The realized proposed antenna is shown in Fig. 3.6.
(a)
(b)
Figure 3.6: Photograph of the fabrication for the proposed antenna. (a) Top view. (b) Bottom view.
Zoom in photo for antenna:
Ground size: 200mm × 300mm Zoom in photo for antenna:
Ground size: 200mm × 300mm
In Fig. 3.7, the dotted line represents the measured result and solid line represents the simulated result of dipole antenna with 1-D EBG structure. The measured bandwidth ranges from 802MHz to 968MHz with a reflectivity less than −6 dB, and the simulated and measured results come to a great agreement.
Figure 3.7: Simulated and measured results of dipole antenna with 1D-EBG structure.
Fig. 3.8 (b) depicts the measured and simulated radiation pattern of dipole antenna with ground in yz plane. In general, the radiation pattern of dipole antenna in this plane should be omnidirectional. However, the short distance between antenna and ground plane cause the mirror current on the ground plane opposite to the current on the antenna, and thus eliminate each other. This leads to skew radiation pattern. Fig. 3.8(a) is the measured and simulated radiation patterns of dipole antenna with 1-D EBG structure in yz plane. Compared Fig. 3.8 (a) to Fig. 3.8 (b), the radiation pattern becomes more identical to the radiation pattern of theoretical dipole antenna after adding 1-D EBG structure. This is because the mirror current on the ground is no longer reversed-phase when 1-D EBG structure is added. Therefore, 1-D EBG structure is able to improve the skewed pattern of the original problem.
Frequency (GHz)
0.8 0.9 1
Measurement Simulation
0.95 0.85
25 15 10 5 0
Ret urn L oss (dB)
20
Figure 3.8: Radiation patterns in yz plane of (a) dipole antenna with 1-D EBG structure.
(b) Dipole antenna with ground..
In the design of traditional antennas which work in free space, the measured parameters are often the reflection coefficient S11 and the radiation patterns measured in the E or H planes around the antenna. The reflection coefficient describes how much of the available power is reflected at the antenna port, but it does not give any information about whether the remainder of the power is radiated or dissipated in the antenna. Therefore, the reflection coefficient alone cannot determine if the antenna is a good or a poor radiator. The efficiency is defined as being the total radiated power divided by the maximum available power when the antenna is impedance matched. The antenna efficiency includes the effects of mismatch, as well as absorption in the antenna and it’s near field environment such as 1-D EBG structure.
As 1-D EBG structure absorbing part of the energy radiated from the antenna, efficiency becomes the key factor in the antenna design. Usually stronger ground current leads to higher energy losses, and moreover, less separation between them introduces stronger coupling effects. It is the in-phase reflection feature given by 1-D
-35-30-25-20-15-10 -5 0 5
EBG structure that effectively decreases the energy absorbed by the ground, thus improving the antenna efficiency.
Fig. 3.9 indicates the antenna efficiency in the given conditions: dipole antenna with ground and dipole antenna with 1-D EBG structure. As illustrated in those results, the dipole antenna with 1-D EBG structure has peak antenna efficiency 81.7% at 860MHz, when the dipole antenna is placed close to the ground without 1-D EBG structure, the antenna efficiency peak drops to 50.4% at 920MHz. In the bandwidth of GSM (824 MHz -960 MHz), the antenna efficiency of the dipole antenna with 1-D EBG structure are all above 55.6%, which has at least 10% enhancement to the case without 1-D EBG structure.
Frequency (GHz)
0.80 0.85 0.90 0.95 1.00
Antenna Efficiency(%)
0 20 40 60 80 100
Dipole with 1-D EBG
Dipole without 1-D EBG with ground
Figure 3.9: Antenna efficiency of dipole antenna with ground and dipole antenna with 1-D EBG structure.
Chapter 4 A P ENTA - BAND G ROUND -P ROXIMITY
M ONOPOLE A NTENNA
In the previous chapter, a ground-proximity dipole antenna was proposed and the efficiency in GSM band has at least a 10% improvement by adding 1-D EBG structure.
So we have confirmed that the 1-D EBG structure can shorten the necessary distance between antenna and ground plane, and allows antennas to maintain the original radiation characteristics. However, the total size of the dipole antenna proposed in Chapter 3 (10 mm 153 mm) is still too large for commercial applications. Also the bandwidth only covers GSM band, which is not enough for WWAN antennas. Therefore, a more compact antenna with wider bandwidth is certainly demanded.
In this chapter, a penta-band ground-proximity monopole antenna is designed for laptop computer applications. Two 1-D EBG structures are applied to achieve low bandwidth while the structures can radiate in high band. In section 4.1, all design considerations and simulations of two new 1-D EBG structures will be illustrated.
Those two 1-D EBG structures will be applied to ground-proximity monopole antennas respectively in section 4.2. Because either of the antenna does not have enough bandwidth for GSM operation, we combine those two 1-D EBG structures together in section 4.3. Furthermore, we adjusted part of the 1-D EBG structures slightly for impedance matching and create resonance at high frequencies, thus achieve penta-band requirement. And in the end of the section, an additional size reduction design will be proposed. The final design of the penta-band monopole antenna with proximate ground plane is shown in Fig. 4.1, and the total size of proposed antenna is 10 mm 63 mm, which is more compact and applicable than the dipole antenna proposed in Chapter 3.
The measured results are presented in section 4.4.
Figure 4.1: A printed penta-band ground-proximity monopole antenna designed using 1-D EBG structures.
(a) Top view. (b) Bottom view.