Download Electronically Scanned Composite Right/Left Handed Microstrip

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 14, NO. 6, JUNE 2004
277
Electronically Scanned Composite Right/Left Handed
Microstrip Leaky-Wave Antenna
Sungjoon Lim, Student Member, IEEE, Christophe Caloz, Member, IEEE, and Tatsuo Itoh, Fellow, IEEE
Abstract—A novel electronically scanned periodic microstrip
leaky-wave (LW) antenna based on the concept of composite
right/left-handed (CRLH) metamaterials is presented. This antenna includes varactors modulating the capacitive loading of the
unit cell and therefore the propagation constant of the structure,
which results in voltage scanning of the radiated beam. An
accurate circuit model is proposed. The antenna is demonstrated
experimentally to exhibit continuous scanning in the dominant
mode from backward to forward angles. The scanning range is
at 35 V to
at 0 V, and broadside occurs at 9 V,
from
at the fixed frequency of 3.23 GHz.
10
+7 5
Index Terms—Composite right/left handed (CRLH) structure,
electronical scanning, leaky-wave (LW) antenna, metamaterials.
I. INTRODUCTION
M
ODERN commercial and military communication frequency bands are getting saturated. In this context, despite their unique features of high directivity and broad-range
scanning, conventional leaky-wave (LW) antennas [1], [2] are
often not suitable because of the excessive frequency band they
use for scanning.
Recently, we proposed a novel transmission line approach of
left-handed (LH) metamaterials in [3] and [4] and introduced a
generalized composite right/left handed (CRLH) concept [generalized metamaterial with negative at low frequencies (LH)
and positive at high frequencies (RH)] in [5] and [6]. Based
on this concept, a backfire-to-endfire LW antenna operating in
the dominant mode was developed and shown to exhibit backfire to endfire radiation capabilities in the dominant mode [7],
while conventional LW antennas operate in a higher mode and
cannot efficiently radiate at broadside. However, this antenna is
also scanned by frequency.
A LW antenna that can be scanned at a fixed frequency is
often preferable to a frequency-scanned one. Much effort was
directed toward developing an electronically-scanned LW antenna [8]–[10]. The scanning angle from broadside is given
by the well-known equation [11]
(1)
space wavelength, is the space harmonic
,
and is the period of the structure. Equation (1) shows that
the radiation angle is determined by the guided wavelength
or by the period . Horn et al. [8] electronically changed
by modulating p–i–n diodes in a dielectric waveguide antenna,
but the resulting antenna allows only two discrete radiation angles because the diodes have only two states (biased / unbiased). Maheri et al. [9] also controlled but by using a biasing
magnetic field in a corrugated ferrite slab configuration, but a
dc magnetic field supply is not practical for most applications.
Huang et al. [10] reported a different approach, in which the
period of the structure was controlled by using p–i–n diodes
switches, but their antenna was also restricted to two discrete
states.
In this letter, we present a novel type of fixed-frequency electronically-scanned LW antenna. The proposed antenna is based
on the CRLH metamaterials concept. The radiated beam of this
antenna can be continuously scanned. In addition, the antenna
is implemented in microstrip technology, whereas all aforementioned tunable antennas are waveguide antennas. Such a lowprofile characteristic makes the proposed antenna easier to fabricate and preferable for the integrated millimeter wave systems.
II. PRINCIPLE OF ELECTRONICALLY-SCANNED LW ANTENNA
The proposed antenna is designed based on the CRLH structure which consists of a series capacitor and a shunt inductor
plus a parasitic series inductor and a parasitic series capacitor
[4], [5]. In the CRLH structure, the propagation constant is
dependent on the inductive and capacitive loadings of the line.
The radiation angle is given by (1), and can be determined once
is known. The underlying idea of our antenna is based on the
observation that, even though propagation constant is dependent
on frequency, it can also be modulated by tuning the inductances
and capacitances in the CRLH structure.
In the lossless case of ideal CRLH-TL [6], the equivalent circuit model is shown in Fig. 1(a) and the propagation constant
can be obtained by
(2)
where is propagation constant of the waveguide,
is the guided wavelength,
space wavenumber,
is the free
is the free
Manuscript received October 27, 2003; revised February 19, 2004. This
work was supported by the MURI program “Scalable and Reconfigurable
Electromagnetic Meta-materials and Devices,” the Department of Defense
(N00014-01-1-0803), and the U.S. Office of Naval Research. The review of
this letter was arranged by Associate Editor A. Weisshaar.
