Tuning Range Optimization of a Planar Inverted F ... - Semantic Scholar
Tuning Range Optimization of a Planar Inverted F Antenna for the LTE low frequency bands Samantha Caporal Del Barrio, Mauro Pelosi, Ondrej Franek, Gert F. Pedersen, Section of Antennas, Propagation and Radio Networking (APNet), Department of Electronic Systems, Faculty of Engineering and Science, Aalborg University, DK-9220 {scdb, mp, of, gfp}@es.aau.dk
Abstract— This paper presents a Planar Inverted F Antenna (PIFA) tuned with a fixed capacitor to the low frequency bands supported by the Long Term Evolution (LTE) technology. The tuning range is investigated and optimized with respect to the bandwidth and the efficiency of the resulting antenna. Simulations and mock-ups are presented. Keywords – tunable antenna, reconfigurable antenna, small antenna, LTE band, frequency tuning, LTE, PIFA.
I.
INTRODUCTION
Due to customers needs and rapid progress in wireless communications, Electrically Small Antennas (ESA) need to be designed to work in CDMA, GSM, WCDMA, GPS, Bluetooth, among other bands. The next generation of cellular th technologies, 4 Generation (4G), promises data rates in the order of 100Mb/s in downlink, using primarily the 790-862 MHz band in Europe. In order to merge these service bands, the antenna design needs to be revised and should be either multiband or frequency reconfigurable. The multiband type of antennas for handheld devices has been largely investigated during the past years, leading to tri-band, quad-band or five bands antenna designs and more with multilayer patches [1]-[5]. The main drawback of the multiband technique is the increased volume dedicated to the antenna, when there is a trend towards slimmer, smaller and lighter devices. One of the main challenges of implementing low frequencies LTE (Long Term Evolution) in small mobile devices with already limited space is the antenna size for the 700 MHz band. The advantages of the frequency reconfigurable type compared to the multiband type are its compact design, similar radiation pattern and proper gain for all desired frequency bands [6]. The reconfigurability has been investigated with PIN diodes, varactor diodes or FET components [6]-[11] in order to manipulate the current distributions as well as the resonant frequency. However, they are not enough to cover all frequency bands and often only address the high frequency range. The Planar-Inverted-F-Antenna (PIFA) is commonly used into compact and small devices because of its low profile, reduction of Specific Absorption Rate (SAR), easy fabrication
and low-cost [12]. Also the insertion of a slot provides dualband operation, which is of particular interest for the wireless applications that use two different frequency bands for receiving and transmitting. The uni-planar nature of these antennas allows for simple integration of lumped components and they are therefore good candidates for tunability investigation. The tuning method presented in this paper focuses on the achievable tuning range of a PIFA for the low frequencies LTE700/GSM850/GSM900 since they are the most challenging for small handsets. The frequency tuning range of the antenna will be optimized by the chosen lumped element and not by changing its geometrical parameters. The proposed technique is based on loading the radiating element with a capacitor along the edge of the PIFA. The finite Quality factor (Q) of the device, unlike PIN diodes or MEMS, degrades the radiation efficiency less. Section II presents the antenna design used to investigate the proposed tuning concept. Section III addresses the tuning range, Q, efficiency and gain of the simulations. Following this in Section IV the measured return losses, and total efficiencies are presented. Finally the conclusion is drawn in Section V. II.
ANTENNA DESIGN
A. Simulations Methodology Each simulation was performed with the Finite Difference Time Domain (FDTD) method using a 1 mm cell and an energy based termination criterion. B. Reconfigurable PIFA Geometry Fig. 1 illustrates the proposed reconfigurable antenna. It consists of a two arms radiating element within a 40 x 22 x 2 mm3 volume. The size of the ground plane is 40 x 100 mm2. For the considered frequencies, it is mainly the board that resonates and since the antenna size, bandwidth and efficiency are strongly interdependent [13] the described antenna is a tough design. The dimensions of the Printed Circuit Board (PCB) considered here are usual for the bar-type application.
