Dual-Frequency Electric–Magnetic–Electric
Microstrip Leaky-Mode Antenna of
a Single Fan Beam
Yu-Chiao Chen, Student Member, IEEE, Ching-Kuo Wu, Student Member, IEEE, and
Ching-Kuang C. Tzuang, Fellow, IEEE
Abstract—This paper presents a dual-frequency electric–mag-netic–electric (EME) microstrip exhibiting two leaky-wave regions of similar radiation characteristics like the microstrip 1mode. The EME microstrip incorporates a photonic bandgap (PBG) structure, which is a two-dimensional array consisting of unit cell made of coupled coils connected by a via. The PBG structure employed in the EME prototype conducts at dc and shows the first stopband between 8.8–12.4 GHz, thus rendering the so-called magnetic surface. The EME microstrip is essentially made by substituting the PBG cells for the metal strip of a conventional microstrip. The finite-element method (FEM) analyses of the PBG structure show that the first and second modes are TM-like and TEM-like, respectively. The latter is leaky between 12.4–12.9 GHz and is found to be responsible for the second leaky region of the EME microstrip. The dispersion characteristics of the EME microstrip are obtained by two theoretical methods, namely, the matrix-pencil method and the FEM. Both show excellent agreement in the two leaky regions. Furthermore, the measured far-field radiation patterns of the two leaky regions also validate the dispersion curves. The first leaky region is of 1 type and between 5.05–5.45 GHz. The second leaky region radiates a frequency-scanning fan beam between 11.95–13.0 GHz, similar to those of the 1 mode. Detailed modal current analyses show even and odd symmetry along longitudinal and transverse plane of EME microstrip, respectively, further confirming the two leaky regions behave like the well-known 1leaky mode. The proposed EME microstrip enriches the modal characteristics of the conventional, uniform microstrip and is thus a manifestation of application of PBG structure for new guiding device.
Index Terms—Dual frequency, leaky-mode antenna, photonic bandgap (PBG).
I. INTRODUCTION
P
HOTONIC bandgap (PBG) structures are artificially engineered two- or three-dimensionally periodic struc-tures. Although early research focused on the optical field [1], [2], PBG structures are readily scaleable and applicable to a broad range of frequencies, including those of microwaves and millimeter waves. Electromagnetic waves that propagate in ar-tificial, multidimensional periodic structures resemble electron waves that propagate in natural crystals. These materials can prevent the omnidirectional propagation of electromagnetic Manuscript received April 6, 2002; revised August 21, 2002. This work was supported by the Ministry of Education of Taiwan under Grant 89-E-FA06-2-4, and by the National Science Council under Grant NSC 91-22213-E-009-063.The authors are with the Institute of Electrical Communication Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. (e-mail: [email protected]).
Digital Object Identifier 10.1109/TMTT.2002.805201
waves within certain frequency bands and exhibit passbands and stopbands for either guided waves along transmission lines or surface waves in dielectric substrates [3]–[15]. PBG struc-tures also exhibit two distinctive physical feastruc-tures. First, PBG structures typically consist of continuous metal and dc current conducted therein and can induce a magnetic surface (MS) at a forbidden frequency band. In the stopband, the tangential magnetic field is weak, even with a large electric field along the surface, and the surface impedance is very high. Such a structure is occasionally called a high impedance surface or an MS. In contrast to normal conductors, a high impedance surface can effectively suppress the propagation of surface waves and reflect electromagnetic waves without reversing their phase [5], [6]. Second, PBG cells or periodic devices latticed on the ground plane of a microstrip structure exhibit great versatility in controlling the dispersion characteristics of electromagnetic waves. Not only does the passband forbid wave propagating in the stopband, but the passband of the microstrip PBG structures display the slow-wave effect and has been used to reduce the dimensions of circuits. Moreover, the broad stopband characteristics have been applied to effectively suppress spurious transmission in the passband of filters and leakage from guided-wave structures [7]–[9]. The feasibility of applying PBG phenomena to both the optical and the microwave/millimeter-wave domains for industrial use has been extensively investigated [3]–[15]. PBG structures have attracted much interest among microwave engineers, primarily owing to their potential applications in several important areas such as microwave antennas, compact spurious-free filters, and frequency-selective surfaces (FSSs). For example, a uniplanar compact photonic bandgap (UC-PBG) structure was invented and its applications have proliferated in various fields of microwave engineering [6]–[10]. The gain of a microstrip antenna can be considerably increased by using a PBG layer as a substrate or by latticing PBG cells on the ground plane [8], [12]. A high impedance ground plane has been developed using metal plates with vertical vias [5].
