**1. Introduction**

[22] L. Qi, H. M. Salgado, and J. R. Pereira, "Printed fractal monopole antenna array for

[23] K. Sekine, and H. Iwasaki, "USB Memory Size Antenna for 2.4 GHz Wireless LAN

[24] A. Gummalla, M. Achour, G. Poilasne *et al.*, "Compact Dual-Band Planar Metamate‐ rial Antenna Arrays for Wireless Lan," *2008 Ieee Antennas and Propagation Society In‐*

[25] K. Jaesoon, K. Dongho, L. Youngki *et al.*, "Design of a MIMO antenna for USB dongle

[27] L. Qi, C. Quigley, J. R. Pereira *et al.*, "Inverted-L antennas array in a wireless USB

[28] Z. N. Chen, "Note on impedance characteristics of L-shaped wire monopole anten‐ na," *Microwave and Optical Technology Letters,* vol. 26, no. 1, pp. 22-23, 2000.

[29] C. H. Ku, L. K. Li, and W. L. Mao, "Compact Monopole Antenna with Branch Strips for Wlan/Wimax Operation," *Microwave and Optical Technology Letters,* vol. 52, no. 8,

[30] S. H. Chang, and W. J. Liao, "A Broadband LTE/WWAN Antenna Design for Tablet PC," *Ieee Transactions on Antennas and Propagation,* vol. 60, no. 9, pp. 4354-4359, Sep,

[31] M. Komulainen, M. Berg, H. Jantunen *et al.*, "Compact varactor-tuned meander line monopole antenna for DVB-H signal reception," *Electronics Letters,* vol. 43, no. 24, pp.

[32] H. F. Abutarboush, R. Nilavalan, S. W. Cheung *et al.*, "Compact Printed Multiband Antenna With Independent Setting Suitable for Fixed and Reconfigurable Wireless Communication Systems," *Ieee Transactions on Antennas and Propagation,* vol. 60, no. 8,

[33] C. H. See, R. A. Abd-Alhameed, Z. Z. Abidin *et al.*, "Wideband Printed MIMO/Diver‐ sity Monopole Antenna for WiFi/WiMAX Applications," *Ieee Transactions on Ant*en‐

nas and Propagation, vol. 60, no. 4, pp. 2028-2035, Apr, 2012.

[26] C. Luxey, "Design of multi-antenna systems for UMTS mobile phones." pp. 57-64.

WLAN." pp. 1-4.

84 Progress in Compact Antennas

and UWB." pp. 1173-1176.

pp. 1858-1861, Aug, 2010.

pp. 3867-3874, Aug, 2012.

2012.

1324-1326, 2007.

*ternational Symposium, Vols 1-9*, pp. 4595-4598, 2008.

application using common grounding." pp. 313-316.

dongle for MIMO application." pp. 1909-1912.

The need of both mobility and communication leads to the integration of antennas in miniature devices so far non-connected (particularly in medical areas). The dedicated volume for the antenna, including its ground plane, has to be kept at its acceptable minimum, involving low bandwidth. Moreover, due to their poor impedance bandwidth, small antennas tend to be very sensitive to the environment. Indeed, they are directly affected by their immediate surround‐ ings, which disturb their working band, their radiation and their performances [1]. To counter the low bandwidth of the antenna and to adapt it to variable conditions and surroundings, it can integrate active components.

Thus, active components become highly suitable for the development of modern wireless communications. Indeed, they allow the miniaturization, shifting the antenna working frequency to be matched over a wide bandwidth by covering only the user channel and the adaptation of antennas to variable operating conditions and surroundings. It is in this framework that authors will propose in this chapter to detail the integration of active compo‐ nents in antennas to be more compact, smart and integrated.

The first part will address an overview of the most common used techniques for compact antennas to become active. In this goal, active antennas state-of-the-art will be presented:


A second part will show relevant parameters for active antennas studies. It will exhibit both challenges and how to integrate active components in order to maximize the antenna per‐

formances and efficiency. This part will be supported by concrete examples. Therefore, depending on their intended applications, readers will be prepared to find the best trade-offs between the agility method, the miniaturization and antenna performances.

