TABLE 1 Performance Comparisons of Different Array
Patterns
Performance
3 dB beam width ( )
Max left SLL (dB)
Max right SLL (dB)
IEP
11.1
29.8
29.7
AEP
10.6
24.8
22.5
AEPC
11.1
30
29.8
corresponding elements, which is indicated that the mutual coupling is operated by space path among elements.
The calculated MCM is applied to compensate the mutual coupling in pattern synthesis for this sectoral cylinder array to evaluate its precision. Three different array patterns are shown in Figure 7, and the performance of each one is presented in
Table 1. The calculated 30 dB low sidelobe pattern using isolated element patterns is regarded as the desired target pattern, and Taylor 30 dB excitation is applied by the null points adjusting method [15]. Apparently, the mutual couplings among elements are not considered in this process of the synthesis. While using the active element patterns with the same excitations, the maximum sidelobe level of the array pattern is deteriorated by nearly 8 dB due to the mutual coupling. After using the MCM to compensate the mutual coupling [13], the required sidelobe level is achieved, and this actual result is quiet close to the desired pattern, which demonstrates the validity and accuracy of the proposed method in calculating MCM.
5. CONCLUSIONS
The improved calculation method for mutual coupling in conformal array based on the experiment is investigated. For confirming the validity of this method, the 70 sectoral cylinder array composed of 12 cavity-backed stacked microstrip antennas is fabricated. The phase pattern calibration and direction weight modified techniques are applied to decrease the effect of error from element complex patterns’ measuring. Moreover, the LS method is adopted for simplifying the solution process for
MCM. The good agreement between the array pattern modified by calculated MCM and the anticipated target indicates the validity of the method. This method, which is accurate, simple, and robust, should be helpful for analyzing and compensating the mutual coupling in conformal array with digital beamforming system.
ACKNOWLEDGMENTS
The authors express their appreciation to Dr. YE Liang-Feng of
National University of Defense Technology (NUDT) for discussions on this work. They thank Master CAO Rui of NUDT for building the complex pattern measured system for the conformal array.
REFERENCES
1. R.J. Maillouw, Phased array antenna handbook, Artech House
Antennas and Propagation Library, Boston/London, 2007.
2. L. Josefsson and P. Persson, Conformal array antenna theory and design, Wiley-Interscience, New York, 2006.
3. J.R. James, P.S. Hall, and C. Wood, Microstrip antenna theory and design, IEE Press, Stevenage, UK, 1981.
4. S.D. Targonski and R.B. Waterhouse, Design of wide-band aperture-stacked patch microstrip antennas, IEEE Trans Antennas
Propag 46 (1998), 1245–1251.
5. N.C. Karmakar and M.E. Bialkowski, Circularly polarized aperture-coupled circular microstrip patch antennas for L-band applications, IEEE Trans Antennas Propag 5 (1999), 933–940.
6. D.W. Boeringer and D.H. Werner, Efficiency-constrained particle swarm optimization of a modified Bernstein polynomial for conformal array excitation amplitude synthesis, IEEE Trans Antennas
Propag 53 (2005), 2662–2673.
7. J. Fondevila-Gomez, J.A. Rodriguez-Gonzalez, and J. Bregains,
Very fast method to synthesise conformal arrays. Electron Lett 43
(2007), 856–857.
8. W. Li, S. Liu, X. Shi, and Y. Hei, Low-sidelobe pattern synthesis of spherical array using the hybrid genetic algorithm, Microwave
Opt Technol Lett 51 (2009), 1487–1491.
9. L. Landesa, J.M. Taboada, F. Obelleiro, et al. Design of on-board array antennas by pattern optimization, Microwave Opt Technol
Lett 21 (1999), 446–448.
10. P. Persson and L. Josefsson, Calculating the mutual coupling between apertures on a convex circular cylinder using a hybrid,
IEEE Trans Antennas Propag 49 (2001), 672–677.
11. W. Li, Y. Hei, X. Shi, et al. An extended particle swarm optimization algorithm for pattern synthesis of conformal phased arrays, Int
J RF Microwave Comput Aided Eng 20 (2010), 190–199.
12. P. Persson, L. Josefsson, and M. Lann, Investigation of the mutual coupling between apertures on doubly curved convex surfaces:
Theory and measurements, IEEE Trans Antennas Propag 51
(2003), 682–692.
13. H. Steyskal and J.S. Herd, Mutual coupling compensation in small array antennas, IEEE Trans Antennas Propag 38 (1990),
1971–1975.