[mm]
Feed and short location Varying capacitor Radiating element
Ground plane
distributions at 960 MHz. On the radiating element surface, the dominant field is Jy plotted on Fig. 3(a). On the other hand, the dominant field distribution on the PCB is Jz shown on Fig. 3(b). On the PIFA the strongest currents are concentrated along the slot in the low arm - Fig. 3(a). The highest points reach values about 4 mA/m. The plot of the PCB in Fig. 3(b) shows high concentration of current at one particular location corresponding to the end of the slot on the PIFA. This is due to very high coupling between the 2 mm high patch and the PCB. Jz
Jy
Figure 1. Design of the proposed high Q reconfigurable PIFA.
The antenna element is located above the top area of the ground plane. The distance between the radiating element and the ground plane is 2 mm in order to have a high Q antenna. On the top edge of the design there is a shorting line, a feeding conductor and a capacitor – which will tune the resonance frequency. The capacitor operates between 1 pF and 2 pF for a tuning range achieving the 900 MHz -GSM band and the 700 MHz LTE band. The antenna design was optimized for operating in the GSM900 and GSM1800 bands. The parameters are described in Fig. 2.
(a)
(b)
Figure 3. Surface current distributions of the proposed reconfigurable PIFA at 960 MHz (Unit [A/m])
III.
TUNING RANGE INVESTIGATION
Fine tuning can be achieved in two ways. The first one is to vary the value of the capacitor, and therefore reduce the inductance of the shorting line. This results in a change of the resonance frequency without any change in the overall structure. This mechanism can also be understood theoretically with the following definition of the resonance frequency: √
(1)
where L and C are the overall inductance and capacitance, respectively, of the resulting antenna. Figure 2. Radiating element geometry (Unit [mm])
In the proposed structure, 50 Ω impedance matching is easily obtained by proper location of the shorting line with respect to the feeding point. C. Surface Current Distributions In order to investigate the tuning range of the proposed PIFA, the excited surface current distributions on the radiating element and on the PCB are studied. Fig 3 shows Jy and Jz distributions, with respect to the axes shown on Fig. 1. The investigation concentrates on the low frequencies bands. Fig. 3 shows simulations results of the surface current
Fig. 4 shows the dual resonance behavior of the PIFA. The varying capacitor is placed on the low arm, between the radiating element and the ground plane. Its capacitance is varied with 1/8 pF steps to ensure coverage of every frequency in the targeted band. By increasing the capacitance from 1/8 pF to 11/8 pF the resonance frequency is tuned down from 960 MHz to 880 MHz. At the same time, the high resonance center frequency remains almost constant. Note that the decrease of the resonant frequency is not uniform and depends on the capacitor’s location. Therefore tuning can also be achieved by only changing the location of the lumped element. This method is presented below.
11/8 pF
1/8 pF
Figure 4. Low band tuning with a capacitor value from 1/8 pF to 11/8 pF
The second way of tuning the antenna resonance frequency is to always use the same capacitor but to change its location. In this way the currents running through it are different and the same mechanism than described for the first way of tuning occurs.
Figure 5. Resonance frequency shift with respect to the 1 pF capacitor’s positions.
This second method of tuning the antenna resonance frequency has been investigated with a 1/8 pF capacitor at different locations on the edge of the PIFA. Table I shows the currents, resonance frequencies (fr), Q and bandwidths (BW) variations for three different positions. The capacitor is placed at 6, 16 and 31 mm away from the source location. TABLE I.