The last part will be dedicated to present limitations of actual and most common solutions proposed for active and compact antennas. In this framework, new approaches will be detailed to overcome these physical limitations.

Varactor diode

Ground plane

Slot antennas are also good candidates for the frequency agility [7-9]. N. Behdad and K. Sarabandi presented in [9] a dual band reconfigurable slot antenna. The schematic of its proposed dual-band slot antenna is shown in Figure 2. Matching is performed by choosing appropriately the location of the microstrip feed and the length of the open circuited line.

Figure 3 shows the simulated and measured dual-band responses of the antenna where by applying the appropriate combination of bias voltages (V1 and V2) the frequency of the first band is kept fixed and that of the second band is tuned. Similarly, as shown in Figure 3, it is possible to keep the frequency of the second band stationary and sweep the frequency of the

Feeding probe

Substrate

**Figure 1.** Patch antenna integrating varactor diodes [28]

**Figure 2.** Dual reconfigurable slot antenna [9]

first band.

Top hat

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 87

#### **2. Overview of compact active antennas**

Very small size antennas are needed for future dense wireless network deployment, for example in WBAN (Wireless Body Area Network) where the size is limited to dimensions much smaller (hearing aid, implants) than wavelength (λ0=12.2 cm at 2.45GHz) or for the DVB-H (Digital Video Broadcasting – Handheld) application, where the miniaturization aspect is even more critical because it is a low frequency standard (λ0=60 cm at 470 MHz). Therefore, the antenna has to be carefully optimized with trading off fundamental size limitations with its characteristics (especially bandwidth and efficiency). In addition, the environment of miniature antennas will be highly variable resulting in large antenna impedance and propa‐ gation channel changes. Several challenges have to be addressed. One of them is the adaptive antenna technology for compensating both low bandwidths and detuning effects.

The most commonly cited performance criterion is the achievable frequency tuning range (TR) defined as:

*TR*(*%*)= 2( *f* max − *f* min) *f* max + *f* min .100 where *f* max and *f* min are respectively the upper and the lower antenna operating frequency. The frequency tuning can either be continuous or discrete. The continuous frequency tuning is able to continuously cover each channel of a same standard while the discrete frequency tuning can only switch between different standards. This part will present the most common methods to target frequency tunable antennas design.

#### **2.1. Integration of active components**

#### *2.1.1. Varactor diodes*

For continuous frequency tuning, the integration of varactor diodes within an antenna is the most common approach [2-5]. P. Bhartia et al. were the first to publish antenna integrating varactor diodes [6]. Indeed, they presented a microstrip patch antenna with varactor diodes at the edges of the structure, as illustrated Figure 1. Both rectangular and circular tunable patches were studied, results reveal that 22% and 30% of bandwidth can respectively be achieved by varying the DC-bias-voltage between 0V and 30V.

**Figure 1.** Patch antenna integrating varactor diodes [28]

formances and efficiency. This part will be supported by concrete examples. Therefore, depending on their intended applications, readers will be prepared to find the best trade-offs

The last part will be dedicated to present limitations of actual and most common solutions proposed for active and compact antennas. In this framework, new approaches will be detailed

Very small size antennas are needed for future dense wireless network deployment, for example in WBAN (Wireless Body Area Network) where the size is limited to dimensions much smaller (hearing aid, implants) than wavelength (λ0=12.2 cm at 2.45GHz) or for the DVB-H (Digital Video Broadcasting – Handheld) application, where the miniaturization aspect is even more critical because it is a low frequency standard (λ0=60 cm at 470 MHz). Therefore, the antenna has to be carefully optimized with trading off fundamental size limitations with its characteristics (especially bandwidth and efficiency). In addition, the environment of miniature antennas will be highly variable resulting in large antenna impedance and propa‐ gation channel changes. Several challenges have to be addressed. One of them is the adaptive

antenna technology for compensating both low bandwidths and detuning effects.

