High-Isolation Leaky-Wave Array Antenna Based on CRLH-Metamaterial Implemented on SIW with ±30 o Frequency Beam-Scanning Capability at Millimetre-Waves

: The paper presents a feasibility study on the design of a new metamaterial leaky-wave antenna (MTM-LWA) used in the construction of a 1 × 2 array which is implemented using substrate-integrated waveguide (SIW) technology for millimetre-wave beamforming applications. The proposed 1 × 2 array antenna consists of two LWAs with metamaterial unit-cells etched on the top surface of the SIW. The metamaterial unit-cell, which is an E-shaped transverse slot, causes leakage loss and interrupts current flow over SIW to enhance the array’s performance. The dimensions of the LWA are 40 × 10 × 0.75 mm 3 . Mutual-coupling between the array elements is suppressed by incorporating a metamaterial shield (MTM-shield) between the two antennas in the array. The LWA operates over a frequency range of 55–65 GHz, which is corresponding to 16.66% fractional bandwidth. The array is shown to exhibit beam-scanning of ±30° over its operating frequency range. Radiation gain in the backward ( − 30°), broadside (0°), and forward (+30°) directions are 8.5 dBi, 10.1 dBi, and 9.5 dBi, respectively. The decoupling slab is shown to have minimal effect on the array’s performance in terms of impedance bandwidth and radiation specifications. The MTM-shield is shown to suppress the mutual coupling by ~25 dB and to improve the radiation gain and efficiency by ~1 dBi and ~13% on average, respectively.


Introduction
Leaky-wave antennas (LWAs) are travelling wave antennas with electrically large radiating aperture [1,2]. Such antennas can provide high gain directive beam without using a complex feeding network [3,4]. The advantage of LWA over a conventional array antenna is a simple feed structure with low loss [5][6][7]. Conventional planar LWAs radiate higher order modes in the forward direction [8]. However, the periodic structure-based LWA can radiate in both forward and backward directions. It has been shown that metamaterial-based LWA designs can achieve a continuous main beam scanning from the backward to the forward direction as a function of frequency [9]. Various LWA designs based on metamaterial structures have been considered in the past. Such designs include (1) an LWA with a composite right/left-hand (CRLH)-folded substrate-integrated waveguide (SIW) structure that is shown to provide beam scanning from of -58° to 65° with a gain of 1 dBi [10]; (2) an interdigital-shaped slotted-SIW-based LWA that is reported to achieve a scanning angle of −60° to 70° with gain of around 8 dBi [11]; (3) a CRLH LWA based on a rectangular waveguide structure that has been demonstrated for a continuous main beam scanning range from −70° to 70° with gain of 8.64 dBi [12]; and (4) a planar slotted SIW LWA that provides a scanning range of −66° to 78° with consistent gain [13].
In this paper, a new type of LWA in array configuration is proposed based on SIW with metamaterial inclusions for millimetre-wave applications. Mutual coupling between the closelyspaced antennas in the array can undermine the array's performance. To circumvent this, a metamaterial shield is embedded between the two LWAs. With this approach, mutual coupling is shown to reduce by an average of ~25 dB over the array's operating frequency range. Engraved on the upper layer of the SIW LWA are several metamaterial unit-cells comprising of transverse Eshaped slots. Dimensions of the LWAs were modified for optimum array performance.

Design Process of The Proposed Mtm-Lwa Array Based On Siw
The proposed 1 × 2 array antenna based on MTM-LWA implemented on SIW technology is designed on a RO3003 dielectric substrate with = 3.0, tang = 0.0010 and thickness of 0.75 mm. Figure 1 displays the layout of the proposed array structure that is constructed with two MTM-LWA on SIW. Engraved on the top of each SIW antenna are several metamaterial unit-cells consisting of transverse E-shaped slots. Leakage loss at the slots interrupts the current flow over SIW, which is shown to enhance the array's impedance bandwidth and beamwidth that scans as a function of frequency. In the structure, the transverse slots behave as series left-handed capacitance and the grounded via holes acts as shunt left-handed inductors. The structural parameters of the MTM-LWA array are summarized in Table 1. Each antenna has dimensions of 40 × 10 × 0.75 mm 3 . The overall ground plane dimensions are 50 × 35 × 0.75 mm 3 . The S-parameter responses of the proposed array antenna are exhibited in Figure 2, which shows it operates throughout the frequency range of 55-65 GHz, which corresponds to 16.66% fractional bandwidth.  Radiation gain patterns of the proposed array antenna at three spot frequencies within its operating frequency range are plotted in Figure 3. It is evident that the array antenna is capable of beam-scanning from −30° to +30° with backward radiation at −30°, broadside radiation at 0°, and forward radiation at +30°. In backward, broadside, and forward directions, the gain is 8.5, 10.1, and 9.5 dBi, respectively.

