**4. Pattern reconfigurable antenna**

Pattern reconfigurable antenna is another type of reconfigurable antenna. Such antenna provides dynamic radiation coverage and mitigates multi-path fading. In this section, we introduce a design of pattern reconfigurable antenna with compact feeding structure. The benefit by using pattern reconfigurable antennas in the MIMO system is also proved by experiment of channel capacity measurement.

The configuration of the pattern reconfigurable antenna is shown in Fig. 21 (a). It is composed of an elliptical topped monopole, two Vivaldi notched slots and a typical CPW feed with 2 PIN diodes. The antenna is printed on the both sides of a 50 x 50 mm2 Teflon substrate, with εr=2.65, tanδ=0.001 and thickness is 1.5 mm. The CPW is connected to the microstrip at the back side through several via holes. A 0.2 mm wide slit is cut from the ground on the front side for DC isolation. Three curves are used to define the shape of antenna, fitted to the coordinates in Fig. 21 (a). Curve 1 is defined by equation (2) and curve 2 is defined by equation (3). Curve 3 and curve 2 are symmetrical along X axis.

Fig. 21. Geometry of the proposed loop antenna. (L1=1.74 mm, L2=28.52 mm, L3=10.74 mm, L4=25 mm; L5=8 mm, L6=16.8 mm, L7=10mm, L8=10mm, W=50 mm, Lp=2 mm, W1=4 mm, S1=0.3 mm. Reprinted from (Li et al, 2010c) by the permission of IEEE).

$$\left[\frac{\chi}{W/2}\right]^2 + \left[\frac{y - (L\_4 - \alpha \* W/2)}{\alpha \* W/2}\right]^2 = 1\tag{2}$$

where 4 4 *L W yL* 2 , and 0.4 .

$$y = \mathbb{C}\_1 e^{\mathbf{c} \cdot \mathbf{x}} + \mathbb{C}\_2 \tag{3}$$

where C1=14, C2=0.26, c=0.16.

#### **4.1 CPW-slot transition design**

Different radiation patterns are provided by different work states of the same antenna. In order to achieve different work states, a switchable CPW-to-slotline transition with two PIN diodes is proposed and sown in Fig. 21 (b). Three feeding modes are achieved in this structure by varying the states of PIN diodes. When both PIN diodes are OFF, the elliptical topped monopole is fed through a typical CPW and a nearly omni-directional radiation pattern is achieved in XZ plane. When PIN 1 is OFF and PIN 2 is ON, the right slotline is shorted. The left Vivaldi notched slot is fed through the left slotline (LS) of the CPW, and a unidirectional radiation pattern is formed along the –X axis. In the same way, when PIN 1 is ON and PIN 2 is OFF, a unidirectional beam along the +X axis is obtained in the right Vivaldi notched slot through the right slot (RS). The proposed CPW-to-slotline transition is able to achieve good switching from the CPW to slotline with any other extra structures. Compared with this design, the CPW-to-slotline transition reported in (Wu et al, 2008; Kim et al, 2007; Ma et al, 1999) all required extra structures for mode convergence, including λ/2 phase shifter (Ma et al, 1999)and λ/4 matching structures (Wu et al, 2008; Kim et al, 2007), which occupy considerable space in the feed network. Such structures are not suitable for the space-limited systems. The proposed CPW-to-slotline transition here is designed to reduce the overall dimensions of the antenna.

Fig. 21. Geometry of the proposed loop antenna. (L1=1.74 mm, L2=28.52 mm, L3=10.74 mm, L4=25 mm; L5=8 mm, L6=16.8 mm, L7=10mm, L8=10mm, W=50 mm, Lp=2 mm, W1=4 mm,

> 2 2 <sup>4</sup> ( 2) ( )[ ] 1 2 2 *x yL W W W*

1 2

Different radiation patterns are provided by different work states of the same antenna. In order to achieve different work states, a switchable CPW-to-slotline transition with two PIN diodes is proposed and sown in Fig. 21 (b). Three feeding modes are achieved in this structure by varying the states of PIN diodes. When both PIN diodes are OFF, the elliptical topped monopole is fed through a typical CPW and a nearly omni-directional radiation pattern is achieved in XZ plane. When PIN 1 is OFF and PIN 2 is ON, the right slotline is shorted. The left Vivaldi notched slot is fed through the left slotline (LS) of the CPW, and a unidirectional radiation pattern is formed along the –X axis. In the same way, when PIN 1 is ON and PIN 2 is OFF, a unidirectional beam along the +X axis is obtained in the right Vivaldi notched slot through the right slot (RS). The proposed CPW-to-slotline transition is able to achieve good switching from the CPW to slotline with any other extra structures. Compared with this design, the CPW-to-slotline transition reported in (Wu et al, 2008; Kim et al, 2007; Ma et al, 1999) all required extra structures for mode convergence, including λ/2 phase shifter (Ma et al, 1999)and λ/4 matching structures (Wu et al, 2008; Kim et al, 2007), which occupy considerable space in the feed network. Such structures are not suitable for the space-limited systems. The proposed CPW-to-slotline transition here is designed to

(2)

*c x y Ce C* (3)

S1=0.3 mm. Reprinted from (Li et al, 2010c) by the permission of IEEE).

