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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, t1, of 0.508 mm and dielectric constant, εr1}, of 3.38. The length of the outer square split ring, s1, is 9.8 mm and the length of the inner square split ring, s2, is 7.4 mm. The width of both rings, s3, is 0.6 mm, and the split width, s4, is 0.6 mm.

UWB Arrays Employ TCAs, Fig. 2

When the unit cell with period d = 10.1 mm and resistivity, R5 = 33 Ω is placed a distance h1 = 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    

UWB Arrays Employ TCAs, Fig. 3

As can be seen from the expression:

ZGP = jη0tan(2πh/λ)

the shunt impedance becomes ZGP = 0 when h = λ/2. So, the TCA is short circuited at frequency fmax = 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, εr1, of 3.38 placed a distance h above the ground plane. A cross WAIM layer with length, lf, width, wf, and gap, gf, was printed on RO4003 circuit material with thickness, t2, of 4 mm and dielectric constant, εr2, of 2.2.

UWB Arrays Employ TCAs, Fig. 4

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 h1 above the ground plane is represented by resistance R and reactance Xr, while the WAIM layer is represented by reactance Xr, 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:

ZGP = η0(1 + Γ)/(1 - Γ)

The input impedance is:

Zin = jωL + 1/(jωC) + η0 || ZGP

where the reflection coefficient is Γ.

The most important parameters for the novel TCA with RFSS is the film resistivity, R5, and the bowtie antenna element gap, de, with de = d - la. 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).

UWB Arrays Employ TCAs, Fig. 5

Figure 5(a) shows that the VSWR increases in the middle frequency band as the film resistivity, R5, increases, while the VSWR increases in the low- and high-frequency bands as film resistivity R5 decreases. As a consequence, it is very important that a proper value of R5 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.

UWB Arrays Employ TCAs, Table

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