The most commonly cited performance criterion is the achievable frequency tuning range (TR)

antenna operating frequency. The frequency tuning can either be continuous or discrete. The continuous frequency tuning is able to continuously cover each channel of a same standard while the discrete frequency tuning can only switch between different standards. This part

For continuous frequency tuning, the integration of varactor diodes within an antenna is the most common approach [2-5]. P. Bhartia et al. were the first to publish antenna integrating varactor diodes [6]. Indeed, they presented a microstrip patch antenna with varactor diodes at the edges of the structure, as illustrated Figure 1. Both rectangular and circular tunable patches were studied, results reveal that 22% and 30% of bandwidth can respectively be

will present the most common methods to target frequency tunable antennas design.

achieved by varying the DC-bias-voltage between 0V and 30V.

.100 where *f* max and *f* min are respectively the upper and the lower

between the agility method, the miniaturization and antenna performances.

to overcome these physical limitations.

86 Progress in Compact Antennas

defined as:

2( *f* max − *f* min) *f* max + *f* min

**2.1. Integration of active components**

*2.1.1. Varactor diodes*

*TR*(*%*)=

**2. Overview of compact active antennas**

Slot antennas are also good candidates for the frequency agility [7-9]. N. Behdad and K. Sarabandi presented in [9] a dual band reconfigurable slot antenna. The schematic of its proposed dual-band slot antenna is shown in Figure 2. Matching is performed by choosing appropriately the location of the microstrip feed and the length of the open circuited line.

**Figure 2.** Dual reconfigurable slot antenna [9]

Figure 3 shows the simulated and measured dual-band responses of the antenna where by applying the appropriate combination of bias voltages (V1 and V2) the frequency of the first band is kept fixed and that of the second band is tuned. Similarly, as shown in Figure 3, it is possible to keep the frequency of the second band stationary and sweep the frequency of the first band.

**Figure 3.** Measured |S11| parameters for different combination of bias voltages [9] Figure 3. Measured |S11| parameters for different combination of bias voltages [9]

Another example show a 3D Inverted F Antenna [10] designed to cover the entire DVB-H band going from 470 MHz to 862 MHz. To be integrated in a mobile handheld device, the antenna allocated volume had to be very compact. A good trade-off between small sizes and the impedance bandwidth was to choose a structure based on the IFA design. Indeed, the radiating monopole of this kind of structure can be folded all around a material to become more compact (see Figure 4). Another example show a 3D Inverted F Antenna [10] designed to cover the entire DVB-H band going from 470 MHz to 862 MHz. To be integrated in a mobile handheld device, the antenna allocated volume had to be very compact. A good trade-off between small sizes and the impedance bandwidth was to choose a structure based on the IFA design. Indeed, the radiating monopole of this kind of structure can be folded all around a material to become more compact (see Figure 4).

shown Figure 6. In order to improve the quality and the reliability of wireless links, the final

(a) (b) Figure 6. DC bias Tee with SMD components and its integration upstream from the antenna (a),

Figure 7 presents respectively the variation of the input impedances and |S11| parameters of the antenna versus frequency for different values of the varactor diode bias voltages.

Figure 6. DC bias Tee with SMD components and its integration upstream from the antenna (a),

Figure 7 presents respectively the variation of the input impedances and |S11| parameters of

Figure 7 presents respectively the variation of the input impedances and |S11| parameters of the antenna versus frequency for different values of the varactor diode bias voltages.

**Figure 6.** DC bias Tee with SMD components and its integration upstream from the antenna (a), integration of two

(a) (b)

 (a) (b) Figure 7. Measured input impedances (a) and |S11| parameters (b) for several DC bias voltages [10] Therefore, the antenna working band is continuously tuned all over the whole DVB-H band. In the worst case, i.e. for a 2V DC bias voltage, the antenna is matched with |S11| < - 6dB in a bandwidth which is covering more than one channel of the DVB-H band at -6 dB (largely

 (a) (b) Figure 7. Measured input impedances (a) and |S11| parameters (b) for several DC bias voltages [10]

**Figure 7.** Measured input impedances (a) and |S11| parameters (b) for several DC bias voltages [10]

**-35 -30 -25 -20 -15 -10 -5 0**

Therefore, the antenna working band is continuously tuned all over the whole DVB-H band. In the worst case, i.e. for a 2V DC bias voltage, the antenna is matched with |S11| < - 6dB in a bandwidth which is covering more than one channel of the DVB-H band at -6 dB (largely

**459 MHz**

**-35 -30 -25 -20 -15 -10 -5 0**

**-6 dB**

**|S11| (dB)**

**459 MHz**

**|S11| (dB)**

**-6 dB**

the antenna versus frequency for different values of the varactor diode bias voltages.