14. N. Kojima, K. Hariu, and I. Chiba, Low sidelobe pattern synthesis using projection method with mutual coupling compensation, 2003.
15. S. Makino and Y. Konishi, Pattern synthesis and calibration methods for conformal, 2006.
2011 Wiley Periodicals, Inc.
Hyoungsuk Yoo,
1
1
Se-Hee Lee,
2 and Hongjoon Kim
2
Department of Biomedical Engineering, School of Electrical
Engineering, University of Ulsan, Ulsan, South Korea
2
Department of Electrical Engineering, Kyungpook National
University, Taegu, South Korea; Corresponding author: hongjoon@knu.ac.kr
Received 30 March 2011
ABSTRACT: We present a compact and broadband balun based on the concept of a synthetic left-handed transmission line and right-handed transmission line. The compact balun uses lumped, discrete LC elements only to reduce the circuit size dramatically. For the frequency range of
0.6–1.2 GHz, this circuit maintains 180
6
9.5
phase difference at the outputs, whereas the maximum insertion loss is 1.78 dB, and the minimum return loss is 10.2 dB at all three ports, yet the circuit size is merely 12 8 mm
2
. As far as we know, this is the smallest broadband balun reported in the literature, and the size can be further reduced, if the circuit is made with a monolithic microwave integrated circuit technology.
V 2011 Wiley Periodicals, Inc. Microwave Opt
Technol Lett 54:203–206, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26468
Key words: broadband; balun; left-handed; transmission line; righthanded; Wilkinson; compact power divider; compact
1. INTRODUCTION
A balun is an essential element for many microwave circuits.
Balanced mixers, image-rejection mixers, balanced amplifiers, and feeding networks of antenna arrays require the conversion of an unbalanced signal into a balanced signal.
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 203
To be useful in actual microwave circuits and systems, this type of circuit usually requires both a small size and a broad bandwidth. Especially, in the modern communication systems where the small size for compact design and the broad bandwidth for high data rate are essential, these properties are indispensible. A way to minimize the size is using lumped elements instead of long transmission lines [1], but such a structure is suffering from a narrow bandwidth.
Recently, a lot of attention has been paid to the microwave applications of the left-handed transmission line (LHTL) where the phase propagation is the opposite direction of a conventional transmission line [2–7]. In Refs. 5–7, the authors implemented broadband baluns by utilizing a broadband phase differentiation between a conventional right-handed transmission line (RHTL) and an artificial LHTL. However, those circuits require either a composite right/left-handed line or a long microstrip line, and thus, inevitably, the size of the circuit is too large to be used in modern communication/microwave systems. Furthermore, the
Wilkinson power divider implemented with the conventional microstrip lines for the power split occupies too much area.
As seen in Figures 1 and 2, the balun suggested here uses lumped-element RHTLs and LHTLs only that the size is totally dependent on the size of the elements. As these lumped elements can be implemented easily with standard monolithic microwave integrated circuit (MMIC) process, the whole circuit can be further minimized and integrated with other circuits easily. In this article, we describe the detailed theories and experimental results of the suggested circuit.
2. THEORY FOR THE LUMPED ELEMENT, BROADBAND
BALUN
2.1. RHTL and LHTL Theory
Figure 3(a) shows a section of a RHTL. A section is constructed with two series inductors and a shunt varactor. As this is a lowpass filter structure, a periodic cut-off frequency (Bragg cut-off frequency) exists as Eq. (1), when the several identical sections are cascaded [2].
f
R
Bragg
¼ p p ffiffiffiffiffiffiffiffiffiffiffi
L
R
C
R
(1)
Here, L
R is a series inductor, and C
R is a shunt capacitor.
In this synthetic transmission line, the characteristic impedance ( Z
0R
) also can be approximated as
Figure 1 The compact, broadband balun (left), and lumped-element
Wilkinson power divider (right).The size of the balun is 12 8 mm
2 without input and output microstrip lines and connectors. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]
Figure 2 Schematic diagram of the broadband balun shown in
Figure 1
Z
0R
¼ r ffiffiffiffiffiffi
L
R
C
R
The phase propagation constant ( b
R
) is given as Eq. (3) [2],
(2) cos
ð b
R
Þ ¼
1
ð
2 p f
Þ 2
L
R
C
R
2
(3)
The frequency at which b
R frequency.
becomes 180 is a Bragg cut-off
Figure 3(b) shows a section of LHTL. The Bragg cut-off frequency, the characteristic impedance ( Z gation constant ( b
L
) [2] are as follows,
0L
) and the phase propaf
L
Bragg
¼
4 p p ffiffiffiffiffiffiffiffiffiffiffi
L
L
C
L
(4) and
Z
0L
¼ r ffiffiffiffiffiffi
L
L
C
L
(5) and sin
ð b
L
=
2
Þ ¼
4 p f p
1 ffiffiffiffiffiffiffiffiffiffiffi
L
L
C
L
(6)
The frequency at which b
L becomes 180 is a Bragg cut-off frequency. The structure is a high-pass filter with a negative phase propagation as shown in Eq. (6).