INFLUENCE OF THE CAPACITOR’S POSITION Currents [A]
Location
1/8pF
6 mm 16 mm 31 mm
Short 6,25 e-5 6.15 e-5 5.90 e-5
Capacitor 7,18 e-7 1.63 e-6 2.40 e-6
fr [MHz]
Q
BW [MHz]
974 967 956
21 23 24
46 43 39
As expected, the currents in the short drop when the capacitor is moved away from it, and the currents running in the capacitor increase. At the same time, the resonance frequency decreases and therefore the Q of the antenna gets higher. The bandwidth is affected too; it is because of overcoupling with the ground plane when the resonance is shifted down.
On the Smith chart (Fig. 6) the behavior of the low resonance while tuned towards lower frequencies is presented. Both tuning methods are investigated, with the capacitor’s location variation or the capacitor’s value variation.
The same simulations are reproduced with a 1 pF capacitor shifted all the way along the edge of the PIFA. Fig. 5 shows that the resonance frequency varies from 920 MHz to 800 MHz by only changing the capacitor’s position.
The above figure depicts the resonance for the three locations investigated in Table I and sustains that the overall capacitance of the resulting antenna structure, at resonance, increases when the capacitor is moved away from the source.
TABLE II.
1pF
RESONANCE FREQUENCY DEPENDING ON THE CAPACITOR’S VALUE AND POSITION Location
fr [MHz]
Q
BW [MHz]
6 mm 16 mm 31 mm
964 917 852
20 16 20
47 56 42
Figure 6. Smith chart representing the frequency behaviour while tuned towards lower frequencies.
Fig. 7 shows the simulated radiation patterns at the resonance frequency of the proposed PIFA, tuned with a 1/8 pF capacitor or a 1 pF capacitor, for the three locations investigated in Table I. The characteristics of the low band resonance are summarized in Table I and Table II.
As shown in Fig. 7 the simulated tunable antenna exhibits a radiation pattern – at the low frequency bands – with dipolelike characteristics. Moreover the gains are very similar for both tuning methods.
(a) Figure 8.
(b) Mock-up of the proposed antenna
The capacitors have high Q (Q=200) and their Equivalent Series Resistance (ESR) is reported in Table III.
Figure 7. Radiation patterns of the tunable PIFA with 1/8 pF capacitor or 1 pF capacitors on the three locations investigated in Table I
IV.
MEASUREMENTS
After a parametric study in simulations, a mock-up of the proposed PIFA has been built in order to measure the efficiencies when tuned down in frequency. A. Efficiencies When the value of the capacitor becomes larger, the operation frequency becomes lower and the efficiency degrades, because the power consumption in the parasitic resistance of the capacitor has become larger due to the increase of the current in the capacitor [14]. To verify the simulation results and measure the resulting efficiency of the tunable antenna the proposed design has been built (see Fig. 8(a)). The antenna, originally resonating at 960 MHz, was tuned to 867 MHz with a fixed capacitor and measured. Lowering the resonance frequency of the mock-up was investigated in the two cases described below and compared. In the first case a 4 pF capacitor is placed on the edge of the PIFA, 16 mm away from the source (Fig. 8(b)) ; in the second case a 1 pF capacitor was chosen and placed further away from the source, at 31 mm. In both situations the antenna resonates at 867 MHz and the measured return loss is shown in Fig. 9. The high resonance is no longer constant when the capacitor is switched from 1 pF to 4 pF. This can be due to a board extra resonance or to higher harmonics detuned to the plotted frequencies.
Figure 9. Measured return loss of the mock-up for the proposed PIFA tuned to 867 MHz with either a 4 pF capacitor placed at 16 mm from the source or a 1 pF capacitor placed at 31 mm away from the source
In both cases the low band resonance is at 867 MHz with the same bandwidth and a very good matching, 30 MHz and 40 dB respectively. The minimum of the |S11| curve is roughly the same for the 1 pF and the 4 pF case, even though the simulations predict a worse matching to the source impedance for the 4 pF case. The very good matching observed is due to losses in the component and the mock-up. For each tuning situation the mock-up was measured in the anechoic chamber and the results are shown in Table III. TABLE III.
