### I. Introduction

*S*- parameters, and scan blindness [8]. The key factor involved in these issues is the mutual coupling among the radiating elements. This occurs dominantly between the adjacent radiating elements due to the surface wave flowing through the ground plane [9]. This not only degrades the isolation characteristics, but also causes deterioration of the various previously mentioned PAA characteristics.

*S*-parameters, and scan blindness.

*S*-parameter (or impedance) characteristics by employing a broadside coupled split ring resonator (BC-SRR) [17]. The ring resonator with a chip capacitor on it, the SRR using the side gap capacitance, and the BC-SRR using the capacitance between two layered planar rings have the stopband LC characteristic [18]. Since the two-layer Vivaldi is used for this work, the BC-SRR is the best candidate and is placed between the neighboring radiation elements. We design, fabricate, and measure a 1 × 8 CVA to validate the effects of the use of the BC-SRR in terms of the high isolation, wide scan angle, and active impedance. The full wave simulation results are compared with the measured ones with useful discussions.

### II. Design of the Single Radiating Element

*t*is 3 mm. Fig. 1(b) shows a front view and side view of the designed antenna. The two metal planes have an exponentially tapered slot. The exponentially tapered slot acts as an impedance transformation network for a match between free space (377 Ω) and the slotline (50 Ω) [19, 20]. The tapered slotline dominantly supports a traveling wave. The stripline to slotline transition is obtained by optimizing

*C*

_{1},

*C*

_{2}, and

*θ*in Fig. 1(b).

_{0}is roughly only 15 dB. In this work, we will demonstrate the promising effects of BC-SRRs in the PAA designs.

### III. Design of Array Antennas with Broadside Coupled Split Ring Resonators

### 1. Broadside Coupled Split Ring Resonator

*S*-parameters of type 1 and 2 BC-SRRs in the set-up shown in Fig. 3(c). The

*S*

_{21}transmission coefficients for both are −30 dB at 3.0 GHz. These results demonstrate that the stopband characteristic is well implemented in the structures at the target frequency.

### 2. 1 × 2 Phased Array Antenna

*d*between the two radiating elements is 0.5λ

_{0}. The heights of type 1 and 2 are optimized for minimum mutual coupling. The optimized heights

*h*

*of types 1 and 2 are 12.5 mm and 15 mm, respectively. In an effort to further enhance the isolation, we have increased the number of BC-SRRs (type 3) or changed the shape (type 4) as shown in Fig. 5. Type 3 has three (*

_{SRR}*N*= 3) circular BC-SRRs. The spacing (

*d*

*) between the resonators is 12 mm. In type 4, a rectangular BC-SRR is used instead of a square (type 2). The optimized heights*

_{SRR}*h*

*of types 3 and 4 are 12.5 mm and 12 mm, respectively.*

_{SRR}*S*-parameters and radiation patterns of the 1 × 2 array antenna structures shown in Fig. 5. The

*S*-parameters are plotted as a function of frequency for different types of the resonators. The radiation patterns show very little change. The details are summarized in Table 3. The isolation is enhanced by about 3 dB with type 4. This improvement will be shown to be more obvious with more array elements in Section III – 3.

### 3. Design of 1 × 8 Phased Array Antenna

*×*8 array antennas extended from the 1

*×*2 array antennas. Fig. 7(a)–(c) show the basic 1

*×*8 array antenna, 1

*×*8 array antenna with type 3 (circular BC-SRR,

*N*= 3), and the same with type 4 (rectangular BC-SRR,

*N*= 1), respectively. The spacing

*d*between the radiating elements is 0.5λ

_{0}. The optimized dimensions of types 3 and 4, after many EM-simulations, are as follows:

*R*

*= 6 mm,*

_{SRR}*d*

*= 13 mm (type 3),*

_{SRR}*L*

*= 5.5 mm,*

_{x}*L*

*= 17 mm (type 4),*

_{z}*W*

*= 1 mm,*

_{SRR}*G*

*= 1 mm, and*

_{SRR}*h*

*= 21 mm (types 3 and 4).*

_{SRR}*S*

_{44}and

*S*

_{54}of the antennas in Fig. 7 are plotted. The bandwidth based on a −10 dB reflection coefficient (

*S*

_{44}) is 2.5–3.6 GHz. We can see that it covers the target frequency band (2.8–3.3 GHz) with enough of a margin. The mutual couplings (

*S*

_{54}) at 3 GHz when using type 3 and 4 resonators are shown to be about −23 dB, which is a 6 dB enhancement compared with the basic 1

*×*8 array antenna.

*S*

_{44}(all ports from 1 to 8 excited) and total gains according to the scan angle

*θ*

_{0}for different antenna types.

*θ*=

*θ*

_{0}, the progressive excitation phase

*α*is given by

*β*

_{0}is the propagation constant given by 2π/λ

_{0}and the

*d*is 0.5λ

_{0.}

*S*

_{44}) of the antennas with type 3 (with circular BC-SRR,

*N*= 3) and type 4 (with rectangular BC-SRR,

*N*= 1) are shown to cover the target frequency band (2.8–3.3 GHz) up to

*θ*

_{0}= 50°. The actual scan range of

*θ*

_{0}is from −53° to +53° based on the – 10 dB active reflection coefficient.

*θ*

_{0}= 60°, the active

*S*

_{44}’s are shown not to satisfy −10 dB at the target frequency. It is about −8 dB at 3 GHz. The radiation patterns depending on the scan angles do not change. The realized scan angles of

*θ*

_{0}are shown to agree well with the targeted ones. While the total gains are similar for different antenna types, the active

*S*

_{44}’s for the antennas with type 3 and 4 resonators are significantly lowered, especially for large scan angles. The radiation efficiencies decrease as

*θ*

_{0}increases. In Table 4, these results are summarized in detail.

### IV. Fabrication and Measurement of Proposed Antenna

*×*8 array antenna and the proposed 1

*×*8 array antenna (with rectangular SRR,

*N*= 1, type 4). Fig. 10(b) and (c) show the set-ups to measure the reflection coefficients and radiation patterns, respectively.

*S*

_{44}and mutual couplings

*S*

_{54}as a function of frequency. The measured

*S*

_{44}and

*S*

_{54}are shown to have also shifted to the left about 200 MHz from the simulated one. The measured

*S*

_{54}’s of the basic and proposed 1

*×*8 type 4 array antenna are 18 and 23 dB (enhanced by 5 dB), respectively. The measured total gains are 11.7 dBi, lower than the simulated by 1 dB.

### V. Conclusion

*×*8 array antenna with BC-SRRs has been designed, fabricated, and measured. The total gains are maintained around 13 dBi, and the isolation is improved by about 6 dB with the BC-SRRs when compared with the basic 1

*×*8 array antenna without them. The FOV is ±53° based on a −10 dB active reflection coefficient. The operation of the scan angle within 60° is possible with a little larger reflection coefficient of about −8 dB. The proposed design with BC-SRRs is expected to be useful for PAA applications.