**0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 -40**

**Frequency**

**0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 -40**

**Frequency**

**V=1.6 V V=2 V V=3 V V=4 V V=6 V V=8 V V=10 V V=15 V V=22 V**

**875 MHz**

**V=1.6 V V=2 V V=3 V V=4 V V=6 V V=8 V V=10 V V=15 V V=22 V**

**875 MHz**

suitable for the DVB-H standard).

**0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8**

**Frequency (GHz)**

suitable for the DVB-H standard).

**0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8**

**Frequency (GHz)**

antennas in the tablet dedicated to the DVB-H reception (b) [10]

**Re(Zin) V=2V Re(Zin) V=3.5V Re(Zin) V=18V Im(Zin) V=2V Im(Zin) V=3.5V Im(Zin) V=18V**

**Re(Zin) V=2V Re(Zin) V=3.5V Re(Zin) V=18V Im(Zin) V=2V Im(Zin) V=3.5V Im(Zin) V=18V**

**Input impedance Zin ()**

**Input impedance Zin ()**

**2.1.2. Positive Intrinsic Negative (PIN) diodes**

**2.1.2. Positive Intrinsic Negative (PIN) diodes**

integration of two antennas in the tablet dedicated to the DVB-H reception (b) [10]

integration of two antennas in the tablet dedicated to the DVB-H reception (b) [10]

A prototype of the tunable antenna has been realized (Figure 6) and measured. For the diode polarization, a DC bias Tee is optimized with SMD components and measured on the DVB-H band (Figure 6). After being validated, it is integrated upstream from the antenna structure as shown Figure 6. In order to improve the quality and the reliability of wireless links, the final mobile device is integrating two antennas (see Figure 6) for diversity

A prototype of the tunable antenna has been realized (Figure 6) and measured. For the diode polarization, a DC bias Tee is optimized with SMD components and measured on the DVB-H band (Figure 6). After being validated, it is integrated upstream from the antenna structure as shown Figure 6. In order to improve the quality and the reliability of wireless links, the final mobile device is integrating two antennas (see Figure 6) for diversity

**Varactordiode**

**Vias**

**RF + DC**

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 89

mobile device is integrating two antennas (see Figure 6) for diversity operations.

**Figure 5.** Design integrating both magneto-dielectric material and a varactor diode [10]

**DC block capacitor C=100 pF**

operations.

**Coplanar waveguide feeding**

**Varactordiode 0.2pF < C < 2pF**

operations.

**Figure 4.** Inverted F Antenna design (a). Antenna top view (b) [10]

Figure 4. 3D Inverted F Antenna design (a). Antenna top view (b) [10]

However, the more compact the antenna is, the lowest the bandwidth is becoming. To counter this issue, it has been proved that using a magneto-dielectric material rather than a dielectric one allows enhancing the input impedance bandwidth. Basing on this antenna design, the idea was to integrate a varactor diode to tune the impedance matching all over the DVB-H band (see Figure 5). However, the more compact the antenna is, the lowest the bandwidth is becoming. To counter this issue, it has been proved that using a magneto-dielectric material rather than a dielectric one allows enhancing the input impedance bandwidth. Basing on this antenna design, the idea was to integrate a varactor diode to tune the impedance matching all over the DVB-H band (see Figure 5).