EFFICIENCY MEASUREMENTS ESR (Ω)
Efficiency at 867 MHz
Case 1 pF, 31 mm
0.4
-1.6 dB
Case 4 pF, 16 mm
0.3
-2.8 dB
The efficiencies are significantly different from one tuning situation to another. With the 4 pF capacitor the efficiency drops more than 1 dB. This is most likely due to higher currents running through the ESR of the capacitor. B. MIMO The same tuning techniques have been tried on a MIMO unit. A second and identical PIFA has been added at the bottom of the board and the handset has been tuned to 796 MHz with a 1.5 pF capacitor. The measured S parameters of the built handset are presented in Fig. 10 and show relatively good bandwidth and matching, 30 MHz in total for the bandwidth. The two antennas on the PCB are tuned to two different frequencies. The top antenna is centered slightly above – and the bottom antenna below – the 796 MHz center frequency. This is done in order to cover a larger bandwidth and to pre-compensate detuning due to environment and/or user influence. The coupling is – 9 dB due to the big size of the antennas and the small 100 x 40 mm² form factor.
due to the reactance change, the position of the tuning component needs to be chosen carefully. The Q and the losses increase as the resonance frequency is tuned down but they can be minimized with a proper placement of the tuning element. Therefore tuning is a tradeoff between position of the capacitor and capacitance range. ACKNOWLEDGMENT Pevand Bahramzy is acknowledged for his participation in all the practical work and the helpful discussions. REFERENCES [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9] Figure 10. S parameters for the tunable MIMO handset
The handset has been measured in the anechoic chamber and the resulting efficiencies are -3.7 dB for the top antenna and -2.9 dB for the bottom one at 796 MHz. V.
[10]
[11]
CONCLUSION
The position variation of the capacitor with a fixed value or the capacitance variation of the component at a fixed position both change the resonance frequency. Therefore a parametrical study is required before tuning an electrically small antenna. The gains, matching to the source and bandwidths are unaffected by the choice of one or the other tuning method. However the efficiency can vary more than 1 dB. The value of the capacitance can dramatically degrade the overall efficiency of the antenna if it is placed where very high currents are running through it. In order to optimize the tuning
[12]
[13]
[14]
Y. X. Guo, M. Y. W. Chia, and Z. N. Chen, “Miniature built-in quadband antennas for mobile handsets” (in available online), IEEE Antennas Wireless Propagat. Lett., vol. 2, pp. 30–32, 2003. Yong-Xin Guo; Irene Ang; Chia, M.Y.W.; , "Compact internal multiband antennas for mobile handsets," Antennas and Wireless Propagation Letters, IEEE , vol.2, no.1, pp.143-146, 2003 Yong-Xin Guo; Hwee Siang Tan; , "New compact six-band internal antenna," Antennas and Wireless Propagation Letters, IEEE , vol.3, no.1, pp.295-297, Dec. 2004 P. Ciais et al., “Design of an internal quad-band antenna for mobilephones,” IEEE Microwave and Wireless Components Letters, vol. 14, no. 4, pp. 148–150, Apr. 2004 Min-Seok Han; Hong-Teuk Kim; , "Compact Five Band Internal Antenna for Mobile Phone," Antennas and Propagation Society International Symposium 2006, IEEE , vol., no., pp.4381-4384, 9-14 July 2006 Jong-Hyuk Lim; Gyu-Tae Back; Young-Il Ko; Chang-Wook Song; TaeYeoul Yun; , "A Reconfigurable PIFA Using a Switchable PIN-Diode and a Fine-Tuning Varactor for USPCS/WCDMA/m-WiMAX/WLAN," Antennas and Propagation, IEEE Transactions on , vol.58, no.7, pp.2404-2411, July 2010 Peroulis, D.; Sarabandi, K.; Katehi, L.P.B.; , "Design of reconfigurable slot antennas," Antennas and Propagation, IEEE Transactions on , vol.53, no.2, pp.645-654, Feb. 2005 Behdad, N.; Sarabandi, K.; , "A varactor-tuned dual-band slot antenna," Antennas and Propagation, IEEE Transactions on , vol.54, no.2, pp. 401408, Feb. 2006 Symeon Nikolaou; Bairavasubramanian, R.; Lugo, C., et al , "Pattern and frequency reconfigurable annular slot antenna using PIN diodes," Antennas and Propagation, IEEE Transactions on , vol.54, no.2, pp. 439448, Feb. 2006 Lai, M.-I.; Wu, T.-Y.; Hsieh, J.-C.; Wang, C.