiband switching capabilities has been presented in [3]. Compact design of integrated two-element multiband reconfigurable antenna utilizing high-performance GaAs switch has also been reported [4]. This paper presents the design of a single port planar monopole-type frequency reconfigurable antenna consisting of several EM coupled elements and p-i-n diode switches. The operation of this antenna is based on the combination of altering the antenna resonant length and excitation of EM coupled elements thus resulting in frequency reconfigurable performance. The single-layer antenna layout is quite simple with the low cost and very low driving voltage p-i-n diodes being utilized. CST MW Studio EM simulator has been used for the antenna design and optimization.
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ANTENNA GEOMETRY AND DESIGN
Two original designs of frequency reconfigurable antenna are proposed in this work. Their layouts and geometries are depicted in Figs. 1 and 2, respectively. Main L-shaped monopole element is printed on a 1mm-think FR4 (epsr=4.2, tanD=0.01) substrate and excited by a 50-Ohm port. Two additional metal strips are printed on a substrate as shown in Fig. 1. RF switch#1 is placed to connect the end of main monopole to the end of the shorter additional element so that in the off state it is excited by the EM coupling. Another additional strip element can only be excited by activating the switches#2 and 3.
RF switch 3
RF switch 2 RF switch 1 FR4 substrate
PCB GP
Antenna port
Figure 1: Layout of antenna consisting of main monopole element, one coupled element, one RF switch-activated element and 3 RF switches.
RF switch 4
RF switch 3
RF switch 2 RF switch 1 FR4 substrate PCB GP
Antenna port
Figure 2: Layout of antenna consisting of main monopole element, two coupled elements and 4 RF switches.
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When both switch#2 and switch#3 are activated while switch#1 is off the L-shaped loop element is excited by EM coupling from the main monopole. The total length of the main element is 46mm and the L-shaped layout reduces the antenna footprint. As has also been confirmed by EM simulation, L shape of the proposed antenna results in better impedance matching without any additional tuning circuits. The second antenna design is presented in Fig. 2. The same main monopole is used together with two EM coupled strip elements and 4 RF switches being placed between antenna elements as shown in Fig. 2. The metal strips have different lengths so as to produce the antenna resonances at 3 frequency bands when all switches are off. By activating the switches and selecting their specific combinations it is expected that antenna covers multiple frequency bands. The geometry of all the antenna elements depicted in Figs. 1 and 2 has been numerically optimized to provide antenna operation at several bands for GSM850/900, CDMA2000, WCDMA, WiFi and FDD/TDD-LTE wireless services. Utilizing the RF switches requires the dc bias lines being integrated so as to reduce their negative effect on the antenna radiation performance. In this design, the antenna layout shown in Figs. 1 and 2 has been covered with 1mm-thick FR4 layer in order to place all the dc bias lines on its top surface. All the p-i-n diode switches have been integrated with the antenna elements by using the via-holes. 3
switches#1 and 2 shifts the lower band while the high frequency band remains the same. When the switches#3 and 4 and activated the lower band is at 930 MHz and the higher frequency band is shifted to 1.85 GHz. Therefore, the frequency reconfigurable capabilities of the proposed antennas are controlled by the specific sets of the RF switches combinations.
All SWs Off
Figure 3: Calculated S-11 parameters of the antenna shown in Fig. 1.
ANTENNA PERFORMANCE
The frequency performance of the antenna shown in Fig. 1 is illustrated in Fig. 3. Calculated S-parameters are compared for the case of all switches being off and various combinations of activated switches. It can be seen that by activating the p-i-n diodes the initial dual-band antenna performance has been changed. When only switch#1 is activated the lower band is shifted from 1.1 GHz to 930 MHz as a result of longer monopole resonating at different frequency. With the switches#1 and 2 being on, triple band antenna operation is realized. When switches#1 and 3 are activated and antenna operating as a branched monopole the additional frequency band at 900 MHz is produced while the higher band is shifted to 1.9 GHz. It should be noted that to provide antenna operation at 700 MHz LTE band the length of antenna strips can be easily increased. Frequency characteristics of the second antenna design are presented in Fig. 4. Here, with all 4 switches being off, the antenna has 3 frequency bands at 1.7 GHz, 2.0 GHz and 2.45 GHz plus an additional lower band as a result of longer coupled monopole excitation. When switches#1 and 3 are on, the antenna produces 900 MHz and 1.95 GHz bands while the Wi-Fi band is eliminated. Activating the
SW1 & SW3
Figure 4: Calculated S-11 parameters of the antenna shown in Fig. 2. To understand the mechanism of antenna operation, it is informative to analyze the current density distributions related to different antenna modes. Figs. 5, 6, and 7 show the calculated current densities induced on the antenna presented in Fig. 2. All the switches are in off state and the current distributions are calculated at the triple band frequencies of this antenna. At 1.7 GHz, the antenna resonance is due to the excitation of the main monopole as illustrated in Fig. 5. The next band at 2.0 GHz is produced by the current induced on the outside EM coupled strip as its length is shorter than the main monopole. The Wi-Fi band at 2.45 GHz is covered because of the excitation of inner EM coupled strip as shown in Fig. 7. It has been confirmed by the EM simulations that the current densities induced at the multimode frequencies, i.e. at
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900 MHz, 930 MHz, 1.85 GHz, and 1.95 GHz are the results of superposition of the distributions shown in Figs. 5, 6, and 6. The current distributions calculated at the multi resonances of the antenna shown in Fig. 1 look very similar.