Instead of latticing PBG cells on the ground plane of the mi-crostrip structure, we place new PBG cells on the signal line to develop a new microstrip structure. This microstrip struc-ture maintains the slow-wave effect and stopband characteris-tics. The dispersion characteristics of the PBG structure could be adjusted through changing the cell numbers along the trans-verse plane on the strip or tuning the dimensions of the PBG cells. The new PBG cells exhibit a high-impedance-surface state 0018-9480/02$17.00 © 2002 IEEE
(a)
(b)
Fig. 1. EME microstrip. (a) EME transmission line.h = 0:508 mm, " =
3:38, h = 0:203 mm, " = 3:38, W = W = 0 mm, W = W = 8:536 mm, W = 1:6 mm, L = 0:5 mm, and L = 256 mm. (b) Unit cell
incorporated in the MS.h = 0:508 mm, h = 0:203 mm, Lx = Ly =
2:134 mm, a = 0:127 mm, b = 0:508 mm, c = 0:864 mm, d = a = b = 0:254 mm, and V = 0:406 mm.
in a certain frequency band and behave like an MS by reflecting the incoming electromagnetic wave without reversing its phase. Electric surface and MS alternate transversely on the signal line, thus forming an electric–magnetic–electric (EME) microstrip, as shown in Fig. 1(a). Fig. 1(b) shows the expanded view of the unit cell incorporated in the EME microstrip. Detailed design in-formation of the unit cell is available in [9]–[11]. The unit cell consists of a high-impedance surface made of coupled induc-tors connected by a via, a substrate of thickness 0.203 mm and relative dielectric constant of 3.38, and a conducting uniform ground plane. The EME microstrip exhibits low-loss slow-wave bound mode propagation [9], [10]. The dispersion characteris-tics of the EME microstrip at zeroth order and first order, named the bound and leaky modes, have been reported in [11]. A reduced-size EME parallel coupled band-pass filter with suppressed spurious responses had been reported in a very sim-ilar way to the filter made of coupled microstrips on UC-PBG ground plane [7], [9].
Replacing the signal strip by the proposed PBG cells, the propagation characteristics of EME microstrip are enriched by the additional degrees of freedom offered by the PBG cells. As
Such an leaky mode can be excited by an out-of-phase differential signal at the opposite ends of the EME microstrip [16]. The same differential feeds can also excite the first higher order TEM-like mode of the PBG cells at higher frequencies, rendering a dual-frequency scanning leaky-mode antenna with a single fan beam for the first time.
The prototype presented in this paper, although not optimized to specific frequencies, is aimed at demonstrating the feasibility of using the EME microstrip as a dual-frequency leaky-mode antenna, with similar frequency scanning and fan beam char-acteristics. Section II reports the dispersion characteristics of the PBG unit cell which are important for designing and un-derstanding the dual-frequency EME leaky-mode antenna. Sec-tion III reports the design principle of the EME leaky-mode mi-crostrip antenna and dispersion characteristics at higher orders. An experiment is performed to validate the reported numerical results and to present the attractive features of the EME mi-crostrip. Conclusions are finally made in Section IV.
II. DISPERSIONDIAGRAM OFUNBOUNDED2-D PBG CELLS The geometry and characteristic dimensions of the recently developed PBG cell, latticed on the strip line of the EME microstrip shown in Fig. 1(a), are sketched in Fig. 1(b). The unisotropic PBG cell has coupled spiral coils on both sides of a printed RO4003 substrate of thickness ( ) 0.203 mm and relative permittivity ( ) 3.38, and connected by the center metal via through this thin substrate. In the design presented here, the unit cell is 2.134-mm wide ( or ) in both the and directions. The PBG cells are connected to adjacent cells on the top layer. More detailed information about the PBG unit cell is available in the caption of Fig. 1.