-H.; Jeng, S.-K.; , "Design of reconfigurable antennas based on an L-shaped slot and PIN diodes for compact wireless devices," Microwaves, Antennas & Propagation, IET , vol.3, no.1, pp.47-54, February 2009 Jong-Hyuk Lim; Gyu-Tae Back; Young-Il Ko; Chang-Wook Song; TaeYeoul Yun; , "A Reconfigurable PIFA Using a Switchable PIN-Diode and a Fine-Tuning Varactor for USPCS/WCDMA/m-WiMAX/WLAN," Antennas and Propagation, IEEE Transactions on , vol.58, no.7, pp.2404-2411, July 2010 L. Z. Dong, P. S. Hall, and D. Wake, “Dual-frequency planar inverted-F antennas,” IEEE Trans. Antennas Propag., vol. 45, no. 10, pp. 1451– 1458, Oct. 1997. Roger F. Harrington, “Effect of Antenna Size on Gain, Bandwidth, and Efficiency,” Journal of Research of the National Bureau of Standards D, Radio Propagation VOL. 64D, No. 1, January-February 1960, pp. 1—12 Tsutsumi, Y.; Nishio, M.; Obayashi, S.; Shoki, H.; Ikehashi, T.; Yamazaki, H.; Ogawa, E.; Saito, T.; Ohguro, T.; Morooka, T.; , "Low profile double resonance frequency tunable antenna using RF MEMS variable capacitor for digital terrestrial broadcasting reception," SolidState Circuits Conference, 2009. A-SSCC 2009. IEEE Asian , vol., no., pp.125-128, 16-18 Nov. 2009
Abstract— This paper presents a Planar Inverted F Antenna (PIFA) tuned with a fixed capacitor to the low frequency bands supported by the Long Term Evolution (LTE) technology. The tuning range is investigated and optimized with respect to the bandwidth and the efficiency of the resulting antenna. Simulations and mock-ups are presented. Keywords – tunable antenna, reconfigurable antenna, small antenna, LTE band, frequency tuning, LTE, PIFA.
I.
INTRODUCTION
Due to customers needs and rapid progress in wireless communications, Electrically Small Antennas (ESA) need to be designed to work in CDMA, GSM, WCDMA, GPS, Bluetooth, among other bands. The next generation of cellular th technologies, 4 Generation (4G), promises data rates in the order of 100Mb/s in downlink, using primarily the 790-862 MHz band in Europe. In order to merge these service bands, the antenna design needs to be revised and should be either multiband or frequency reconfigurable. The multiband type of antennas for handheld devices has been largely investigated during the past years, leading to tri-band, quad-band or five bands antenna designs and more with multilayer patches [1]-[5]. The main drawback of the multiband technique is the increased volume dedicated to the antenna, when there is a trend towards slimmer, smaller and lighter devices. One of the main challenges of implementing low frequencies LTE (Long Term Evolution) in small mobile devices with already limited space is the antenna size for the 700 MHz band. The advantages of the frequency reconfigurable type compared to the multiband type are its compact design, similar radiation pattern and proper gain for all desired frequency bands [6]. The reconfigurability has been investigated with PIN diodes, varactor diodes or FET components [6]-[11] in order to manipulate the current distributions as well as the resonant frequency. However, they are not enough to cover all frequency bands and often only address the high frequency range. The Planar-Inverted-F-Antenna (PIFA) is commonly used into compact and small devices because of its low profile, reduction of Specific Absorption Rate (SAR), easy fabrication
and low-cost [12]. Also the insertion of a slot provides dualband operation, which is of particular interest for the wireless applications that use two different frequency bands for receiving and transmitting. The uni-planar nature of these antennas allows for simple integration of lumped components and they are therefore good candidates for tunability investigation. The tuning method presented in this paper focuses on the achievable tuning range of a PIFA for the low frequencies LTE700/GSM850/GSM900 since they are the most challenging for small handsets. The frequency tuning range of the antenna will be optimized by the chosen lumped element and not by changing its geometrical parameters. The proposed technique is based on loading the radiating element with a capacitor along the edge of the PIFA. The finite Quality factor (Q) of the device, unlike PIN diodes or MEMS, degrades the radiation efficiency less. Section II presents the antenna design used to investigate the proposed tuning concept. Section III addresses the tuning range, Q, efficiency and gain of the simulations. Following this in Section IV the measured return losses, and total efficiencies are presented. Finally the conclusion is drawn in Section V. II.
