Frequency (GHz)
Fig. 3.21. Simulated return loss and radiation efficiency as well as measured return loss for the proposed current driver with a printed interdigital capacitor. The simulated radiation efficiency includes the mismatch loss. All parameters for the current driver are the same as given in TABLE III.
The simulated return loss and radiation efficiency as well as the measured return loss for the proposed current driver with a printed interdigital capacitor are depicted in Fig.
3.21. The size of the ground plane is 50 × 100 mm2. As seen in the figure, the simulated return loss generally agrees with the measured one. The measured return loss shows a wider bandwidth than the simulation predicts. The 10-dB return-loss bandwidth is about 180 MHz with the center frequency occurs at 2.48 GHz, which covers the required band for WLAN 2.4 GHz. Moreover, the simulated radiation efficiency varies from 61% to 74%, which agrees well with the measured one, as will be shown later. Compare the simulated radiation efficiency in Fig. 3.15. It suggested that the main ohmic loss for the current driver come from the printed capacitor.
37
Fig. 3.22. Measured 2-D radiation patterns for the proposed current driver with a lumped capacitor at 2.45 GHz. The driver is fabricated on the ground plane of 50 mm by 100 mm.
(a) xy plane. (b) xz plane. (c) yz plane.
38
Frequency (GHz)
2.35 2.40 2.45 2.50 2.55
Radiation Efficiency (%)
0 20 40 60 80 100
Fig. 3.23. Measured radiation efficiency for the current driver with a printed capacitor.
The measured 2-D radiation patterns in the three principal planes at 2.45 GHz are shown in Fig. 3.22. It is predictable that the patterns are similar to those shown before for the current driver designed with a lumped capacitor. As shown in the figure, the total-power radiation patterns Etotal in the three principal planes are all nearly omin-directional.
The corresponding values of measured peak and average gains in three principal planes are listed in TABLE V. In xz plane, the measured average gain is about 0 dBi, and the measured peak gains in three principal planes are all above 0 dBi. Fig. 3.23 gives the measured antenna radiation efficiency, where the efficiency varies from 64% to 79% over the band for 2.4 GHz WLAN application. It is worth noting that the measured efficiency comes to a good agreement with the simulated results. This is because that the ohmic loss caused by the printed capacitor is considered in the simulation. The current driver is demonstrated to have good radiation characteristics with the lumped capacitor replaced with a printed one. The photograph of the fabricated current driver with a printed capacitor is shown in Fig. 3.24.
39
Current
Driver
(a)
Feed Line
Current Driver
Feed Line
(b)
Fig. 3.24. Photograph of the fabrication for the proposed current driver with a printed capacitor. (a) Top view. (b) Bottom view.
40
Detailed dimensions for the dual-band current driver
parameter Ls1
Ws
1 N1l
1w
1d g = g
eunit(mm) 4 4 10 2 0.5 2 0.2
parameter Ls2
Ws
2 N2l
2w
2s w
cunit(mm) 2.5 2.1 5 1.2 0.5 0.2 0.2
With the rapid development of wireless communications, the hand-held devices must be capable of multiband operation to tackle with different demands of wireless communication. However, as mentioned in Introduction, the overall size of the devices become smaller and smaller, and the space allocated for antenna design is very limited, especially for those in client terminal such as PCMCIA card or USB dongle. Thus, it is necessary to design a light weighted and low cost multiband antenna to integrate various applications. Given the compactness and design flexibilities, the current driver is a good candidate to meet the requirement. In this chapter, a dual-band current driver for WLAN 2.4/5.2 GHz applications is developed by combing two single-band current drivers. Both the simulation and measurement results are provided to evaluate the dual-band current driver.
41
4-1 G EOMETRY
To achieve WLAN 2.4/5.2 GHz applications, the developed dual-band current driver consists of two current drivers, one is designed for 2.4 GHz, and the other is for 5.2 GHz.
They are fabricated on an FR4 substrate with thickness of 0.4 mm and dielectric constant εr = 4.4. The substrate size is the standard size for PCMCIA applications, which is L × W
= 50 mm × 100 mm. The dual-band driver is fabricated on the center of the shorter ground edge L. To reduce cost and increase design flexibilities, the driver is designed by employing a printed interdigital capacitor as discussed in section 3.4. This makes the dual-band current driver easier to fabricate in the standard industrial process.
Fig. 4.1 shows the detailed configuration of the proposed dual-band current driver. The driver is printed on both sides of the substate. As mentioned above, the dual-band current driver is designed by combining two current drivers. As seen in the figure, the larger current driver in the left side is utilized for WLAN 2.4 GHz applications, and the smaller one in the right side is for WLAN 5.2 GHz applications. Each driver comprises a U-shaped metal strip on the top layer and the slot with a printed internal interdigital capacitor on the bottom layer. The two current drivers are connected with a thin metal strip on the top layer with width s and length d. The U-shaped strip is placed right above the slot with the strip edge aligned with the slot edge. The strip width and the arm length of the U-shaped metal strip are designated as w and l, respectively. The interdigital capacitor is defined by the parameter shown below, the figure width wc, the gap between figures g, the gap at the end of the fingers ge, and figure number N. The finger length varies with the slot length Ls. Also, the slot width Ws can be larger than the interdigital capacitor requires. Just ensure that the slot width must be large enough to accommodate N figures kept at the gap distance g. The current driver is fed by a 50 Ω transmission line, which can be a coaxial line or a microstrip line. The feed line is placed in between the current drivers with the positive terminal connected to one arm of the U-shaped metal strip for the 2.4 GHz current driver and the negative terminal connected to the ground plane near the slot edge of the 2.4 GHz current driver. The antenna occupies the size of only 8.5 mm × 4 mm, which is still very compact. The detailed dimension for the antenna is listed in TABLE VI. The design parameters for the 2.4 GHz and the 5.2 GHz current