The authors are with the Electrical Engineering Department, University of
California, Los Angeles, CA 90095 USA.
Digital Object Identifier 10.1109/LMWC.2004.828008
where
(3)
(4)
where
is period of the structure.
1531-1309/04$20.00 © 2004 IEEE
278
IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 14, NO. 6, JUNE 2004
Fig. 1. Equivalent circuit models for the unit cell of (a) purely passive CRLH
LW antenna and (b) the proposed LW antenna including the varactor bias
is the
network. C (V) is the tunable capacitance of the varactor and C
capacitance of the dc block.
Fig. 3. Measured (a) radiation patterns and (b) return losses for the antenna
shown in Fig. 2 at different bias voltages. At 3.23 GHz, the receiving angles
for the reverse voltages 35, 9, and 0 V are 10 , 0 and +7:5 , respectively.
Return losses at 3.23 GHz are 6.263, dB, and 11.402 dB at 35, 9, and 0 V,
respectively.
0
0
0
0
(8)
(9)
III. DESIGN AND EXPERIMENTAL RESULTS
Fig. 2. Prototype of the proposed antenna including shunt varactors.
In the balanced case, defined by
gation constant can be expressed from (2)–(4) as
, the propa(5)
In the proposed antenna, we tune the reactance provided by
with a varactor. For this purpose, a dc block has to be introduced for the bias network, which makes the equivalent circuit
mode slightly more complicated, as shown in Fig. 1(b). But this
structure can still be effectively CRLH in nature. In this case,
the propagation constant and scanning angle are the following
function of the bias voltage
(6)
where
(7)
Fig. 2 shows the prototype of the proposed electronically,
scanned LW antenna built on RT/Duroid5880 (
). For the bias network, ATC 3.9 pF chip capacitors and
Murata 4.7 nH chip inductors are used for dc blocks and dc
feeds, respectively. The varactors are the Metelics MSV34069E28X. The reverse bias voltage is varied from 0 V to 35 V. In
order for the varactor to operate at the reverse bias, positive
voltage is supplied at cathode of the varactors and their anode is
connected to the ground. The output port of the antenna is terminated to 50 in order to prevent reflection inducing spurious
beams on the other side of the normal to the substrate.
Forward, broadside and backward scanning at the fixed frequency of 3.23 GHz are demonstrated in Fig. 3(a), where the
, 0 , and
are obtained at 0, 9, and 35 V, reangles
spectively. The return loss of the antenna is shown in Fig. 3(b).
At 3.23 GHz, they are 11.402, 11.203, and 6.263 dB at 0,
9, and 35 V, respectively. The continuous electronic-scanning
LIM et al.: ELECTRONICALLY-SCANNED COMPOSITE RIGHT/LEFT HANDED MICROSTRIP LEAKY-WAVE ANTENNA
279
tenna as a function of the number of cells at 0 V. The gain from
full-wave simulation was found to increase from 4.8 to 5.7 dB
at 0 V as the number of cells is increased from four to 27 cells.
IV. CONCLUSION
Fig. 4. Scanning angle versus reverse bias voltage relationship. The transition
reverse voltage is 9 V in the measurement.
A novel fixed-frequency electronically-scanned microstip
leaky-wave antenna, based on the concept of CRLH metamaterials, is proposed and demonstrated experimentally. The CRLH
equivalent circuit model, including the varactors bias network,
is proposed, and the principle of the antenna is explained based
on this model. At the fixed frequency 3.23 GHz, the beam
of the antenna can be continuously steered from backward to
forward angles by tuning the reverse voltage of the varactors.
Wider scanning angle ranges can be achieved by introducing a
varactor in series in addition to the shunt varactor used in the
reported antenna.
REFERENCES
Fig. 5. Full-wave simulated half-power beamwidth (HPBW) as a function of
number of cells at 0 V.
capability of the antenna is shown in Fig. 4. The numerical and
full-wave simulated results are compared with the measured re) four-cell structure shown in Fig. 3
sults. A short (0.753
was used for the proof of concept. As a consequence of this very
short length, the half-power beamwidth (HPBW) is 73 at 0 V.
The measured gains are 6.1, 5.8, and 5.6 dB at 0, 9, and
35 V, respectively. As in conventional LW antennas, the HPBW
and gain can be improved by simply increasing the length of
the line. Fig. 5 shows the full-wave simulated HPBW of the an-
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