Two-dimensional beam steering array using planar eight-element composite right/left-handed leaky-wave antennas



[1] A two-dimensional beam steering array composed of an eight-element antenna array using composite right/left-handed leaky-wave antennas fed by an 8 × 8 Butler matrix network is designed at X-band. An eight-way beam switching in one direction by input port switching and a continuous beam steering in the other direction by frequency sweep are achieved. A wide range beam steering operation covering from −55 to +53 degrees by port switching and from −37 to +27 degrees by frequency sweep is demonstrated with the maximum gain of 9.2 dBi.

1. Introduction

[2] Beam steering antennas are required in modern radar/sensor systems. Especially, low cost two-dimensional (2-D) wide range beam steering antennas will expand an application range of the systems. 2-D beam steering antenna arrays using composite right/left-handed (CRLH) metamaterial leaky-wave antennas (LWAs) have been proposed [Allen et al., 2006; Kaneda et al., 2006] to reduce the total cost of the systems drastically compared with conventional ones using phased array antennas. The CRLH LWAs have a unique feature of having a wide range backward-to-forward beam steering functionality [Liu et al., 2002; Grbic and Eleftheriades, 2002] including broadside direction when they are designed under the balanced CRLH condition [Sanada et al., 2004a, 2004b] with which the periodic CRLH LWAs operate at the Γ-point (β = 0) with an infinite wavelength wave with a nonzero group velocity.

[3] A 2-D beam steering operation has successfully been demonstrated by an all-passive four-element array of balanced CRLH LWAs fed by a 4 × 4 Butler matrix (BM) [Butler and Lowe, 1961] in a microstrip configuration [Kaneda et al., 2006]. The antenna gain is improved by the array effect, and more importantly, the beam is steered two-dimensionally; in one direction by changing frequency and in the other direction by changing one of the 4 input ports. The beam steering is continuous in one direction by changing frequency, however, it is limited to be only in four discretized directions by changing the input port. In order to increase the resolution of the beam, a larger number of antenna elements with a larger scale feeding network are required.

[4] In this paper, in order to increase the resolution of the 2-D beam, an eight-element array of balanced CRLH LWAs fed by an 8 × 8 BM is designed in a microstrip configuration. A balanced CRLH LWA element is designed by full-wave simulation using HFSS® and tested experimentally. In addition, an 8 × 8 BM on the same substrate is individually designed using Sonnet®, and the scattering parameters of the BM alone are tested experimentally. Then the overall eight-element CRLH LWA array is fabricated and tested to demonstrate a 2-D beam steering operation.

2. Antenna Array Configuration

[5] Figure 1a shows the prototype of the proposed antenna array. The array is composed of 8 CRLH LWA elements fed by an 8 × 8 BM feeding network. The array is fabricated on a dielectric substrate with the thickness of t = 0.64 mm, the relative permittivity of ɛr = 10.2, and the loss tangent of tanδ = 0.0028. The antenna array is designed at 10 GHz band. The total size of the footprint is 101 mm × 158 mm. Each antenna element is terminated by a 50 Ohm coaxial SMA termination.

Figure 1.

Eight-element CRLH LWA array operating at 10 GHz band. The substrate thickness and relative permittivity are t = 0.64 mm and ɛr = 10.2, respectively. The footprint size is 101 mm × 158 mm. (a) Overall configuration. (b) Unit cell (unit: mm). (c) Equivalent circuit of the unit cell.

[6] The CRLH LWA element is composed of a periodic array of unit cells with a metallic pattern shown in Figure 1b. The unit cell has a structure of a microstrip line type via-free CRLH structure [Sanada et al., 2004c]. The series capacitance between the adjacent metallic patterns leads to effective negative permeability. The two shunt open stubs in the unit cell are designed to be inductive looked at from the joint and they play a role of effective negative permittivity. Therefore, the guided waves in the periodic structure can possess left-handedness [Sanada et al., 2004c].