2.2. Lumped-Element Wilkinson Power Divider
1 (right) and 2 is based on the RHTL theory. To replace p ffiffiffi
2 Z
0 k
/4 microstrip sections with lumped-element RHTL, the total
, phase propagation of the lumped-element RHTL Eq. (3) should be 90 at the desired frequency. If the Bragg frequency is close to the desired frequency, one should use the smaller values of
L
R and C
R to increase the cut-off frequency at the expense of less phase propagation at the desired frequency. In this case, several sections need to be cascaded to achieve 90 phase shift at the frequency.
2.3. Broadband, Constant Phase Differentiation
Figure 4 shows the theoretical phase variations of a RHTL and a LHTL, when two sections of each are cascaded. The
204 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 DOI 10.1002/mop
Figure 3 Lumped-element transmission lines. (a) A section of a
RHTL. (b) A section of LHTL characteristic impedance of these lines is Z
0
. The frequency at which the phase variation becomes 360 is the Bragg cut-off frequency for the RHTL, and 360 is for the LHTL. The phase difference (bold line) between them is also shown. From 0.61 to
1.25 GHz, 180
6
10 phase difference is maintained. A broadband and constant phase differentiation is possible between a
RHTL and a LHTL.
3. REALIZATION AND MEASUREMENTS
First, we had to verify the performance of a lumped-element Wilkinson power divider based on a RHTL theory. The Wilkinson power divider is fabricated on a FR4 board whose thickness and relative permittivity are 1.58 mm and 4.34, respectively. Johanson
Technology chip inductors and capacitors whose values are specified in Figure 2 are soldered on the board. The measured performance of the Wilkinson power divider is shown in Figure 5. With the values specified in Figure 2, the 90 phase propagation occurs at 0.86 GHz according to Eq. (3). The result agrees with the
Figure 5 The performance of the lumped-element Wilkinson power divider based on the RHTL theory theory. For the frequency range of 0.6–1.2 GHz, the insertion loss varies from 0.43 to 1.45 dB, whereas the minimum return loss and isolation are maintained below 11.1 dB.
Figure 4 Theoretical phase propagation of a RHTL and a LHTL and their difference. For each RHTL and LHTL, two sections are cascaded with CR
¼
2.7 pF, LR
¼
6.7 nH, CL
¼
4.9 pF, and LL
¼
12 nH
Figure 6 The performance of the lumped element, broadband balun.
(a) Insertion loss, return loss, and isolation. (b) The phase difference between Ports 2 and 3
DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 205
To have a broadband 180 phase differentiation, we added two sections of a RHTL and a LHTL to the lumped-element
Wilkinson power divider as shown in Figure 2. The result is shown in Figure 6. For the same frequency range mentioned above, the insertion loss varies from 0.51 to 1.78 dB, whereas the minimum return loss at all ports and the isolation are below 10.2 dB. The output phase difference for the frequency range is just 6 9.5
only as shown in Figure 6(b), which agrees very well with the theory (Fig. 4).
4. CONCLUSIONS
In this article, we present a compact and broadband balun made with lumped elements only. Unlike the previous works, we achieved both a small size and a broadband phase differentiation. Also, only this work is compatible with the modern integrated circuit design. The detailed theory suggested, and the measured results agree very well. The insertion loss and return loss can be better, if an optimized design is incorporated in the
MMIC design. Also, the size can be further minimized in such a design. Applications exist in many microwave systems where a broadband and balanced signal generation is required.
ACKNOWLEDGMENT
This work was supported by the Kyungpook National University internal fund.
REFERENCES
1. H.S. Nagi, Miniature lumped element 180 Wilkinson divider, In:
Microwave Symposium Digest, 2003 IEEE MTT-S International,
Philadelphia, PA, 8–13 June 2003, Vol. 1, 2003, pp. 55–58.
2. H. Kim, S.-J. Ho, M.-K. Choi, A.B. Kozyrev, and D.W. van der
Weide, Combined left- and right-handed tunable transmission lines with tunable passband and 0 phase shift, IEEE Trans Microwave
Theory Tech 54 (2006), 4178–4184.