To investigate the propagation and stopband characteristics of the PBG cells incorporated in the EME microstrip, the theoretical dispersion diagram of the unbounded 2-D PBG periodical structure with unit cell shown in Fig. 1(b) is obtained by invoking the finite-element method [8]. Fig. 2 shows the dispersion diagram tracing along three sides of the irreducible Brillouin zone (BZ) triangle, as indicated in the inset of Fig. 2. The straight solid line and dashed line, referred to as the air-filled TEM line and the dielectric-filled TEM line, respec-tively, represent TEM wave propagations in free space and in a dielectric material of relative permittivity of 3.38, respectively. The first mode in dotted symbols deviates from the dashed line up to a certain frequency and then becomes relatively flat. Fig. 3(a) and (b) plots the electric field and magnetic field of
Fig. 2. Dispersion diagram of the MS using the unit cell of Fig. 1(b).Lx andLy denote the lengths of the PBG unit cell in the x and y directions, respectively.k and k denote the wave numbers of the PBG unit cell in the x andy directions, respectively.
(a)
(b)
Fig. 3. TM-like mode at the X point of the BZ. (a) Electric field. (b) Magnetic field.
the X point of the BZ (see Fig. 2) below the top metal coils at mm, respectively, showing that the first mode is pre-dominantly a TM mode. The electric field of the first mode has strong components along the longitudinal direction of the wave propagation and localized at the edge of the PBG unit cells in the direction; the magnetic field has a major component transverse to the propagation direction. Notice that the axis of the PBG cell coincides with the longitudinal direction of EME microstrip as seen in Fig. 1(a) and (b). The second mode begins at 12.4 GHz and crosses the air-filled TEM line at 12.9 GHz. Between 12.4–12.9 GHz, the second mode leaks in the forward direction with an angle from broadside to horizon as frequency increases. Fig. 4(a) and (b) plots the electric and magnetic
(a)
(b)
Fig. 4. TEM-like mode at the X point of the BZ. (a) Electric field. (b) Magnetic field.
fields of the X point of the BZ at mm, indicating that the second mode is predominantly a TEM mode, since both electric and magnetic fields are predominantly perpendicular to the direction of wave propagation. The second mode has strong electric field components in the direction and magnetic field components mainly along the direction. The TEM mode, after crossing the air-filled TEM line and before entering the dielectric-filled TEM line, becomes leaky in the surface. When the frequency is higher than 16.3 GHz, the TEM surface leaky wave becomes a bound mode and reaches the second stopband near 21.4 GHz. A complete bandgap exists between the first and second modes, which spans the frequency range from 8.8 to 12.4 GHz, and a second narrower stopband could be observed between the second and third modes, centered at 22.5 GHz and spanning from 21.4 to 23.6 GHz.
III. DUAL-FREQUENCYEME LEAKY-MODEANTENNA
A. Design and Operational Principle
A short EME microstrip prototype with mm
( mm) and mm, with
approxi-mately one guided wavelength at 5.25 GHz, is consid-ered. A metal strip with the same width ( mm), but 0.5-mm long ( mm) is added to each end of the EME microstrip to facilitate the connection to the feeding structure. The small uniform microstrip feed line, at one of the four cor-ners of Fig. 1(a), is 1.6-mm wide ( mm) and 0.5-mm long ( mm) and acts as a 50- microstrip on the com-bined substrates.
A conventional microstrip leaky line is fed with an out-of-phase differential signal, and the first higher order leaky mode is appropriately excited. The differential feed is also applied to
Fig. 5. Theoretical two-port scattering parameter and RPA value of the EME microstrip under differential excitations.
the EME microstrip, similarly. Proper widths ( , , and ) of the electric surfaces and MSs can be selected to obtain the required propagation constants and the useful operating fre-quencies of the leaky-mode antenna. A special EME microstrip, referred to as an MS microstrip, filled with PBG cells on the strip of the EME microstrip ( mm) is ap-plied in our work. The width of the EME microstrip incorpo-rated in the prototype is 47.5% less than that of a uniform mi-crostrip leaky-wave antenna, which is 16.256 mm at the same 5.45-GHz onset frequency [11]. Below 5.45 GHz, the onset fre-quency of the mode of the EME microstrip, the first leaky mode is excited and the EME microstrip antenna can ra-diate with a single frequency-scanning fan beam, like a conven-tional microstrip leaky-wave antenna [11].
Increasing the operating frequency to around 12.4 GHz (the upper corner of the first complete stopband of the PBG MS shown in Fig. 2) excites the TEM mode of the PBG cell without changing the feeding structure of the EME microstrip. The TEM mode of the PBG MS induces the second leaky mode of the EME microstrip and radiates in the forward direction between 12.4–12.9 GHz (see Fig. 2). As will become clearer in the fol-lowing descriptions, the current distribution of the second leaky mode of the EME microstrip resembles the current distribution of the conventional microstrip leaky-wave antenna. In contrast to the second or third higher order leaky modes of the uni-form microstrip with multiple beams, the second leaky mode of the EME microstrip radiates with a single fan beam. Thus, a dual-frequency EME microstrip antenna can be designed with frequency-scanning and single fan beam properties.