A prototype of the tunable antenna has been realized (Figure 6) and measured. For the diode polarization, a DC bias Tee is optimized with SMD components and measured on the DVB-H band (Figure 6). After being validated, it is integrated upstream from the antenna structure as

**V=1.6 V V=2 V V=3 V**

A prototype of the tunable antenna has been realized (Figure 6) and measured. For the A prototype of the tunable antenna has been realized (Figure 6) and measured. For the **Figure 5.** Design integrating both magneto-dielectric material and a varactor diode [10]

operations.

operations.

**150 200** **Re(Zin) V=3.5V Re(Zin) V=18V Im(Zin) V=2V**

**Figure 3.** Measured |S11| parameters for different combination of bias voltages [9]

Figure 4. 3D Inverted F Antenna design (a). Antenna top view (b) [10]

**Figure 4.** Inverted F Antenna design (a). Antenna top view (b) [10]

Figure 3. Measured |S11| parameters for different combination of bias voltages [9]

(see Figure 4).

88 Progress in Compact Antennas

more compact (see Figure 4).

the DVB-H band (see Figure 5).

(see Figure 5).

Another example show a 3D Inverted F Antenna [10] designed to cover the entire DVB-H band going from 470 MHz to 862 MHz. To be integrated in a mobile handheld device, the antenna allocated volume had to be very compact. A good trade-off between small sizes and the impedance bandwidth was to choose a structure based on the IFA design. Indeed, the radiating monopole of this kind of structure can be folded all around a material to become more compact

Another example show a 3D Inverted F Antenna [10] designed to cover the entire DVB-H band going from 470 MHz to 862 MHz. To be integrated in a mobile handheld device, the antenna allocated volume had to be very compact. A good trade-off between small sizes and the impedance bandwidth was to choose a structure based on the IFA design. Indeed, the radiating monopole of this kind of structure can be folded all around a material to become

(a) (b)

However, the more compact the antenna is, the lowest the bandwidth is becoming. To counter this issue, it has been proved that using a magneto-dielectric material rather than a dielectric one allows enhancing the input impedance bandwidth. Basing on this antenna design, the idea was to integrate a varactor diode to tune the impedance matching all over

However, the more compact the antenna is, the lowest the bandwidth is becoming. To counter this issue, it has been proved that using a magneto-dielectric material rather than a dielectric one allows enhancing the input impedance bandwidth. Basing on this antenna design, the idea was to integrate a varactor diode to tune the impedance matching all over the DVB-H band

A prototype of the tunable antenna has been realized (Figure 6) and measured. For the diode polarization, a DC bias Tee is optimized with SMD components and measured on the DVB-H band (Figure 6). After being validated, it is integrated upstream from the antenna structure as shown Figure 6. In order to improve the quality and the reliability of wireless links, the final mobile device is integrating two antennas (see Figure 6) for diversity operations. DVB-H band (Figure 6). After being validated, it is integrated upstream from the antenna structure as shown Figure 6. In order to improve the quality and the reliability of wireless links, the final mobile device is integrating two antennas (see Figure 6) for diversity diode polarization, a DC bias Tee is optimized with SMD components and measured on the DVB-H band (Figure 6). After being validated, it is integrated upstream from the antenna structure as shown Figure 6. In order to improve the quality and the reliability of wireless

links, the final mobile device is integrating two antennas (see Figure 6) for diversity

diode polarization, a DC bias Tee is optimized with SMD components and measured on the

integration of two antennas in the tablet dedicated to the DVB-H reception (b) [10] Figure 7 presents respectively the variation of the input impedances and |S11| parameters of **Figure 6.** DC bias Tee with SMD components and its integration upstream from the antenna (a), integration of two antennas in the tablet dedicated to the DVB-H reception (b) [10] (a) (b)

Figure 6. DC bias Tee with SMD components and its integration upstream from the antenna (a),

Figure 6. DC bias Tee with SMD components and its integration upstream from the antenna (a),

**-10 -5**

the antenna versus frequency for different values of the varactor diode bias voltages.

