Configuration and Design

Figure 4.1 shows the three–dimensional (3-D) structure and geometry of the proposed antenna. The antenna is a loop structure. The input signal fed from the feed strip first enters the patch then passes through the shorting strip to the ground plane. The antenna is soldered on the PCB using a single folded metal plate formed by several bending manufacture procedure. The 12 edges polygonal radiation patch is a quasi-ellipse shape with the major axis and the minor axis are 25mm and 17 mm, respectively The feed point is denoted as point A as shown in Figure 4.1. A 50 Ω

microstrip line of 1.5 mm width is connected to the antenna as the feed line. The substrate chosen is FR4 substrate, whose dielectric constant is 4.4, loss tangent 0.02, and thickness 0.8 mm. The polygon patch size of the antenna wa × A and the ground _a size is W × L. The shorting strip is offset located at the corner of the patch opposite to the feed strip of the antenna and its width denotes as ws. There are two L-shaped slits embedded on the ground plane, the size of the right slit is w₁ × A₁ and the left slit is w₂

× A₂. Also two cut portion of the ground plane are w3 × A . The gap width of the ₃ L-shaped slit is denoted as g. The feed depth and the position are d and s, respectively.

x

z

Figure 4.1 Geometry of the low-profile UWB antenna. (a) 3-D structure, (b) side view of x-z plane, (c) side view of y-z plane, and (d) top view of x-y plane.

Figure 4.2 shows the simulated return loss of the proposed antenna with and without the embedded L-shaped slits. The patch size of the antenna wa × A = 25 mm × 17 mm _a and the ground size is W × L = 34 mm × 75 mm. The width and height of the shorting strip are ws = 5.85 mm and h = 5 mm, respectively, while those of the feed strip are 1.5 and 5 mm, respectively. The depth and the position of the feed line are d = 2.5 mm and s

left side one is w2 × A₂ = 9.5 mm × 4 mm. The two cut portions of the ground plane are w3 × A = 4.5 mm × 17 mm. The gap width g of the L-shaped slit is 1mm. As show ₃ in Figure 4.2, there is poor impedance bandwidth and only has one resonance (dashed line). The total impedance bandwidth, determined by a 10-dB return loss, is about 15.75

% from 3.45 to 4.04 GHz. But when we embedded two L-shaped on the ground plane (solid line), the total impedance bandwidth is about 44.53 % from 2.95 to 4.64 GHz.

There are three resonances, with resonant frequencies at 3.19, 3.71, and 4.41, sustaining the full band.

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Frequency (GHz) 40

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Return Loss (dB)

Proposed antenna Antenna w/o slits

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Proposed antenna Antenna w/o slits

Figure 4.2 Simulated return loss for the proposed antenna shown in Figure 4.1 with and without slits. wa

×A_a = 25 mm × 17 mm, W × L = 34 mm × 75 mm, w1 ×A₁= 9 mm × 3 mm, w2 ×A₂= 9.5 mm × 4 mm, w3 × A₃ = 4.5 mm × 17 mm, ws = 5.85 mm, h = 5mm, d = 2.5 mm, s = 6.75 mm, and g =1 mm.

For further comprehension on the embedded L-shaped slits mechanisms of the proposed antenna. Figure 4.3 compares the simulated return loss results of the antenna with and without the embedded L-shaped slit on the ground plane. The embedded arrangement have two position i.e., right or left side on the ground plane. As show in Figure 4.3 the dotted line means the antenna without embedded slits. It can be seen from the figure that, there is only one resonance across full band. But when a slit embedded

on the ground plane in different arrangement, both of them result in an additional resonance at different frequency. The simulated return loss of the embedded slit at right side (solid line) and left side (dashed line) as show in Figure 4.3 for comparison. When the slit is adding at the right side on the ground plane, the additional higher resonance frequency (denote as f3) is appeared. But when the slit is embedded at the left side on the ground plane, the lower resonance frequency (denote as f1) is obtained. Besides, according to the results in the Figure 4.3, while the slit is embedded on different position, the second resonance frequency (denote as f2) maintain its resonant surrounding 3.78 to 3.98 GHz.

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w/o slits left side right side f₁

f₂ f₃

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Frequency (GHz) 40

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w/o slits w/o slits left side left side right side right side f₁

f₂ f₃

Figure 4.3 Comparison of the simulated return loss for the proposed antenna with different arrangement of the embedded slits. Other geometric parameters are the same as given in Figure 4.2.

Figure 4.4 (a) shows the effect on the resonance frequencies with various slit length A2. The influence on the second resonant frequencies is negligibly changed comparing the first one. It can be seen that the first resonance frequency decrease as A₂ increase.

When A₂ varies from 9 to 15 mm, the first resonance frequency moves from 3.31 to 2.92 GHz. And the effect on the resonance frequencies with various slit length A₁ is

shown in Figure 4.4 (b). Notably, the third resonant frequency decreases quite obviously

Figure 4.4 The effect on the resonance frequencies with various slit length of (a)A₂and (b)A₁.

Figure 4.5 (a)-(c) shows the surface current distribution at 3.19, 3.71, and 4.41 GHz to illustrate the wideband mechanism, respectively. As seen in Figure 4.5 (a) at lower

frequency region, the primary current mainly concentrated and flow through the left side of the patch then surrounding the left side slit. Hence, the increase of the length would alter the current distribution condition. Notably, the first resonant frequency decreases quite obviously as the increase of the length A₂ (shows in Figure 4.4(a)).