Notice that the number of PBG cells in the transverse direc-tion of the EME microstrip should be large enough to apply the BZ dispersion data, which assumes that the PBG MS is infinite. In the EME prototype of Fig. 1, four cells will be shown to be adequate to emit the second leaky mode near 12.4 GHz.
Theoretical scattering analyses of the short EME microstrip are first conducted using differential excitations. Fig. 5 presents the theoretical results obtained by a commercial full-wave inte-gral equation solver IE3D. The radiation efficiency of the EME microstrip can be estimated by the expression for the relative power absorbed (RPA), which is [17]. The value of the relative power absorbed (RPA) represents the total power losses of the electromagnetic device under investigation.
to the first complete forbidden band of the dispersion diagram from 8.8 to 12.4 GHz. The RPA results show that three peak regions have RPA values greater than 30%, wherein much en-ergy is lost. The frequency of the first peak of the RPA value is approximately 5.35 GHz, consistent with the design of the EME microstrip antenna with the 5.45-GHz onset frequency [11]. The PBG structure could reduce the significant crosstalk or transmission in the stopband, but a considerable amount of coupling or crosstalk is consistently presented in the passband, especially at the corner of the stopband. The second peak of the RPA value, at around 8 GHz, is near the lower corner of the stopband of the EME microstrip, and energy vanishes due to the surface wave loss [18]. The third peak region appears between 12.1–13.1 GHz, very close to the TEM leaky wave region of the dispersion diagram of Fig. 2.
Closely examining the current distributions on the ground plane at mm provides valuable physical information con-cerning the behavior of the EME microstrip at higher orders, such as the dispersion characteristics of the modes. Fig. 6(a) shows the current distributions on the ground plane of the EME microstrip excited differential at the left-hand side while the right-hand side port is terminated by a 50- load at 5.25 GHz. The current vectors are primarily transverse to the direction of the waves that propagate along the EME microstrip. Fig. 6(b) plots the current distribution on the ground plane of the EME microstrip line at 12.35 GHz. In both leaky regions, the currents vectors on the ground plane of the EME microstrip are primarily transverse to the direction of the waves that propagate along the EME microstrip.
B. Dispersion Characteristics of EME Microstrip at Higher Orders
The excited surface currents at the ground plane ( mm) of the EME microstrip record the information of the space har-monics [19]–[21]. The excited electromagnetic wave compo-nents that propagate on the EME microstrip are the superposi-tion of space harmonics due to the presence of a periodic PBG. The space harmonic component is denoted by its complex prop-agation constant ( ), which represents a trav-eling-wave component of the th higher order space compo-nent associated with the microstrip mode: the superscript ( ) signifies the forward- (backward-) traveling-wave com-ponents. In the matrix-pencil method [22], [23], a number of significant decimal digits provides a sanity check during the noise-eliminating process. Four significant digits applied in our analyses of the particular EME microstrip indicate that the
am-(a)
(b)
Fig. 6. Current distribution on the ground plane of the EME microstrip (z =
0 mm). The dashed rectangle indicates the position of the metal strip of the EME
microstrip. (a) At 5.25 GHz. (b) At 12.35 GHz.
plitudes of the higher order space harmonics ( ) are ap-proximately less than 1% of the amplitude of the dominant space harmonic ( ), both in the forward and the backward direc-tions. Therefore, a forward traveling wave, represented by the complex propagation constant , and a backward traveling wave, denoted by , are the two major wave components be-tween 4 –14 GHz. Both and have the same magnitude, but opposite signs. Outside this frequency range, only the for-ward traveling wave contributes markedly to the total field because of the high attenuation constant.