ANTENNA DESIGN
A. Simulations Methodology Each simulation was performed with the Finite Difference Time Domain (FDTD) method using a 1 mm cell and an energy based termination criterion. B. Reconfigurable PIFA Geometry Fig. 1 illustrates the proposed reconfigurable antenna. It consists of a two arms radiating element within a 40 x 22 x 2 mm3 volume. The size of the ground plane is 40 x 100 mm2. For the considered frequencies, it is mainly the board that resonates and since the antenna size, bandwidth and efficiency are strongly interdependent [13] the described antenna is a tough design. The dimensions of the Printed Circuit Board (PCB) considered here are usual for the bar-type application.
[mm]
Feed and short location Varying capacitor Radiating element
Ground plane
distributions at 960 MHz. On the radiating element surface, the dominant field is Jy plotted on Fig. 3(a). On the other hand, the dominant field distribution on the PCB is Jz shown on Fig. 3(b). On the PIFA the strongest currents are concentrated along the slot in the low arm - Fig. 3(a). The highest points reach values about 4 mA/m. The plot of the PCB in Fig. 3(b) shows high concentration of current at one particular location corresponding to the end of the slot on the PIFA. This is due to very high coupling between the 2 mm high patch and the PCB. Jz
Jy
Figure 1. Design of the proposed high Q reconfigurable PIFA.
The antenna element is located above the top area of the ground plane. The distance between the radiating element and the ground plane is 2 mm in order to have a high Q antenna. On the top edge of the design there is a shorting line, a feeding conductor and a capacitor – which will tune the resonance frequency. The capacitor operates between 1 pF and 2 pF for a tuning range achieving the 900 MHz -GSM band and the 700 MHz LTE band. The antenna design was optimized for operating in the GSM900 and GSM1800 bands. The parameters are described in Fig. 2.
(a)
(b)
Figure 3. Surface current distributions of the proposed reconfigurable PIFA at 960 MHz (Unit [A/m])
III.
TUNING RANGE INVESTIGATION
Fine tuning can be achieved in two ways. The first one is to vary the value of the capacitor, and therefore reduce the inductance of the shorting line. This results in a change of the resonance frequency without any change in the overall structure. This mechanism can also be understood theoretically with the following definition of the resonance frequency: √
(1)
where L and C are the overall inductance and capacitance, respectively, of the resulting antenna. Figure 2. Radiating element geometry (Unit [mm])
In the proposed structure, 50 Ω impedance matching is easily obtained by proper location of the shorting line with respect to the feeding point. C. Surface Current Distributions In order to investigate the tuning range of the proposed PIFA, the excited surface current distributions on the radiating element and on the PCB are studied. Fig 3 shows Jy and Jz distributions, with respect to the axes shown on Fig. 1. The investigation concentrates on the low frequencies bands. Fig. 3 shows simulations results of the surface current
Fig. 4 shows the dual resonance behavior of the PIFA. The varying capacitor is placed on the low arm, between the radiating element and the ground plane. Its capacitance is varied with 1/8 pF steps to ensure coverage of every frequency in the targeted band. By increasing the capacitance from 1/8 pF to 11/8 pF the resonance frequency is tuned down from 960 MHz to 880 MHz. At the same time, the high resonance center frequency remains almost constant. Note that the decrease of the resonant frequency is not uniform and depends on the capacitor’s location. Therefore tuning can also be achieved by only changing the location of the lumped element. This method is presented below.
