**5.2 Receiving antennas**

40 Wireless Communications and Networks – Recent Advances

(elliptical polarization). It possesses all the characteristics of planar structure along with

The proposed outer-wall loop antenna (Yun et al., 2010.) makes maximal use of the capsule's outer surface, enabling the antenna to be larger than inner antennas. As shown in Figure 10(a), the antenna is part of the outer wall of the capsule, thus decreasing volume and increasing performance, and uses a meandered line for resonance in an electrically small area. The capsule shell with the relative permittivity of 3.15 has the outer and the inner radius of the capsule as 5.5mm and 5mm, respectively. Its length is 24 mm. The height of the meander line and gap between meander patterns are set to 7mm and 2.8mm, respectively. The opposite side of the loop line is meandered in the same way. Although capsule size is reduced, the radius of sphere enclosing the entire structure of the antenna is increased.

(a) (b)

(c)

body phantom fluid, antenna efficiency is measured to 43.7% (3.6 dB).

and measured return losses; (c) azimuth pattern at 500MHz.

Fig. 10. Outer-wall loop antenna (Yun et al., 2010.): (a) the geometric structure; (b) simulated

Figure 10(b) shows that the proposed antenna has an ultra wide bandwidth of 260 MHz (from 370MHz to 630 MHz) for VSWR<2 and an omnidirectional radiation pattern at azimuth plane (as shown in Figure 10(c)). Using identical antenna pairs in the equivalent

polarization diversity. **Outer-wall loop antenna** 

The receiving antennas are operating outside of human body, which is no longer limited by its size. Therefore, the design of receiving antennas is less challenge than the design of transmitting antennas. In this subsection, several types of receiving antenna are selected as examples.

### **Narrow bandwidth antenna for receiver**

A narrow bandwidth receiving antenna is designed using microstrip loop structure (Shirvante et al. 2010). The antenna is patterned using a milling machine on a Duroid 5880 substrate with a relative permittivity *εr* of 2.2 and a thickness of 500μm as shown in Figure 11(a). The overall length of the wire is approximately a quarter wavelengths: **λ**air /4 = 187mm at 402MHz for air medium.

Fig. 11. Rectangular microstrip loop antenna (Shirvante et al. 2010): (a) the geometric structure; (b) simulated and measured return losses; (c) azimuth pattern at 403MHz.

Figure 11(b) shows the simulated and measured return losses of the proposed antenna. The return loss shows a deep null of -30dB at 403MHz. The directional rational pattern as shown in Figure 11(c) provides the possibility to aim the receiver to human body area, where the transmitter sends signals from. Therefore, for narrow bandwidth applications, such as the ASK or FSK modulation, the line loop antenna is a good choice.

#### **Miniaturized microstrip planar antenna**

To accommodate the antenna in a small communication unit, a meander line style structure is used (Babar et el., 2009). The antenna's radiating part is shorted with the ground plane, to further decrease the size of the antenna structure. The reduction of the size of the antenna by shortening also reduces the gain of the antenna, as decreasing the size of the antenna more than its wavelength affects the efficiency of the antenna.

The antenna was fabricated on a double sided copper FR4 – printed circuit board, with 1.6mm thickness as shown in Figure 12(a). The excitation is given through an SMA connector from the opposite direction of the PCB to the antenna structure. The total size of the antenna structure is 20mm x 37mm. There is no ground plane present on the opposite side of the PCB, where the antenna structure is present, which helps in getting an omnidirectional radiation pattern.

Fig. 12. Microstrip planar antenna (Babar et al. 2009): (a) the geometric structure; (b) simulated and measured return losses; (c) radiation patterns at 433MHz.

Figure 12(b) presents that the operating frequency of the antenna is 433 MHz with the bandwidth of 4MHz. Figure 12(c) shows the radiation pattern of the antenna's E and Hplane. The achieved max gain from the antenna was around -6.1 dBi.

#### **Receiver antenna with buffer layer**

42 Wireless Communications and Networks – Recent Advances

Figure 11(b) shows the simulated and measured return losses of the proposed antenna. The return loss shows a deep null of -30dB at 403MHz. The directional rational pattern as shown in Figure 11(c) provides the possibility to aim the receiver to human body area, where the transmitter sends signals from. Therefore, for narrow bandwidth applications, such as the

To accommodate the antenna in a small communication unit, a meander line style structure is used (Babar et el., 2009). The antenna's radiating part is shorted with the ground plane, to further decrease the size of the antenna structure. The reduction of the size of the antenna by shortening also reduces the gain of the antenna, as decreasing the size of the antenna more

The antenna was fabricated on a double sided copper FR4 – printed circuit board, with 1.6mm thickness as shown in Figure 12(a). The excitation is given through an SMA connector from the opposite direction of the PCB to the antenna structure. The total size of the antenna structure is 20mm x 37mm. There is no ground plane present on the opposite side of the PCB, where the antenna structure is present, which helps in getting an omni-

(a) (b)

(c)

Fig. 12. Microstrip planar antenna (Babar et al. 2009): (a) the geometric structure; (b)

simulated and measured return losses; (c) radiation patterns at 433MHz.

ASK or FSK modulation, the line loop antenna is a good choice.

than its wavelength affects the efficiency of the antenna.

**Miniaturized microstrip planar antenna** 

directional radiation pattern.

The dual pentagon loop antenna having circularly polarization is proposed (Park, S. et al., 2008). The configuration of the proposed dual pentagon loop antenna is shown in Figure 13(a). The proposed antenna and the feeding structure were etched on the front and the back of a substrate (Figure 13(b)). And a-a' are b-b' are shorted as follows. The proposed antenna was designed a dual loop type to enhanced H-field since the current direction of each of loops is different. And there is a gap on each of loops to make a CP wave (Morishita & Hirasawa 1994; Sumi et al., 2004 as cited in Park, S. et al., 2008). The strip widths of the primary loop and of the CPW are 0.80 mm; the used substrate is R/flex 3850; L1 = 12.93 mm, L2 = 10.97 mm, L3 = 10.21 mm, G = 0.49 mm, S1 = 26.01 mm, S2 = 1.65 mm, W1 = 5.80 mm, W2 = 1.70 mm. The CPW feeding line on the back of substrate is used to efficiently excite balanced signal power which makes to have a broadband.

Fig. 13. Receiver antenna with buffer layer (Park, S. et al., 2008): (a) the pentagon dual loop antenna; (b) feeding structure; (c) simulated and measured return losses.

Figure 13(c) presents that the bandwidth of the receiver antenna is from 400 MHz to 600 MHz for VSWR≤2. As a wave in air meets a medium of which relative permittivity is very high over air, much reflection is inevitably generated. So we designed the buffer layer having *εr* between air and human body for reducing the reflection, artificially. The buffer layer which is added a little bit loss is attached on the back of the proposed antenna for reducing a size of antenna and back lobe power.