Fig. 7 presents the normalized propagation constant of the forward traveling-wave component for the EME microstrip. The solid line represents the normalized phase constant and the dashed line represents the normalized attenuation constant. Below the first onset frequency at 5.45 GHz, the first higher order mode leaks, and its attenuation constant increases with a decreasing frequency. Therefore, the EME microstrip radiates between 5.05–5.45 GHz, as shown in Fig. 5. The second leaky region could be observed between approximately 12.0–13.0 GHz, which closely correlates to the leaky-wave region of the second TEM mode of the PBG MS of Fig. 2 and the peaked RPA region of Fig. 5. Above 5.45 GHz, the normalized phase constant is in excess of unity, as frequency is increased toward 8.0 GHz. If the frequency is increased further over 8 GHz, then the propagation constant becomes complex again, with a decreasing normalized phase constant and changing normalized attenuation constant between 8–12 GHz, until the curve again enters a leaky region, wherein a fast wave propagates with a complex propagation constant. However, the
Fig. 7. Normalized phase constant =k and normalized attenuation constant
=k of the leaky mode of the EME microstrip propagating in the forward
direction.
leaky mode is not easily excited in this region because of high normalized attenuation constant ( ). These findings closely correspond to the scattering analyses shown in Fig. 5.
Next, the finite-element HFSS is applied to confirm the the-oretical results obtained by the matrix-pencil technique. The EME microstrip prototype, used in the finite-element method simulation, has four cells in the transverse direction and one cell in the longitudinal direction with the linked boundary condi-tion (LBC) at the cell sidewalls to simulate periodicity. An open boundary transverse to the propagation direction can be mod-eled accurately by perfectly matched layers (PMLs). The nor-malized phase constants are given only in the radiation regions. Fig. 7 shows that excellent agreement is achieved for data ob-tained by the matrix-pencil and finite-element methods.
The inset of Fig. 7 exhibits the correlation between the num-bers of the PBG cells, transverse to the longitudinal direction that waves propagate, and the position of the second radiation region. The solid lines with triangles, squares, and circles rep-resent the normalized phase constants of the EME microstrip containing five PBG cells, six PBG cells, and seven PBG cells transverse to the propagation in the second radiation region. The inset shows that various curves are nearly the same in the leaky region of interest, although the EME microstrip with transverse cells greater than four is wider than the prototype. The EME microstrip with four cells along the transverse direction can ex-hibit the modal behavior of the infinite-periodic 2-D PBG array, shown in Fig. 2.
Fig. 8(a) presents the transverse and longitudinal modal cur-rents obtained by the matrix-pencil method at 5.25 GHz. Cur-rents are normalized to a maximum of unity. The central plane of the signal line latticed by the PBG cells is at mm and the width mm. Real and imaginary parts of the longitudinal current ( ) are odd-symmetric and have null values on the central – plane. Real and imaginary parts of the transverse current ( ) are even-symmetric and have max-imum values on the central plane. Fig. 8(b) illustrates the trans-verse and longitudinal modal currents on the ground plane of the EME microstrip at 12.35 GHz. Currents are normalized to a maximum of unity. Real and imaginary parts of the longitu-dinal current ( ) are odd-symmetric and real and imaginary parts of the transverse current ( ) are even-symmetric. In both
(a)
(b)
Fig. 8. Normalized current distribution of the higher order leaky modes for the EME microstrip. The metal strip of the EME microstrip is located from
y = 04:268 mm to y = 4:268 mm. (a) At 5.25 GHz with the normalized
propagation constant =k = 0:06 + j0:67. (b) At 12.35 GHz with =k =
0:13 + j0:64.
leaky regions, the longitudinal currents ( ) are odd-symmetric and the transverse currents ( ) are even-symmetric. The cur-rent vectors on the ground plane of the EME microstrip antenna are primarily transverse to the direction of the waves that prop-agate along the EME microstrip, consistent with the first higher order leaky mode of the uniform microstrip. Therefore, the leaky-wave characteristics of the EME microstrip antenna at both bands are similar to those of a uniform microstrip at a higher order mode.
C. Measurements
A dual-frequency EME microstrip leaky-mode antenna is fabricated and tested based on the theoretical analyses outlined in the previous subsection. The experimental antenna is fed asymmetrically with a 50- microstrip 1.6-mm wide
( mm) and 0.5-mm long ( mm). The
antenna has the same dimensions as that in Fig. 1(a) with a length of 256 mm ( mm) and a width of 8.536 mm ( mm). The length of the EME microstrip is 4.5 (free-space wavelength at 5.25 GHz) and exceeds the required 3.9 if more than 90% of the electromagnetic energy leaks [24]. Fig. 9 displays a photograph of the dual-frequency EME microstrip leaky-mode antenna, showing a typical asym-metric feed reported by Menzel [25]. The asymasym-metric feed for
Fig. 10. Theoretical and measured far-field H-plane radiation patterns of the asymmetrically-fed EME microstrip leaky-mode antenna at 5.25 GHz. presents the tilt angle of the main beam from the broadside direction. an exciting leaky microstrip mode has been a well-known technique.