[7] Each CRLH LWA element works as a beam steering LWA when the frequency is swept. With a frequency sweep within a fast-wave region where the phase constant of the guided mode is smaller than the wave number in free space, the direction of radiation changes from backward to forward. With appropriate metallic pattern dimensions, the unit cell can be designed to be a balanced CRLH structure [Sanada et al., 2004a, 2004b] for which the upper limit frequency of the left-handed (LH) band and the lower limit frequency of the right-handed (RH) band are degenerated. At the balanced frequency, the guided wave number becomes zero (β = 0) with a nonzero group velocity, therefore the broadside radiation occurs.

[8] The array of the CRLH LWA elements is fed by a BM feeding network realized on the same substrate. The BM ideally provides 8 equal-amplitude outputs with a certain phase difference Δϕ between the adjacent ports [Butler and Lowe, 1961]. The phase difference Δϕ is supposed to be −22.5, +157.5, −112.5, +67.5, −67.5, +112.5, −157.5 and +22.5 degrees for input ports from #1 to #8, respectively. Therefore, an eight-way beam steering can be realized by the input port switching. Theoretically, when 8 omni-directional antenna elements are arrayed with the interval of a half wavelength in free space at the center frequency, the beam steering angle is theoretically from −61 to +61 degrees.

[9] It should be noted that the fast-wave region of the CRLH LWA where a radiation occurs has to be designed within the frequency band of the BM. The fast-wave region is controlled by controlling the equivalent circuit parameters of the CRLH LWA.

3. CRLH LWA Element Design

[10] The unit cell structure is designed by full-wave simulations using commercial software HFSS®. In order to design the LWA, three conditions should be considered; the balanced CRLH condition, the bandwidth of leakage, and the Bloch impedance for matching. As for the balanced condition, we introduce the equivalent circuit for the unit cell as shown in Figure 1c. When the power dissipated by radiation as well as by conductor and dielectric losses per unit cell is small enough (i.e., Rserad, Rshrad, rseloss and rshloss are small compared with the reactances due to the other reactive elements), the balanced condition for the structure is given by [Sanada et al., 2004c]

equation image

The unit cell pattern is designed to electrically satisfy the condition of (1) with assistance of full-wave simulation. Besides, the dispersion characteristics of the LWA are designed so that the LWA works within the operation bandwidth of the BM. Leakage occurs when the guided phase constant β is smaller than the wave number in free space, k0. Generally speaking, the bandwidth of the LWA is determined by the equivalent circuit parameter values, and as the right-handed contributions ωLR and 1/(ωCR) become dominant compared with the left-handed 1/(ωCL) and ωLL, respectively, the LH operation bandwidth becomes small. This is also taken into account in the optimization of the unit cell pattern. In addition, the Bloch impedance is matched to the characteristic impedance of the feed line 50 Ohm. For a balanced CRLH LWA, the Bloch impedance ZB at the balanced frequency becomes

equation image

It is noted that frequency characteristics of ZB become broad in the vicinity of the balanced frequency.

[11] In order to satisfy these three conditions simultaneously, we optimize the metallic pattern of the unit cell using full-wave simulations. The period is chosen to be 4.0 mm (∼0.15λ0) and the unit cell metallic pattern is optimized to satisfy the balanced condition of (1) at 11.0 GHz within an error range of the simulations. The optimized parameters are shown in Figure 1b. The simulated dispersion characteristics of the designed CRLH LWA are shown in Figure 2. It is seen from the figure that the CRLH LWA is balanced approximately at 11 GHz and the leakage occurs from 10.7 GHz to 11 GHz for the LH bandwidth and from 11 GHz to 11.7 GHz for the RH bandwidth. The simulated Bloch impedance at the balanced frequency is calculated to be 46 Ohm from full-wave simulations.

Figure 2.

Simulated dispersion characteristics of the CRLH LWA.

[12] A single CRLH LWA element with 20 unit cells is fabricated and its radiation characteristics are measured. The total length of the LWA is 80 mm excluding the feeding and terminating lines. The CRLH LWA is fed by a 50 Ohm microstrip feed line and is terminated at the end. A broadside radiation is observed approximately at 10.4 GHz as opposed to the fact that the balanced frequency is designed to be 11.0 GHz. The reason why the frequency is shifted 600 MHz lower than the simulated balanced frequency is considered to be due to errors in our fabrication process and the deviation of the permittivity of the substrate. Measured antenna gain is 3.8 dBi and the return loss is −8.9 dB at 10.4 GHz. A −30-degree angle backward radiation (seen from the broadside direction) is observed at 10.2 GHz with the gain of 4.8 dBi, and a +25-degree angle forward radiation is obtained at 10.7 GHz with 4.8 dBi.