**-6 dB**

the antenna versus frequency for different values of the varactor diode bias voltages. **250 Re(Zin) V=2V 0** Figure 7 presents respectively the variation of the input impedances and |S11| parameters of the antenna versus frequency for different values of the varactor diode bias voltages. integration of two antennas in the tablet dedicated to the DVB-H reception (b) [10] Figure 7 presents respectively the variation of the input impedances and |S11| parameters of

Figure 7. Measured input impedances (a) and |S11| parameters (b) for several DC bias voltages [10]

Therefore, the antenna working band is continuously tuned all over the whole DVB-H band. In the worst case, i.e. for a 2V DC bias voltage, the antenna is matched with |S11| < - 6dB in a bandwidth which is covering more than one channel of the DVB-H band at -6 dB (largely

**Figure 7.** Measured input impedances (a) and |S11| parameters (b) for several DC bias voltages [10]

**2.1.2. Positive Intrinsic Negative (PIN) diodes**

**2.1.2. Positive Intrinsic Negative (PIN) diodes**

suitable for the DVB-H standard).

suitable for the DVB-H standard).

Therefore, the antenna working band is continuously tuned all over the whole DVB-H band. In the worst case, i.e. for a 2V DC bias voltage, the antenna is matched with |S11| <-6dB in a bandwidth which is covering more than one channel of the DVB-H band at-6 dB (largely suitable for the DVB-H standard).

#### *2.1.2. Positive Intrinsic Negative (PIN) diodes*

PIN diodes are using as switches:


Selected antenna's types for integrating PIN diodes are often slot antennas or printed antennas (e.g. printed monopole, Inverted F Antenna, …). J-M. Laheurte presented in [11] a slot antenna including pin diodes for multi-frequency operation within a frequency octave.

> a suitable PIN switch has been grasped before integrate it and show its effects on the antenna performances. Indeed, to implement the electronic reconfigurability, the ideal shunt switches must be replaced by real PIN diodes. Therefore, the RF equivalent circuit of the diode has been studied (see Figure 10) for both the ON and OFF states. The reactive components Cp and Lp are modeling the packaging effect, while the others come from the electric properties of the diode junction in the ON and OFF positions. Then, the switch bias network was presented as

> **Figure 9.** Measured |S11| parameters for different states of diodes: all diodes OFF (i), diodes 1, 8 ON (ii), diodes 1, 2, 7, 8

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 91

antenna performances. Indeed, to implement the electronic reconfigurability, the ideal shunt switches must be replaced by real PIN diodes. Therefore, the RF equivalent circuit of the diode has been studied (see Figure 10) for both the ON and OFF states. The reactive components Cp and Lp are modeling the packaging effect, while the others come from the electric properties of the diode junction in the ON and OFF positions. Then, the switch bias network was presented as an inductor of 470 nH and three 10 pF capacitors (Figure 10).

(a) (b)

Finally, a reconfigurable slot antenna design (Figure 11) is presented in this paper. Four switches are used in order to tune the antenna over a range of 540–950 MHz. The integration of the fours PIN diodes allows choosing the operating frequency of the antenna (Figure 11).

Finally, a reconfigurable slot antenna design (Figure 11) is presented in this paper. Four switches are used in order to tune the antenna over a range of 540–950 MHz. The integration of the fours PIN diodes allows choosing the operating frequency of the antenna (Figure 11).

(a) (b)

Figure 11. Reconfigurable slot antenna (a) and its measured |S11| parameters (b) [13]


**2.1.3. MicroElectroMechanical systems (MEMS)**

MEMS components can allow:

Figure 10. RF equivalent circuit of the PIN diode (a) and the switch bias network (b) [13]

**Figure 10.** RF equivalent circuit of the PIN diode (a) and the switch bias network (b) [13]

an inductor of 470 nH and three 10 pF capacitors (Figure 10).

ON (iii) and diodes 1, 2, 3, 6, 7, 8 ON (iv) [11]

**Figure 8.** Switchable slot antenna including eight pin diodes [11]

As shown Figure 8, this antenna integrates eight PIN diodes and according to their ON or OFF states combination, the antenna can operate at different and discrete frequency bands (see Figure 9). Instantaneous impedance bandwidths are between 8% and 21% depending on the diodes' states combination.

