**5.1 Transmitting antennas**

32 Wireless Communications and Networks – Recent Advances

actually proportional to the frequency if the loss tangent is fixed; where the attenuation

The dominant feature of radio wave propagation in media is that the attenuation increases

*γ = jωμ(σ + jωε)*

× E *<sup>j</sup> H = ωμ*

It can find out that the power of E plane and H plane reduce with high dielectric constant and conductivity. The total power is consumed easily in human body. The efficiency of

in a high dielectric material, the electrical length of the antenna is elongated. Compare dipole antenna in the air and in the body material, they have same physical length but electrical lengths are not same. Because of the high permittivity, the antenna in the body material has longer electrical length. The time-averaged power density of an EM wave is

which leads to high power density in human body. The intrinsic impedance of the material and is determined by ratio of the electric field to the magnetic field (Huang & Boyle, 2008).

Based on wave equation *2 2 E - γ E=0* , A and B in the wave propagating trigonometric form *E = xAcos(ωt - βz)+ yBsin(ωt - βz)* can be determined. With the relationship of A and B, it can

The multi-layered human body characteristic can be simplified as one equivalent layer with dielectric constant of 56 and the conductivity of 0.8 (Kim & Rahmat-Samii, 2004). So, with

*α = ω με 1+ -1 2 2 2 ε ω* 

*1/2 <sup>2</sup> <sup>1</sup> <sup>σ</sup>*

. (2)

, (3)

, (6)

*<sup>j</sup>ωt-γ<sup>z</sup> E=E e <sup>0</sup>* , (4)

*1/2*

*<sup>1</sup> <sup>ε</sup> <sup>2</sup> S= E av <sup>0</sup> <sup>2</sup> <sup>μ</sup>* , (7)

*<sup>j</sup>ωμ <sup>η</sup><sup>=</sup> <sup>σ</sup> + jωε* . (8)

*2 2 2 <sup>1</sup> <sup>σ</sup> <sup>2</sup><sup>π</sup> <sup>β</sup> <sup>=</sup> ω με 1+ +1 =*

 

*2 ε ω λ*

, (5)

constant is

with the frequency. With the formula

confirm shape of polarization.

antenna becomes lower than free space. With the formula

*1 v =*

*με* and The capsule camera system is shown in Figure 2. One of the key challenges for ingestible devices is to find an efficient way to achieve RF signal transmission with minimum power consumption. This requires the use of an ultra-low power transmitter with a miniaturized antenna that is optimized for signal transmission through the body. The design of an antenna for such a system is a challenging task (Norris et al., 2007). The design must fulfill several requirements to be an effective capsule antenna, including: miniaturization to achieve matching at the desired bio-telemetric frequency; omni-directional pattern very congruent to that of a dipole in order to provide transmission regardless of the location of the capsule or receiver; polarization diversity that enables the capsule to transmit efficiently regardless of its orientation in the body; easy and understandable tuning adjustment to compensate for body effects. Types of transmitting antenna are used such as the spiral antennas, the printed microstrip antennas, and conformal antennas as shown in following subsections.

## **5.1.1 Spiral antennas**

A research group from Yonsei University, South Korea, proposed a series of spiral and helical antennas providing ultra-wide bandwidth at hundreds of megahertz.

#### **Single arm spiral antenna**

The first design is a miniaturized normal mode helical antenna with the conical structure (Kwak et al., 2005). To encase in the capsule module, the conical helical antenna is reduced only in height with the maintenance of the ultra-wide band characteristics. Thus, the spiral shaped antenna is designed with the total spiral arm length of a quarter-wavelength. The configuration of the designed antenna is shown in Figure 3(a). It is composed of a radiator and probe feeding structure. The proposed antenna is fabricated on the substrate with 0.5-oz copper, 3 mm substrate height, and dielectric constant of 2.17. The diameter of the antenna is 10.5 mm and 0.5 mm width conductor.

Fig. 3. Single arm spiral antenna (Kwak et al. 2005): (a) the geometric structure; (b) simulated and measured return losses; (c) azimuth pattern at 430MHz.

The simulated and the measured return losses of the antenna surrounded by human body equivalent material are shown in Figure 3(b). It can be observed that the bandwidth of the proposed spiral shaped antenna for S11<-10dB is 110 MHz of 400-510 MHz and the fractional bandwidth is 24.1 %, which is larger than 20%, the reference of the UWB fractional bandwidth. The measurement result of the azimuth radiation pattern is shown in Figure 3(c). The normalized received power level is varying between 0dB to -7dB, which can be considered as an omni-directional radiation pattern.

#### **Dual arm spiral antenna**

The dispersive properties of human body suggested that signals are less vulnerable when they are transmitted at lower frequency range. Therefore, a modified design is proposed to provide ultra-wide bandwidth at lower frequency range (Lee et al. 2007). Figure 4(a) shows the geometry of a dual spiral antenna. The newly proposed antenna is composed of two spirals connected by the single feeding line. The radius of designed antenna is 10.1mm and

and measured return losses; (c) azimuth pattern at 430MHz.

considered as an omni-directional radiation pattern.