Suppress the Mutual Coupling Between the Closely Spaced Mtm-Lwa Arrays
Mutual coupling between closely-spaced radiation elements can severely undermine the array's radiation performances. Here the isolation between the two MTM-LWAs is increased by introducing a metamaterial shield which is based on the SIW structure, as indicated in Figure 4. It comprises of transverse slots that are tapered. The slots have a width of 0.5 mm and essentially play the role of the series left-handed capacitances (CL), where the metallic via-holes with diameter of 0.25 mm act as shunt left-handed inductances (LL). The MTM-shield suppresses surface waves created by the LWAs to increase isolation between the two antennas in the array. The overall dimensions of the shield are 40 × 4 mm 2 . Figure 5 shows the S-parameter response before and after applying the MTM-shield. After applying MTM-shield, the minimum, average and maximum suppression observed are 8 dB, ~25 dB, and 42.5 dB, respectively. This shows the effectiveness of the proposed isolation technique. The shield has no influence on the reflection coefficient response which is S11 −10 dB.
Radiation patterns of the proposed antenna arrays with no and with MTM-shield through its operational bandwidth at spot frequencies of 55 GHz, 60 GHz & 65 GHz are plotted in Figure 6. It is clear from this figure that with the shield the cross-polarized radiation over its operating range is substantially reduced. The average gain of the co-polarized radiation is only marginally affected. All details are tabulated in Table 2.
In addition, the radiation gain and efficiency curves over frequency bandwidth for both antennas without and with the proposed shield are shown in Figure 7. Obviously, after realizing the metamaterial shield based on SIW, the radiation gain and efficiency performances improved by ~1dBi and ~13% on average, respectively.
Surface current density distributions without and with the MTM-shield are exhibited in Figure  8. This figure shows that the MTM-shield is an effective EM band-gap structure to remarkably block surface currents from EM interacting with adjacent radiation elements in the antenna array. Destructive influences of surface currents in the antenna are dramatically suppressed from effecting the far-field of the array antennas.

