CHAPTER 4 The Proposed Broadband Antenna
4.1 Design Concept of the Proposed Antenna…
Figure 4.1 The purpose of each design step.
The design purpose of each major step is shown in Figure 4.1. In step one, the main work is to determine the rough dimension of the antenna according to the required center frequency. In step two, the broadband characteristic was created. In step three, the work is to match the desired frequency band by adjusting the geometrical parameters of the antenna. In step four, coupling effects between the ground plane and the antenna was discussed. In all steps, FR4 with thickness of 0.8 mm is used for computing by HFSS. Width of the feed line is 1.5 mm.
Chapter 4 The Proposed Broadband Antenna
Step One : Determine the rough dimension of the antenna according to the required center frequency
The resonant (center) frequency of the simple planar rectangular antenna shown in Figure 4.2 is given by [16]
respectively, as shown in Figure.4.2. The wanted center frequency will be achieved by properly tuning Lr and Wr. The computed results using Eq. (4.1) and HFSS are compared in Table 4.1 (Up to 4GHz) for different combinations of Lr and Wr. Here, W = 50 mm, L = 45 mm and G = 1.6 mm are fixed for simplification. The comparison shows that Eq. (4.1) at least determines one of resonant frequencies with reasonable accuracy. Therefore, Eq. (4.1) is proposed as a rough model for determining the resonant frequency of the simple patch antenna. Sizes of Lr and Wr will affect the bandwidth of the planar rectangular antenna. Figure 4.3 (a), (b) and (c) show its return loss versus frequency for Wr= 25mm, 35 mm and 45 mm, respectively. In each figure, it is found that the bandwidth is increased withLr. It is also found that when Wr=25 mm, more than one resonant frequency may be created. In Table 4.1, the one of specific size of Lr and Wr, which are underlined, represents its resonant frequencies in the range of 1.7 - 1.8 GHz to fulfill the desired bands.
Chapter 4 The Proposed Broadband Antenna
Figure 4.2 The geometry of the simple planar rectangular antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Chapter 4 The Proposed Broadband Antenna
Figure 4.3 The computed return loss versus frequency of the simple planar rectangular antennas (a) Wr =25 mm (b) Wr =35 mm (c) Wr =45 mm.
Chapter 4 The Proposed Broadband Antenna
Table 4.1 Resonant frequency comparison using Eq. (4.1) and HFSS.
Computed resonant frequency
Chapter 4 The Proposed Broadband Antenna
Step Two : Broaden the bandwidth
In step two, the chosen antennas (the underlined one) shown in Table 4.1, are shaped into the one as shown in Figure 4.4, which is called the planar binomial antenna [17]. The binomial antenna shows ultra-wideband characteristics. The formula of the binomial curve is shown below [17].
4 1 1 simplification. The wanted bandwidth will be achieved by properly tuning Wb1 and
1
Lb . From the computed return loss versus frequency as shown in Figure 4.5, it is found that the bandwidth is increased with Wb1. From the computed covering frequency band using HFSS listed in Table 4.2 (Up to 4GHz) for different sizes of Wb1 and Lb1, it is found that the operational frequency range is increased with Wb1. The low bound of the operational frequency range becomes smaller when Lb1 increases. By comparing between Figure 4.3 and Figure 4.5, it is found that a binomial curve structure provides with a wider bandwidth than a simple rectangular one.
Chapter 4 The Proposed Broadband Antenna
Figure 4.4 The geometry of the planar binomial antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Figure 4.5 The computed return loss versus frequency of the planar binomial antennas with Lb1=25 mm
.
Chapter 4 The Proposed Broadband Antenna
Table 4.2 Computed bandwidths of the planar binomial antennas.
Chapter 4 The Proposed Broadband Antenna
Step Three : Shaping the antenna
In step three, a planar binomial antenna is shaped into the one as shown in Figure 4.6, which is called the planar hybrid-binomial antenna. Current mainly flow in edges of an antenna. Frequency response is changed severely by modifying a shape of edges.
Therefore, some edges of the selected planar binomial antenna are selected to cut off.
Another binomial formula is selected to cut the end edge of the selected planar binomial antenna in order to sustain the broadband characteristic. The formula is shown below [17] respectively, as shown in Figure.4.6. W = 50 mm and L = 45 mm are fixed in this step.
G = 1.6 mm and Wb2 =25 mm are fixed for simplification. The wanted bandwidth will be achieved by properly tuning L . From the computed return loss versus frequency b2 shown in Figure 4.7, it is found that the bandwidth is decreased with larger L . As b2
2
L is varied from 5 to 20 mm, the lower bound of the covering frequency band will b
shift to a higher value. The components of high frequencies will also cut off. Although a stop frequency band is created in high frequencies, the lower bound of the covering frequency band shift to a higher value. This is not my desired result. So we make a second adjustment.
Chapter 4 The Proposed Broadband Antenna
Figure 4.6 The geometry of the planar hybrid-binomial antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Figure 4.7 The computed return loss versus frequency of the first adjustment,
2 25
Chapter 4 The Proposed Broadband Antenna
In the second adjustment, we would like to cut off frequency response higher than 2.9 GHz by cutting some parts of a planar binomial antenna and also compensate lower frequency response of the one. The geometry of the modified antenna is shown in Figure 4.8 which is called the planar eagle-shaped antenna. Here, W and c L are the c width and length of the cutting area, respectively, as shown in Figure.4.8. G = 1.6 mm are fixed for simplification. The wanted bandwidth will be achieved by properly tuning W and c L . From the computed return loss versus frequency shown in Figure 4.9, it is c found that the bandwidth is decreased with larger L . The resonant frequency will also c shift to a lower value with larger W . From Figure 4.9, we find that there are two c purposes by tuning W and c L . One is to compensate the components of lower c frequencies; another is to cut off the components of higher frequencies. The computed current distribution of the unmodified antenna in f = 1.575 GHz is shown in Figure 4.10.
From Figure4.10, it is found that the current mainly flows along edges of the antenna and the strength of the current magnitude is stronger near the feed point. The computed current distribution of the modified antenna in f = 1575 MHz is shown in Figure 4.11.
From Figure 4.11, it is found that the current flows along the cutting area. The miniaturization is achieved due to increasing resonant length formed by the cutting area.
Chapter 4 The Proposed Broadband Antenna
Figure 4.8 The geometry of the planar eagle-shaped antenna.
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Chapter 4 The Proposed Broadband Antenna
Figure 4.9 The computed return loss versus frequency of the second adjustment (a) Wc =2 mm(b) Wc =6 mm.
Figure 4.10 The computed current distribution of the unmodified antenna;
Chapter 4 The Proposed Broadband Antenna
Figure 4.11 The computed current distribution of the modified antenna;
f=1.575GHz, phase=320 degree.
G is a gap between the ground plane and the antenna as shown in Figure.4.9. In the following discussion, the influence on frequency response will be studied by properly tuning G.Wc =4mm and Lc =6 mm are fixed in this discussion. From the computed return loss versus frequency shown in Figure 4.12, it is found that the resonant frequency will shift to a lower value with larger G. It is simply because that there is a coupling effect between the ground plane and the antenna. The coupling effect is created by two currents flowing on the ground plane and the antenna.
Chapter 4 The Proposed Broadband Antenna
Figure 4.12 The computed return loss versus frequency of the planar eagle-shaped antenna by varying G.