4. The 8 × 8 Butler Matrix

[13] The BM is designed on the same substrate of the antenna element. The BM is realized by a combination of microstrip branch-line couplers, intercrossing circuits, and phase shifters as seen in Figure 3. The intercrossing circuit is realized by a tandem connection of two identical blanch-line couplers. These constituents are designed and optimized individually by full-wave simulations at the center frequency of 10.5 GHz with the reflection coefficient less than −30 dB.

Figure 3.

The 8 × 8 BM. (a) Block diagram. (b) Microstrip line implementation.

[14] Characteristics of the BM are confirmed experimentally. The BM is fabricated and the scattering parameters are measured. Figure 4 shows the measured frequency characteristics of the scattering parameters and the results are summarized in Table 1. Here, only scattering parameters with input ports from #1 to #4 are shown due to the symmetry of the structure. It is seen from the measured results that the operation frequency is approximately 10.4 GHz, slightly lower than the designed frequency, 10.5 GHz. This is considered to be due to errors in the fabrication and permittivity of the substrate. The worst amplitude deviation is ±2.7 dB and the worst phase deviation is −4.3 deg (0.4%) as seen in the table. The worst return loss in the frequency band from 10.2 GHz to 10.8 GHz of interest is less than −14 dB.

Figure 4.

Measured scattering parameters of the BM. (a) Amplitudes. (b) Phases.

Table 1. Measured Characteristics of the BM (10.3 GHz)
Input Port Number1234
Amplitude deviations (dB)±1.2±2.7±1.4±2.7
Average phase difference (deg) (Theory)−22.4 (−22.5)+157.0 (+157.5)−108.2 (−112.5)+71.2 (+67.5)
Phase error (deg)−0.1+0.5−4.3−3.7

5. Experiments

[15] An eight-element CRLH LWA array is fabricated by combining the designed antenna elements and the BM, and its radiation characteristics are measured. Figure 5a shows the measured radiation characteristics for the cases with different input ports from Port #1 to Port #8 at 10.4 GHz where the broadside radiation is expected. As seen in the figure, an eight-way beam switching from −59 deg to +50 deg is confirmed. The maximum gain is approximately 9.2 dBi when the input power is applied to Port #1 and Port #8, which is 5.4 dB larger than that of a single element. On the other hand, the maximum gain is 3.8 dB smaller than the numerical prediction from the array factor, and the degradation corresponds to the loss of the BM. The measured beam angles by port switching are from −59 deg to +50 deg agree well with the theoretical prediction, −55 deg to +53 deg.

Figure 5.

Radiation characteristics. (a) Beam steering by port switching. Measured radiation patterns (left) and calculated ones by a measured element radiation pattern and its array factor (right). (b) Beam steering by frequency sweep (measured).

[16] Figure 5b shows the beam steering characteristics of the antenna array when frequency is swept. The beam covers from −37 deg to +27 deg when frequency is swept from 10.4 GHz to 10.8 GHz. It is noted that broadside radiations are obtained approximately at 10.55 GHz for each input port.

[17] Figure 6 shows radiation characteristics in a half-space. The gain distribution on the ϕ-ψ plane (refer to the inset of Figure 6 for the coordinate system) is shown in the figure. The rows correspond to the different input port cases and the columns correspond to the cases with different frequencies. It is seen from the figure that positions of maximum gain change individually by the port switching and the frequency sweep, and a wide range 2-D beam steering operation is confirmed.

Figure 6.

Measured gain on a half ϕ-ψ plane. Rows correspond to the input port and columns correspond to frequency.

6. Conclusions

[18] A fully passive eight-element array of CRLH LWAs at X-band has been fabricated and its 2-D beam steering operation has been confirmed experimentally. The wide range 2-D beam steering operation covering from −55 to +53 degrees in one plane by port switching and from −37 to +27 degrees in the other plane by frequency sweep has been demonstrated with the maximum gain of 9.2 dBi.