Circuit Model of The Proposed MTM-Lwa Array and its Dispersion Phenomenon
One way to explain the metamaterials (MTMs) is the transmission-line theory in terms of the circuit models. The concept of the composite right/left-handed metamaterial transmission lines (CRLH-MTM TLs) is investigated and realized based on this approach. This solution has been broadly recognized and adopted as a powerful analysis tool for the understanding and modelling of MTM devices. By considering the right-handed (RH) effects within a purely left-handed (LH) circuit, it demonstrates a general configuration of a practical MTM-TL. The circuit model of a generic symmetrical CRLH transmission-line unit-cell has exhibited in Figure 9 where the loss is neglected for simplicity. The series capacitance (CL) and the shunt inductance (LL), which have been realized by the slots and via-holes, respectively, contribute to the left-handedness while the series inductance (LR) and the shunt capacitance (CR), which have been implemented by the unwanted currents flowing on the patches and the gap space between the patches and ground plane, respectively, actualize its righthanded (RH) dual counterpart. The one indicated in Figure 9a is called T-type model with the LH capacitance placed at the two ends. The mushroom unit cell belongs to this type [14]. The circuit exhibited in Figure 9b is called -type model with the LH capacitance in the centre. One example is the CRLH-SIW unit cell [15][16][17]. Therefore, each unit cell of the proposed leaky wave array antennas is based on the -type model, which has been identified in Figure 4b. By using the periodic boundary conditions corresponding to the Bloch-Floquet theorem, these two CRLH transmission line unit-cells basically become equal and their dispersion relevance is determined to be [9]: where p = 5 mm is the length of the unit-cell and Seemingly, there are two frequency spots referred to as the infinite-wavelength points ( = 0) with a bandgap in between. In the balanced case ( = ), the bandgap vanishes. Generally, just one particular zeroth-order resonance will be excited that depends on the boundary conditions and the circuit values. For the short-ended resonator, it is determined by , while for the open-ended case, it is represented by [9]. Multiplex resonances containing the negative-, zeroth-, and positive-order resonances can be produced by cascading more than one unit-cell. Those resonance frequencies of different order modes for an M-stage CRLH-transmission line can be discovered on the dispersion diagram when the following condition is satisfied The proposed leaky wave antenna (LWA) array applying the CRLH-SIW unit-cell as shown in Figures 1 and 4 has an -type circuit model with the two ends terminated by the LH inductances (LL) realized by the metallic via-holes. The E-shaped transverse and the tapered transverse slots have presented within the circuit model as the LH capacitances (CL). RH contribution comes from the distributed shunt capacitor (CR) and the series inductor (LR). Figure 10 shows the dispersion curves of the proposed leaky wave array antennas based on the CRLH-SIW unit-cell that has been extracted by the CST Microwave Studio package and the equivalent circuit model exhibited in Figure 9b. It has been observed that the results achieved from CST Microwave Studio package and the circuit elements are in an excellent coherence with each other, and also it illustrates the dispersion relation of the unitcell very well. There is one zeroth-order resonance frequency occurring at 61 GHz that defines the upper and lower edges of the stop-band. This LW antenna array falls into the short-ended case because it is operated below the original waveguide cutoff-frequency and the metallic via-holes offer the short-ended condition. According to Equations (6) and (7), there is one resonance that can be excited comprising the zeroth-order resonance at = 61 GHz. Figure 5 shows the reflection coefficient which has verified the proposed model. Notice that this resonance frequency can be simply controlled by engineering the dispersion diagram. Magnitudes of the equivalent circuit parameters, which were determined from full-wave EM simulation using CST Microwave Studio package, are LL = 6.45 nH, CL = 8.69 pF, LR = 1.53 nH, and CR = 4.12 pF. This data was then utilized to determine the equivalent circuit-model of the antenna, displayed in Figure 9b, which was verified applying ADS (RF-circuit solver). The proposed LWA is exhibited as a traveling-wave antenna, where the current propagates along a guiding structure. Since that, the perturbations are introduced along the structure by implementing the E-shaped transverse and the traveling-wave leaves the structure and radiates into free-space. Therefore, in the ideal case, no energy reaches the end of the structure. In a practical scenario, any energy that reaches the end is absorbed by a matching load. Usually, LWA is designed, in which at the least 90% of the power at the structures leaks away before the traveling-waves reach the end of the antenna. Leaky-wave phenomenon is demonstrated with fast propagating waves only. The propagating wave number is defined by [18,19] = − In this case, = − is for surface-wave or slow-wave, or = is for leaky-wave or fast-wave, where is the longitudinal wave-number and refers to the free-space propagating wavenumber. The complexity of radiation is given by where and are the attenuation and phase constants respectively. Supposing that there is a standard free-space wave equation for the above wave, the waves outside the leaking-structure are given by: If < i.e. if the phase velocity is smaller than the free-space velocity of light, < , then it is a slow-wave and is imaginary. The wave decays exponentially in amplitude along the length of the structure and it is a bounded wave. If, > i.e. if the phase velocity is greater than the free space velocity of light, > , then it is a fast-wave and is purely real; therefore, the real power at an angle is radiated with respect to the normal defined by [20]: Since all of the abovementioned terms are functions of the angular frequency, the angle changes with frequency; hence, this shows frequency scanning behaviour. The main beam-width is If the above equation is applied for large antenna lengths, high directivity can be specified as However, the effect of enhancing directivity is negligible if there is no power left near the end of the structures. To specify this parameter, the attenuation/leakage constant is determined as [20]: Thus, if is sufficiently small so that (1 − > 0), the improvement of directivity is perceptible as length l enhances.

Comparison between This Work and The Literature
Performance parameters of the proposed 1 × 2 MTM-LWA array antenna based on SIW is compared with the recent works employing various mutual coupling suppression techniques. The comparison in Table 3 is for array antennas composed of two radiation elements. Most of the arrays listed in Table 3 exhibit narrow band performance, and to increase isolation between the radiation elements they employ defected ground structures (DGS) to complement their suppression technique. The proposed array antenna presented here has the advantage of (i) symmetry; (ii) very wide bandwidth from 55 GHz to 65 GHz; (iii) simple design; (iv) improved radiation patterns; (v) enhanced radiation gain; (vi) low cross-polarization levels; and (vii) mutual coupling suppression on average of ~25dB over its operating band.

Conclusion
A feasibility study of a new metamaterial leaky-wave array antenna based on substrateintegrated waveguide (SIW) technology with transverse slots and metallic via-holes for operation over 55 GHz to 65 GHz was proposed and investigated. The array antenna provides beam-scanning capability of ±30° with the gain of 8.5, 10.1, and 9.5 dBi at backward (-30°), broadside (0°), and forward (+30°) directions, respectively. To increase the isolation between the array's elements, a metamaterial shield based on SIW was introduced between the antennas, which has reduced the mutual coupling by an average value of ~25 dB. In addition, the proposed MTM shield has increased the radiation gain and efficiency by an average value of ~1dBi and ~13%, respectively. Funding: This work is partially supported by grant agreement H2020-MSCA-ITN-2016 SECRET-722424 and the UK EPSRC under grant EP/E022936/1.