The -plane (the – -plane) radiation pattern measurement is obtained in an anechoic chamber. The two radiation bands, from 5.05 to 5.45 GHz and from 11.95 to 13.0 GHz, are observed. Outside the space-wave leaky region, the radiation pattern contains several lobes and nulls. The far-field theoretical radiation pattern is obtained by invoking the full-wave integral equation solver IE3D. Fig. 10 shows that the theoretical and measured co-polarization far-field patterns along the -plane agree closely at 5.25 GHz. The difference between the two main beam angles is under 1.5 and the measured beamwidth is slightly greater than the theoretical prediction. Fig. 11(a) and (b) depict the far-field radiation patterns of the EME microstrip antenna at two radiation regions. In the first radiation band, the tilt angle of the main beam increases from 26 to 56 , as the operating frequency increases from 5.15 to 5.35 GHz. The tilt angle increases from 25 to 63 in the second radiation band as the frequency increases from 12.15 to 12.75 GHz. The fre-quency-scanning properties of the EME microstrip antenna are similar to those of the uniform microstrip leaky-wave antenna. Additionally, radiation beam with the same tilt angle can be obtained in two radiation regions; for example, the radiation angles are approximately 39 at both 5.25 and 12.35 GHz. The measured 3-dB beamwidth is 19 at 5.25 GHz, which is slightly greater than 14.5 at 12.35 GHz.
(a)
(b)
Fig. 11. Frequency-scanning H-plane radiation patterns of the EME microstrip dual-frequency antenna. (a) First radiation region. (b) Second radiation region.
The normalized phase constant of the higher order mode is related to the angle of the leakage by , where is the free-space wavenumber and the main beam angle is measured from the broadside direction ( axis) [26]. The measured value of the main beam angle at two radiation regions are converted to the normal-ized phase constants ( ) which are superimposed on the normalized phase constant curve in Fig. 7, demonstrating the excellent agreement of normalized phase constants obtained by two theoretical methods and measurement.
IV. CONCLUSION
Substituting the PBG structure for a uniform metal strip of a conventional microstrip adds more degrees of freedom to the design of new guided-wave structure such as the so-called EME microstrip. Mingling the modal properties of the first (TM) mode and the second (TEM) mode of the PBG MS to the EME
microstrip, the leaky properties of the conventional, uniform microstrip are greatly transformed, resulting in dual-band leaky modes, which in many aspects resemble the leaky mode of a conventional microstrip. Both theoretical analyses and ex-perimental results show excellent agreement for the dispersion curve of the EME microstrip, confirming the dual-frequency operation of two leaky -like modes of the EME microstrip.
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Yu-Chiao Chen (S’99) was born in Tainan, Taiwan,
R.O.C., in 1975. He received the B.S. and M.S. degrees in communication engineering from the National Chiao Tung University, Hsinchu, Taiwan, R.O.C., in 1998 and 2000, respectively, and is currently working toward the Ph.D. degree in communication engineering at the same university.
His current research interests include field theory analysis and application designing of periodic struc-tures.
Ching-Kuang C. Tzuang (S’80–M’80–SM’92–
F’99) received the B.S. degree in electronic engi-neering from the National Chiao Tung University, Hsinchu, Taiwan, R.O.C., in 1977, the M.S. degree from the University of California at Los Angeles, in 1980, and the Ph.D. degree in electrical engineering from the University of Texas at Austin, in 1986.
From 1981 to 1984, he was with TRW, Redondo Beach, CA, where he was involved with analog and digital MMICs. Since 1986, he has been with the Institute of Communication Engineering, National Chiao Tung University. His research activities involve the design and development of millimeter-wave and microwave active and passive circuits and the field theory analysis and design of various complex waveguiding structures and large-array antennas. He has supervised 58 M.S. students and 15 Ph.D. students.
Dr. Tzuang helped in the formation of the IEEE Microwave Theory and Techniques Society (IEEE MTT-S) Taipei chapter, and served as secretary, vice chairman, and chairman in 1988, 1989, and 1990, respectively. He has been on the Asia–Pacific Microwave Conference International Steering Committee, where, since 1994, he has represented the Taipei chapter as the international liaison officer.