.

2 , and 0.4

where 4 4 *L W yL* 

where C1=14, C2=0.26, c=0.16.

**4.1 CPW-slot transition design** 

reduce the overall dimensions of the antenna.

In order to explain work principle of the feed transition, the equivalent transmission line model is utilized, illustrated in Fig.22 and 23. The PIN diode is expressed as perfect conductor for 'ON' state and open circuit for the 'OFF' state. Fig. 22 (a) shows the normal CPW structure. By tuning the L5, the radiation resistance Rmonopole of monopole is matched to 50Ω at the feed port. When the right slot is shorted by PIN diode, the antenna is fed through the RS mode. The diagram and equivalent transmission line model are depicted in Fig. 22 (b). The right slotline is used to feed the Vivaldi notched slot, and the shorted left slotline works as a matching branch. The shorted branch which is less than a quarter of wavelength serves as a shunt inductance and its value is determined by its length L5. The position of the PIN diode is not fixed, and it is another freedom for impedance matching of RS feed. As shown in Fig. 23, the value of shunt inductance is jZslottan[βslot(L5-Lp)] and used to match the radiation resistance Rvivaldi. The advantage of this switchable feeding structure is that no extra structure is used in the CPW and slotline transition.

Fig. 22. Feed diagram and equivalent transmission line model. (a) CPW feed; (b) RS feed.

Fig. 23. Matching strategy of RS feed. (a) Feed diagram; (b) Transmission line model.

Fig. 24. Simulated and measured reflection coefficient of the reconfigurable antenna.

The selected PIN diode is Agilent HPND-4038 beam lead PIN diode, with acceptable performance in a wide 1-10 GHz bandwidth. The bias circuit is similar as the PIN diodes used in the last design in Fig. 15. The values of each component are determined by the working current of the PIN diode. The efficiency decreases approximately 0.3 dB by using this PIN diode. All the measurements were taken using an Agilent E5071B VNA. The simulated and measured reflection coefficients of CPW feed, LS and RS feeds are shown in Fig. 24. The measured -10dB bandwidths are 2.02-6.49 GHz, 3.47-8.03 GHz and 3.53-8.05 GHz for CPW feed, LS feed and RS feed, respectively. The overlap band from 3.53 GHz to 6.49 GHz is treated as the operation frequency for the reconfigurable patterns. The measured normalized radiation pattern in XZ and XY planes for CPW, LS and RS feed at 4, 5, 6 GHz are shown in Fig. 25. For the CPW feed, a nearly omni-directional radiation pattern appears in XZ plane and a doughnut shape in XY plane. For the LS or RS feed, a unidirectional beam appears along –X or +X axis, with acceptable front-to-back ratio better than 9.5dB. For the CPW feed, an average gain in the desired frequency range is 2.92 dBi. For the LS and RS feed, the average gains in the 4-6 GHz band are 4.29 dBi and 4.32 dBi. The improved gain is mainly contributed to the directivity of the slotline feed mode, and the diversity gain is achieved by switching the patterns.

Fig. 24. Simulated and measured reflection coefficient of the reconfigurable antenna.

The selected PIN diode is Agilent HPND-4038 beam lead PIN diode, with acceptable performance in a wide 1-10 GHz bandwidth. The bias circuit is similar as the PIN diodes used in the last design in Fig. 15. The values of each component are determined by the working current of the PIN diode. The efficiency decreases approximately 0.3 dB by using this PIN diode. All the measurements were taken using an Agilent E5071B VNA. The simulated and measured reflection coefficients of CPW feed, LS and RS feeds are shown in Fig. 24. The measured -10dB bandwidths are 2.02-6.49 GHz, 3.47-8.03 GHz and 3.53-8.05 GHz for CPW feed, LS feed and RS feed, respectively. The overlap band from 3.53 GHz to 6.49 GHz is treated as the operation frequency for the reconfigurable patterns. The measured normalized radiation pattern in XZ and XY planes for CPW, LS and RS feed at 4, 5, 6 GHz are shown in Fig. 25. For the CPW feed, a nearly omni-directional radiation pattern appears in XZ plane and a doughnut shape in XY plane. For the LS or RS feed, a unidirectional beam appears along –X or +X axis, with acceptable front-to-back ratio better than 9.5dB. For the CPW feed, an average gain in the desired frequency range is 2.92 dBi. For the LS and RS feed, the average gains in the 4-6 GHz band are 4.29 dBi and 4.32 dBi. The improved gain is mainly contributed to the directivity of the slotline feed mode, and the

diversity gain is achieved by switching the patterns.

Fig. 25. Radiation patterns of the reconfigurable antenna.