Moreover, the current direction of the feed strip and shorting strip are the same, hence, the proposed has stronger vertical polarization field as compared to the conventional printed antenna. And, the antenna radiation pattern is omnidirectional in the xy-plane (see Figure 4.17 (a)).

feed (a)

feed (b)

feed (c)

Figure 4.5 Simulated current distributions of the proposed antenna at (a) 3.19, (b) 3.71, and (c) 4.41GHz.

As for the third resonant frequency (4.41 GHz), as shown in Figure 4.5 (c), the primary antenna current mainly distributed on the right side of the patch and flow through the right side slit which arranged on the ground plane. Hence, the increase of the length A₁ would alter the third resonance frequency. It is verified with Figure 4.4 (b), the third resonant frequency decreases quite obviously as the increase of the length A1. Similarly, both feed and shorting strip have the same current direction. Finally, for the second resonant frequency of 3.71 GHz, the current on the ground plane through pass the space between two slits immediately, thus, however the slit length varied, the second resonance frequency has slightly changed. Hence, by comparing Figure 4.4 and Figure 4.5, it can be summarized that, the first and third resonance frequency is controlled by the slit embedded on the left side and right side of the ground plane, respectively.

The effect of the position of the feed strip on antenna performance is investigated.

All of the antenna dimensions are fixed, except for the feed strip position s relative to the antenna patch side wall. Figure 4.6 shows the simulated return loss for the proposed antenna of various feed positions s. It is clear that, the change of the feed position influences the impedance matching of the antenna, mainly at the second and third resonance frequencies. When the feed strip moves along the x direction (i.e., aparting from the right side of the ground plane) the input impedance matching becomes better.

The input impedance can be improved when properly choosing the feed position s especially at high band. The widest bandwidth can be obtained when the s = 6.75 mm.

Figure 4.7 depicts the results of varying the feed depth d (0, 2.5, 5, 7.5 mm). When

d = 0 mm, indicating that the feed strip is located on the patch edge. It can be seen that,

the variation of the feed depth mainly influences the input impedance of the antenna.

When d increases, the input matching becomes better, expect the case of d > 7.5 mm. In the case of d = 7.5 mm as shown in Figure 4.7, it is obvious that, the antenna

performance becomes dual-band operating. The reasons is that, when a deeper feed strip insert to the patch, the current distribution on the patch can provide two current flow paths. One is mainly toward the right side of the patch. The other one enters the left side of the patch and then result in a dual-band operation.

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Figure 4.6 Simulated return loss for the proposed antenna of various feed position s. Other geometric parameters are the same as given in Figure 4.2.

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Figure 4.7 Simulated return loss for the proposed antenna of various feed depth d. Other geometric

Figure 4.8 shows the effect of the position of the short strip on antenna performance.

As show in Figure 4.8, there are four position of the shorting strip arrangement from p1

to p4. At the position p1, there are two resonance frequencies at 3.25 and 4.4 GHz. When the shorting strip moves to the position p3, two resonance frequencies occur at 3.05 and 3.89 GHz. But there is only one resonance frequency at 3.13 GHz while the shorting strip moves to the position p4. It indicates that by properly choosing the position, the antenna structure can provide appropriate current distribution across full band. Thus the three resonance modes can be effectively excited and better input impedance matching can be achieved over the whole frequency band.

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Figure 4.8 Simulated return loss for the proposed antenna of various short position. Other geometric parameters are the same as given in Figure 4.2.

Figure 4.9 shows the results of varying the shorting strip width ws from 2.85 to 5.85 mm step 1 mm. As the simulated result in Figure 4.9, the change of the width mainly affects the impedance matching of the antenna. The decreasing of the second and third resonant frequency, as the width reduced, which is due to the increase of the equivalent

current path, causing the resonance frequency moving toward the low frequency. As refer to the current distributions shown in Figure 4.5, the shorting strip has strong current magnitude at the second frequency than the other two frequencies. Thus, changing the shorting strip width influences the antenna performance obviously, especially at the second resonance frequency. Properly selecting the shorting width can lead to a largest high band edge and thus a widest impedance bandwidth.

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w_s= 2.85 mm w_s= 3.85 mm

w_s= 5.85 mm w_s= 4.85 mm

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Frequency (GHz) 40

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w_s= 2.85 mm w_s= 3.85 mm

w_s= 5.85 mm w_s= 4.85 mm

Figure 4.9 Simulated return loss for the proposed antenna of various shorting strip width ws. Other geometric parameters are the same as given in Figure 4.2.

Figure 4.10 shows that the results of varying the antenna height h from 4 to 7 mm step 1 mm. As the simulated result in Figure 4.10, the change of the height mainly affects the impedance matching of the antenna, especially the third resonant frequency.

It has been observed from the simulation result that the input reactance of the third resonant frequency with h = 4 mm is capacitive. Increasing the antenna height would lengthen the feed strip, thus the input inductance is increased. As a result, a larger antenna height h is associated with better input matching.

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40

h = 4 mm h = 5 mm h = 7 mm h = 6 mm

Figure 4.10 Simulated return loss for the proposed antenna of various antenna height h. Other geometric parameters are the same as given in Figure 4.2.