11/8 pF
1/8 pF
Figure 4. Low band tuning with a capacitor value from 1/8 pF to 11/8 pF
The second way of tuning the antenna resonance frequency is to always use the same capacitor but to change its location. In this way the currents running through it are different and the same mechanism than described for the first way of tuning occurs.
Figure 5. Resonance frequency shift with respect to the 1 pF capacitor’s positions.
This second method of tuning the antenna resonance frequency has been investigated with a 1/8 pF capacitor at different locations on the edge of the PIFA. Table I shows the currents, resonance frequencies (fr), Q and bandwidths (BW) variations for three different positions. The capacitor is placed at 6, 16 and 31 mm away from the source location. TABLE I.
INFLUENCE OF THE CAPACITOR’S POSITION Currents [A]
Location
1/8pF
6 mm 16 mm 31 mm
Short 6,25 e-5 6.15 e-5 5.90 e-5
Capacitor 7,18 e-7 1.63 e-6 2.40 e-6
fr [MHz]
Q
BW [MHz]
974 967 956
21 23 24
46 43 39
As expected, the currents in the short drop when the capacitor is moved away from it, and the currents running in the capacitor increase. At the same time, the resonance frequency decreases and therefore the Q of the antenna gets higher. The bandwidth is affected too; it is because of overcoupling with the ground plane when the resonance is shifted down.
On the Smith chart (Fig. 6) the behavior of the low resonance while tuned towards lower frequencies is presented. Both tuning methods are investigated, with the capacitor’s location variation or the capacitor’s value variation.
The same simulations are reproduced with a 1 pF capacitor shifted all the way along the edge of the PIFA. Fig. 5 shows that the resonance frequency varies from 920 MHz to 800 MHz by only changing the capacitor’s position.
The above figure depicts the resonance for the three locations investigated in Table I and sustains that the overall capacitance of the resulting antenna structure, at resonance, increases when the capacitor is moved away from the source.
TABLE II.
1pF
RESONANCE FREQUENCY DEPENDING ON THE CAPACITOR’S VALUE AND POSITION Location
fr [MHz]
Q
BW [MHz]
6 mm 16 mm 31 mm
964 917 852
20 16 20
47 56 42
Figure 6. Smith chart representing the frequency behaviour while tuned towards lower frequencies.
Fig. 7 shows the simulated radiation patterns at the resonance frequency of the proposed PIFA, tuned with a 1/8 pF capacitor or a 1 pF capacitor, for the three locations investigated in Table I. The characteristics of the low band resonance are summarized in Table I and Table II.
As shown in Fig. 7 the simulated tunable antenna exhibits a radiation pattern – at the low frequency bands – with dipolelike characteristics. Moreover the gains are very similar for both tuning methods.
(a) Figure 8.
(b) Mock-up of the proposed antenna
The capacitors have high Q (Q=200) and their Equivalent Series Resistance (ESR) is reported in Table III.
Figure 7. Radiation patterns of the tunable PIFA with 1/8 pF capacitor or 1 pF capacitors on the three locations investigated in Table I
IV.