3. W. Tang and H. Kim, Low spurious, broadband frequency translator using left-handed nonlinear transmission line, IEEE Microwave
Wireless Compon Lett 19 (2009), 221–223.
4. H. Kim, A.B. Kozyrev, A. Karbassi, and D.W. van der
Weide, Linear tunable phase shifter using a left-handed transmission line, IEEE Microwave Wireless Comp Lett 15 (2005),
366–368.
5. M.A. Antoniades and G.V. Eleftheriades, A broadband Wilkinson balun using microstrip metamaterial lines, IEEE Antennas Wireless
Propag Lett 4 (2005), 209–212.
6. C.-L. Tseng, and C.-L. Chang, Wide-band balun using composite right/left-handed transmission line, Electron Lett 43 (2007),
1154–1155.
7. Y.-H. Ryu, J.-H. Park, J.-H. Lee, and H.-S. Tae, Broadband Wilkinson balun using pure left-handed transmission line, Microwave
Opt Technol Lett 52 (2010), 1665–1668.
2011 Wiley Periodicals, Inc.
Jorge Moreno, Jie Fang, Roberto Quaglia, Vittorio Camarchia,
Marco Pirola, and Giovanni Ghione
Department of Electronics (DELEN), Politecnico di Torino, Corso
Duca degli Abruzzi, 24, Torino 10129, Italy; Corresponding author: roberto.quaglia@polito.it
Received 6 April 2011
ABSTRACT: This article describes the design, realization, and characterization of a set of hybrid medium-power RF power amplifiers, based on a commercial packaged GaN HEMT and developed through a low-cost microstrip process. Two different design solutions suitable for wireless applications are presented: the first, intended for a constant envelope modulation (with reference to the GSM standard) is a Class F amplifier exhibiting at 900 MHz an efficiency of 72% with an output power of 37.5 dBm; the second, optimized for nonconstant envelope signals with high dynamics (with reference to the UMTS WCDMA standard) is a Doherty amplifier showing, at 2.14 GHz, an efficiency higher than 40% at 6 dB of output power back-off with a maximum output power of 40 dBm.
V
2011 Wiley Periodicals, Inc. Microwave Opt
Technol Lett 54:206–210, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26459
Key words: GaN ; power amplifier ; microstrip ; wireless communications ; Doherty
1. INTRODUCTION
As well known, the need to reduce DC power consumption and the dissipation in RF transmitters makes the increase of the efficiency, one of the most important power amplifier (PA) design issues, both for handsets and base-station applications. High-efficiency solutions are derived from standard schemes such as the
Class B [1] in its several variants and are typically based today on more advanced solutions like the Class E [2], Class D [3], or
Class F [4]. However, the optimum choice of the PA implementation in terms of efficiency should also take into account the signal peak to average power ratio (PAPR), that is mainly determined by the modulation format and therefore by the specific wireless standard.
In this study, we focus our attention on the design of medium-power base-station amplifiers having as a target of two well-established wireless signal formats: a quasi-constant envelope modulation scheme working at 900 MHz (the GSM standard) and an advanced modulation scheme with larger PAPR typical of WCDMA applications, working at 2.14 GHz (the UMTS standard). Notice that with the GSM signal format the total
PAPR deriving from the channel statistics in a multicarrier medium-PA is low (0 dB for a single carrier and 3 dB for a multicarrier), whereas in the same conditions it is higher for the
UMTS standard (from 3 dB to more than 10 dB; Refs. 5 and 6).
For the PA implementation, we choose to exploit GaN-based devices. In fact, today high electron mobility transistor (HEMT) field-effect transistors based on the GaN technology [7–9] exhibit very interesting performances for RF wireless applications, as they are able to overcome the well-known limitations of the
Si-based LDMOS [10], today the dominant technology in basestation PAs. The devices exploited in this study were GaN
HEMTs grown on a Si substrate provided by Nitronex [11].
The choice of the PA topology and operating class was suggested by the following remarks. For the 900-MHz GSM application, single-stage PAs have been adopted, and a Class F topology has been selected (after comparison with a Class B topology realized with the same device and substrate), because it exhibits better performances both in terms of efficiency and maximum output power, as expected from the theory.
For the 2.14-GHz WCDMA application, the situation is more complex, because the high PAPR leads to operation with a significant output back-off (OBO) implying a severe penalty in terms of efficiency with respect to its peak values achieved at maximum output power. Combined stages, able to properly
206 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 DOI 10.1002/mop