#### What is in this article?:

- UWB Arrays Employ TCAs
- A Closer Look
- Conclusions
- References

Tightly coupled arrays with resistive frequency-selective surfaces can serve as building blocks in phased-array antennas with wide bandwidths.

## A Closer Look

Download this article in .PDF format This file type includes high resolution graphics and schematics when applicable. |

**Figure 2** shows a unit cell with square-split-ring RFSS structure. The square-split-ring is comprised of resistive film, with the resistor layer printed on a microwave printed-circuit board (PCB) based on RO4003 circuit substrate from Rogers Corp. with thickness, t_{1}, of 0.508 mm and dielectric constant, ε_{r1}}, of 3.38. The length of the outer square split ring, s_{1}, is 9.8 mm and the length of the inner square split ring, s_{2}, is 7.4 mm. The width of both rings, s_{3}, is 0.6 mm, and the split width, s_{4}, is 0.6 mm.

When the unit cell with period d = 10.1 mm and resistivity, R_{5} = 33 Ω is placed a distance h_{1} = 5.5 mm away from the ground plane, **Fig. 3** shows a simulated reflection coefficient as achieved by means of the Microwave Office simulation software from Computer Simulation Technology. As these simulated data show, the square-split-ring RFSS can suppress ground-plane reflections over a wide bandwidth, so this approach can be used in the design of UWB antennas to minimize the influence of the ground plane on bandwidth.^{11}

As can be seen from the expression:

Z_{GP} = jη_{0}tan(2πh/λ)

the shunt impedance becomes Z_{GP} = 0 when h = λ/2. So, the TCA is short circuited at frequency f_{max} = c/2h, severely limiting its bandwidth. To avoid the short-circuit effect and increase the TCA bandwidth, an RFSS can be inserted between the TCA and the ground plane.

**Figure 4(a)** shows a TCA unit cell comprised of a bowtie antenna element, a square-split-ring RFSS, a wide-angle impedance matching (WAIM) layer, and a ground plane. The bowtie antenna element was printed on RO4003 circuit material from Rogers Corp., with thickness, t, of 0.254 mm and dielectric constant, ε_{r1}, of 3.38 placed a distance h above the ground plane. A cross WAIM layer with length, l_{f}, width, w_{f}, and gap, g_{f}, was printed on RO4003 circuit material with thickness, t_{2}, of 4 mm and dielectric constant, ε_{r2}, of 2.2.

**Figure 4(b)** shows the equivalent-circuit model of the unit cell. It can be seen that the square-split-ring RFSS located a distance h_{1} above the ground plane is represented by resistance R and reactance X_{r}, while the WAIM layer is represented by reactance X_{r}, the self-inductance of the bowtie antenna by inductance L, and the coupled capacitance between the adjacent elements by C. Thus, the impedance below the antenna element is:

Z_{GP} = η_{0}(1 + Γ)/(1 - Γ)

The input impedance is:

Z_{in} = jωL + 1/(jωC) + η_{0} || Z_{GP}

where the reflection coefficient is Γ.

The most important parameters for the novel TCA with RFSS is the film resistivity, R_{5}, and the bowtie antenna element gap, d_{e}, with d_{e} = d - l_{a}. To further analyze the effects of such parameters on impedance bandwidth, a TCA with different parameter values was simulated by commercial computer-aided-engineering (CAE) software from CST. Simulated results are shown in **Figs. 5(a) and (b)**.

**Figure 5(a)** shows that the VSWR increases in the middle frequency band as the film resistivity, R_{5}, increases, while the VSWR increases in the low- and high-frequency bands as film resistivity R_{5} decreases. As a consequence, it is very important that a proper value of R_{5} be selected for wideband operation. **Figure 5(b)** depicts that increasing the element gap leads to higher VSWR. After analyzing the effects of those parameters, optimized parameter values were achieved. The film resistivity is R = 50 Ω, and the other values are shown in the **table**.

Download this article in .PDF format This file type includes high resolution graphics and schematics when applicable. |