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RADIATION CHARACTERISTICS
The radiation characteristics of the proposed antennas have been analyzed to confirm their performance for the mobile phone and smartphone applications. Figs. 8 and 9 present the gain radiation patterns of the antenna shown in Fig. 2. XY-plane is the plane of antenna layout. The first gain pattern is calculated at 905 MHz when the switches#1 and 3 are activated and the others are off. Typical monopole-like radiation pattern is produced. Top antenna gain is 1.65dBi while the total efficiency is -0.36dB. Fig. 9 shows the gain pattern of the same antenna at 1.945 GHz for the same combination of RF switches. At this band, both top antenna gain and total efficiency are higher at 3.65dBi and -0.08dB, respectively. It should be noted that the antenna gain and total efficiency have been calculated by including the actual model of a p-i-n diode switch in the EM simulations of the proposed antenna.
Figure 5: Current density distribution on the antenna shown in Fig. 2 when all switches are off at 1.7 GHz.
Figure 8: 3D gain radiation pattern of the antenna shown in Fig. 2 at 905 MHz. Figure 6: Current density distribution on the antenna shown in Fig. 2 when all switches are off at 2.0 GHz.
Figure 9: 3D gain radiation pattern of the antenna shown in Fig. 2 at 1.945 GHz. Figure 7: Current density distribution on the antenna shown in Fig. 2 when all switches are off at 2.45 GHz.
For the reference, consider next the radiation characteristics of the same antenna with all the switches being off. The gain pattern calculated at 1.7
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GHz is presented in Fig. 10 and the one at 2.45 GHz is shown in Fig. 11.
respectively. It has been confirmed by the series of EM simulation that the total efficiency of the proposed antennas remains high for all the modes and combinations of the RF switches within the frequency band of 800 MHz to 2.45 GHz. 5
CONCLUSIONS
The design of planar frequency reconfigurable RF switches-integrated antennas has been presented. The proposed antennas have simple L-shaped layout and a compact footprint. By activating the RF switches in various combinations several frequency bands between 800 MHz and 2.45 GHz are covered. Total efficiency of the proposed antennas remains high at all frequency bands. The reconfigurable antennas presented can be successfully utilized in a variety of smartphones and wireless terminals. Figure 10: 3D gain radiation pattern of the antenna shown in Fig. 2 at 1.7 GHz, all switches as off.
References [1] S. Yang, C. Zhang, H.K. Pan, A.E. Fathy, and V.K. Nair, “Frequency-Reconfigurable Antennas for Multiradio Wireless Platforms”, IEEE Microwave Magazine, February 2009, pp. 66-83. [2] K.R. Boyle, and P.G. Steeneken, “A Five-Band Reconfigurable PIFA for Mobile Phones”, IEEE Trans. Antennas Propagat., vol. 55, no. 11, part 2, pp. 3300-3309, Nov. 2007. [3] J. Cho, C.W. Jung, and K. Kim “FrequencyReconfigurable Two-Port Antenna for Mobile Phone Operating Over Multiple Service Bands”, Electronic Letters, Vol. 45, no. 20, 24 Sept. 2009. [4] Y. Koga, A.S. Andrenko, T. Yamagajo, and M. Shimizu, “Compact Frequency-Tunable Antenna with a GaAs Switch for the Mobile Handset”, 2013 IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications Proc., Torino, Italy, 2013, pp. 736-739.
Figure 11: 3D gain radiation pattern of the antenna shown in Fig. 2 at 2.45 GHz, all switches as off. In the “all-off” mode of operation, the top gain is 2.99dBi and 3.11dBi while the total efficiency is calculated as -0.4dB and -0.9dB at these two bands,
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