MEASUREMENTS
After a parametric study in simulations, a mock-up of the proposed PIFA has been built in order to measure the efficiencies when tuned down in frequency. A. Efficiencies When the value of the capacitor becomes larger, the operation frequency becomes lower and the efficiency degrades, because the power consumption in the parasitic resistance of the capacitor has become larger due to the increase of the current in the capacitor [14]. To verify the simulation results and measure the resulting efficiency of the tunable antenna the proposed design has been built (see Fig. 8(a)). The antenna, originally resonating at 960 MHz, was tuned to 867 MHz with a fixed capacitor and measured. Lowering the resonance frequency of the mock-up was investigated in the two cases described below and compared. In the first case a 4 pF capacitor is placed on the edge of the PIFA, 16 mm away from the source (Fig. 8(b)) ; in the second case a 1 pF capacitor was chosen and placed further away from the source, at 31 mm. In both situations the antenna resonates at 867 MHz and the measured return loss is shown in Fig. 9. The high resonance is no longer constant when the capacitor is switched from 1 pF to 4 pF. This can be due to a board extra resonance or to higher harmonics detuned to the plotted frequencies.
Figure 9. Measured return loss of the mock-up for the proposed PIFA tuned to 867 MHz with either a 4 pF capacitor placed at 16 mm from the source or a 1 pF capacitor placed at 31 mm away from the source
In both cases the low band resonance is at 867 MHz with the same bandwidth and a very good matching, 30 MHz and 40 dB respectively. The minimum of the |S11| curve is roughly the same for the 1 pF and the 4 pF case, even though the simulations predict a worse matching to the source impedance for the 4 pF case. The very good matching observed is due to losses in the component and the mock-up. For each tuning situation the mock-up was measured in the anechoic chamber and the results are shown in Table III. TABLE III.
EFFICIENCY MEASUREMENTS ESR (Ω)
Efficiency at 867 MHz
Case 1 pF, 31 mm
0.4
-1.6 dB
Case 4 pF, 16 mm
0.3
-2.8 dB
The efficiencies are significantly different from one tuning situation to another. With the 4 pF capacitor the efficiency drops more than 1 dB. This is most likely due to higher currents running through the ESR of the capacitor. B. MIMO The same tuning techniques have been tried on a MIMO unit. A second and identical PIFA has been added at the bottom of the board and the handset has been tuned to 796 MHz with a 1.5 pF capacitor. The measured S parameters of the built handset are presented in Fig. 10 and show relatively good bandwidth and matching, 30 MHz in total for the bandwidth. The two antennas on the PCB are tuned to two different frequencies. The top antenna is centered slightly above – and the bottom antenna below – the 796 MHz center frequency. This is done in order to cover a larger bandwidth and to pre-compensate detuning due to environment and/or user influence. The coupling is – 9 dB due to the big size of the antennas and the small 100 x 40 mm² form factor.
due to the reactance change, the position of the tuning component needs to be chosen carefully. The Q and the losses increase as the resonance frequency is tuned down but they can be minimized with a proper placement of the tuning element. Therefore tuning is a tradeoff between position of the capacitor and capacitance range. ACKNOWLEDGMENT Pevand Bahramzy is acknowledged for his participation in all the practical work and the helpful discussions. REFERENCES [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9] Figure 10. S parameters for the tunable MIMO handset
The handset has been measured in the anechoic chamber and the resulting efficiencies are -3.7 dB for the top antenna and -2.9 dB for the bottom one at 796 MHz. V.
[10]
[11]
CONCLUSION
The position variation of the capacitor with a fixed value or the capacitance variation of the component at a fixed position both change the resonance frequency. Therefore a parametrical study is required before tuning an electrically small antenna. The gains, matching to the source and bandwidths are unaffected by the choice of one or the other tuning method. However the efficiency can vary more than 1 dB. The value of the capacitance can dramatically degrade the overall efficiency of the antenna if it is placed where very high currents are running through it. In order to optimize the tuning
[12]
[13]
[14]
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