**Dual arm spiral antenna** 

(a) (b)

(c) Fig. 3. Single arm spiral antenna (Kwak et al. 2005): (a) the geometric structure; (b) simulated

The simulated and the measured return losses of the antenna surrounded by human body equivalent material are shown in Figure 3(b). It can be observed that the bandwidth of the proposed spiral shaped antenna for S11<-10dB is 110 MHz of 400-510 MHz and the fractional bandwidth is 24.1 %, which is larger than 20%, the reference of the UWB fractional bandwidth. The measurement result of the azimuth radiation pattern is shown in Figure 3(c). The normalized received power level is varying between 0dB to -7dB, which can be

The dispersive properties of human body suggested that signals are less vulnerable when they are transmitted at lower frequency range. Therefore, a modified design is proposed to provide ultra-wide bandwidth at lower frequency range (Lee et al. 2007). Figure 4(a) shows the geometry of a dual spiral antenna. The newly proposed antenna is composed of two spirals connected by the single feeding line. The radius of designed antenna is 10.1mm and its height is about 3.5mm. To design a dual spiral antenna, two substrate layers are used. The upper and lower substrate layers have the same dielectric constant of 3.5 and the thicknesses of them are both 1.524mm. Two spirals with the same width of 0.5mm and the same gap of 0.25mm have different overall length. The lower spiral antenna is a 5.25 turn structure and the upper spiral is 5 turns.

Fig. 4. Dual arm spiral antenna (Lee et al. 2007): (a) the geometric structure; (b) measured return losses; (c) azimuth pattern at 400MHz.

The return loss of the proposed antenna was measured in the air and in the simulating fluid of the human tissue as shown in Figure 4(b). Because of considering electrical properties of equivalent material of human body, return loss characteristic in the air is not good but dual resonant characteristic is shown in the air. However, the proposed antenna has low return loss value at operating frequency in the fluid and its bandwidth is 98MHz (from 360MHz to 458MHz) in the fluid, with the fractional bandwidth of about 25%. The simulated radiation pattern as shown in Figure 4(c) is omni-directional at the azimuth plane with 5dB variation.

### **Conical helix antenna**

Extensive studies of the helical and spiral antennas were conducted with modified geometric structures. For example, a conical helix antenna fed through a 50 ohm coaxial cable is shown in Figure 5. Compared to small spiral antenna, conical spiral takes up much space. However, additional space is not necessary because a conical spiral can use the end space of the capsule as shown in Figure 5(a). The radius of the designed antenna is 10mm and the total height is 5 mm. This size is enough to be encased in small capsule.

Fig. 5. Conical helix antenna (Lee et al. 2008): (a) the geometric structure; (b) simulated and measured return losses; (c) azimuth pattern at 450MHz.

The proposed antenna provides a bandwidth of 101MHz (from 418MHz to 519MHz) in the human body equivalent material as shown in Figure 5(b). Its center frequency is 450MHz, so the fractional bandwidth is about 22%. The normalized simulated radiation pattern is shown in Figure 5(c). The proposed antenna has omni-directional radiation pattern with less than 1dB variation.

#### **Fat arm spiral antenna**

Another modified design is the fat arm spiral antenna as shown in Figure 6(a). The spiral arm is 3mm wide and separated from ground plane with a 1mm air gap. The antenna is

Extensive studies of the helical and spiral antennas were conducted with modified geometric structures. For example, a conical helix antenna fed through a 50 ohm coaxial cable is shown in Figure 5. Compared to small spiral antenna, conical spiral takes up much space. However, additional space is not necessary because a conical spiral can use the end space of the capsule as shown in Figure 5(a). The radius of the designed antenna is 10mm

and the total height is 5 mm. This size is enough to be encased in small capsule.

(a) (b)

(c) Fig. 5. Conical helix antenna (Lee et al. 2008): (a) the geometric structure; (b) simulated and

The proposed antenna provides a bandwidth of 101MHz (from 418MHz to 519MHz) in the human body equivalent material as shown in Figure 5(b). Its center frequency is 450MHz, so the fractional bandwidth is about 22%. The normalized simulated radiation pattern is shown in Figure 5(c). The proposed antenna has omni-directional radiation pattern with less than

Another modified design is the fat arm spiral antenna as shown in Figure 6(a). The spiral arm is 3mm wide and separated from ground plane with a 1mm air gap. The antenna is

measured return losses; (c) azimuth pattern at 450MHz.

1dB variation.

**Fat arm spiral antenna** 

**Conical helix antenna** 

simulationally investigated in the air, in the air with capsule shell and in the human body equivalent material.

Fig. 6. Fat arm spiral antenna (Lee et al. 2010): (a) the geometric structure; (b) return losses; (c) azimuth pattern at 450MHz.

The return losses of the antenna in free space, with dielectric capsule shell and in the liquid tissue phantom are plotted in Figure 6(b). The resonant frequency is observed about 800 MHz in the air, and reduced to 730 MHz due to the capsule effects on the effective dielectric constant and matching characteristic. When the proposed antenna is emerged in the equivalent liquid, it shows good matching at a resonant frequency and its bandwidth is 75 MHz (460 ~ 535 MHz) for S11 less than -10dB. The radiation pattern illustrated in Figure 6(c) presents that this antenna also provides omni-directional feature at azimuth plane.

#### **Square microstrip loop antenna**

A square microstrip loop antenna (Shirvante et al. 2010) is designed to operate on the Medical Implant Communication Service (MICS) band (402MHz -405MHz). The antenna is patterned on a Duroid 5880 substrate with a relative permittivity *εr* of 2.2 and a thickness of 500μm as shown in Figure 7(a). The area of the antenna is approximately 25 mm2 which is smaller enough to be encased in a swallowable capsule for children.

Fig. 7. square microstrip loop antenna (Shirvante et al. 2010): (a) the geometric structure; (b) simulated and measured return losses; (c) azimuth pattern at 403MHz.

The simulated and measured return losses as shown in Figure 7(b) presents that the antenna provides enough bandwidth to cover the 402MHz to 405MHz band. At the FSK operating frequency 403MHz, the measured return loss is -13dB. Moreover, the designed antenna shows a large tolerance to impedance variation at the MICS band, in correspondance to *εr* variation. The designed antenna also has an omni-directional radiation pattern at azimuth plane.

## **5.1.2 Conformal antennas**

A conformal geometry exploits the surface of the capsule and leaves the interior open for electrical components including the camera system. Several designs made efficient usage of the capsule shell area are selected as examples and introduced in this subsection.

#### **Conformal chandelier meandered dipole antenna**

The conformal chandelier meandered dipole antenna is investigated as a suitable candidate for wireless capsule endoscopy (Izdebski et al., 2009). The uniqueness of the design is its

patterned on a Duroid 5880 substrate with a relative permittivity *εr* of 2.2 and a thickness of 500μm as shown in Figure 7(a). The area of the antenna is approximately 25 mm2 which is

(a) (b)

(c) Fig. 7. square microstrip loop antenna (Shirvante et al. 2010): (a) the geometric structure; (b)

The simulated and measured return losses as shown in Figure 7(b) presents that the antenna provides enough bandwidth to cover the 402MHz to 405MHz band. At the FSK operating frequency 403MHz, the measured return loss is -13dB. Moreover, the designed antenna shows a large tolerance to impedance variation at the MICS band, in correspondance to *εr* variation.

A conformal geometry exploits the surface of the capsule and leaves the interior open for electrical components including the camera system. Several designs made efficient usage of

The conformal chandelier meandered dipole antenna is investigated as a suitable candidate for wireless capsule endoscopy (Izdebski et al., 2009). The uniqueness of the design is its

The designed antenna also has an omni-directional radiation pattern at azimuth plane.

the capsule shell area are selected as examples and introduced in this subsection.

smaller enough to be encased in a swallowable capsule for children.

simulated and measured return losses; (c) azimuth pattern at 403MHz.

**5.1.2 Conformal antennas** 

**Conformal chandelier meandered dipole antenna** 

miniaturization process, conformal structure, polarization diversity, dipole-like omnidirectional pattern and simple tunable parameters (as shown in Figure 8(a)). The antenna is offset fed in such a way that there is an additional series resonance excited in addition to the parallel resonance (as shown in Figure 8(b)). The two arms with different lengths generate the dual resonances. This additional series resonance provides better matching at the frequency of interest. This antenna is designed to operate around 1395MHz – 1400 MHz wireless medical telemetry services (WMTS) band.

Fig. 8. Conformal chandelier meandered dipole antenna (Izdebski et al., 2009): (a) the geometric structure of the conformal chandelier meandered dipole antenna; (b) Offset Planar Meandered Dipole Antenna with current alignment vectors.

The offset planar meandered dipole antenna is simulated on a 0.127 mm thick substrate with a dielectric constant of 2.2. The antenna is placed in the small intestine and it is observed that there is a lot of detuning due to the body conductivity and the dielectric constant (average body composition has a relative permittivity of 58.8 and a conductivity of 0.84S/m). The series resonance shifts closer to 600 MHz. The antenna is then retuned to the operational frequency of 1.4 GHz by reducing the length of the dipole antenna. The return losses of both the detuned and tuned antenna are shown Figure 9(a). Figure 9(b) shows the radiation pattern of the tuned antenna inside the human body at 1.4 GHz.

Fig. 9. Conformal chandelier meandered dipole antenna (Izdebski et al., 2009): (a) the return losses of detuned and tuned structure in human model; (b) azimuth pattern at 1.4GHz.

The radiation pattern is dipole-like but tilted due to the conformity of the structure. The axial ratio (dB) for the conformal chandelier meandered dipole antenna is about 7dB (elliptical polarization). It possesses all the characteristics of planar structure along with polarization diversity.

#### **Outer-wall loop antenna**

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.

Fig. 10. Outer-wall loop antenna (Yun et al., 2010.): (a) the geometric structure; (b) simulated and measured return losses; (c) azimuth pattern at 500MHz.

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