Eventually, this antenna presents somewhat large dimensions since its main size is higher than à λ0/2 at 2.8 GHz. The literature presents smaller antennas integrating PIN diodes since their main size are lower than à λ0/2 at the working frequency [12].

Peroulis et al. [13] presented a tunable single-fed S-shaped slot loaded with a series of four PIN diodes. The effective length modification allows this antenna to operate in one of four selectable frequency bands between 530 and 890 MHz.

Before directly studying the tunable slot antenna, both single S-shaped slot antenna and PIN diodes were separately presented and studied. By this way, the issue related to the design of

Therefore, the antenna working band is continuously tuned all over the whole DVB-H band. In the worst case, i.e. for a 2V DC bias voltage, the antenna is matched with |S11| <-6dB in a bandwidth which is covering more than one channel of the DVB-H band at-6 dB (largely

**•** The ON state of the diode can be modelled by a zero resistance, i.e. a continuous metal strip

**•** The OFF state of the diode can be modelled as an infinite resistance. The radiating length after the diode is not seen from RF point of view. The effective length of the antenna, and

Selected antenna's types for integrating PIN diodes are often slot antennas or printed antennas (e.g. printed monopole, Inverted F Antenna, …). J-M. Laheurte presented in [11] a slot antenna

**64.7 mm**

As shown Figure 8, this antenna integrates eight PIN diodes and according to their ON or OFF states combination, the antenna can operate at different and discrete frequency bands (see Figure 9). Instantaneous impedance bandwidths are between 8% and 21% depending on the

Eventually, this antenna presents somewhat large dimensions since its main size is higher than à λ0/2 at 2.8 GHz. The literature presents smaller antennas integrating PIN diodes since their

Peroulis et al. [13] presented a tunable single-fed S-shaped slot loaded with a series of four PIN diodes. The effective length modification allows this antenna to operate in one of four

Before directly studying the tunable slot antenna, both single S-shaped slot antenna and PIN diodes were separately presented and studied. By this way, the issue related to the design of

Diode 5

Diode 6

Diode 7

Diode 8

hence its operating frequency, is changing compared with the ON state case.

including pin diodes for multi-frequency operation within a frequency octave.

Diode 4

Diode 3

suitable for the DVB-H standard).

90 Progress in Compact Antennas

PIN diodes are using as switches:

DC-bias voltage

diodes' states combination.

*2.1.2. Positive Intrinsic Negative (PIN) diodes*

across the slot where the diode is integrated.

Diode 1

Ground plane

**Figure 8.** Switchable slot antenna including eight pin diodes [11]

main size are lower than à λ0/2 at the working frequency [12].

selectable frequency bands between 530 and 890 MHz.

Diode 2

**Figure 9.** Measured |S11| parameters for different states of diodes: all diodes OFF (i), diodes 1, 8 ON (ii), diodes 1, 2, 7, 8 ON (iii) and diodes 1, 2, 3, 6, 7, 8 ON (iv) [11]

a suitable PIN switch has been grasped before integrate it and show its effects on the antenna performances. Indeed, to implement the electronic reconfigurability, the ideal shunt switches must be replaced by real PIN diodes. Therefore, the RF equivalent circuit of the diode has been studied (see Figure 10) for both the ON and OFF states. The reactive components Cp and Lp are modeling the packaging effect, while the others come from the electric properties of the diode junction in the ON and OFF positions. Then, the switch bias network was presented as an inductor of 470 nH and three 10 pF capacitors (Figure 10). antenna performances. Indeed, to implement the electronic reconfigurability, the ideal shunt switches must be replaced by real PIN diodes. Therefore, the RF equivalent circuit of the diode has been studied (see Figure 10) for both the ON and OFF states. The reactive components Cp and Lp are modeling the packaging effect, while the others come from the electric properties of the diode junction in the ON and OFF positions. Then, the switch bias

network was presented as an inductor of 470 nH and three 10 pF capacitors (Figure 10).

Finally, a reconfigurable slot antenna design (Figure 11) is presented in this paper. Four **Figure 10.** RF equivalent circuit of the PIN diode (a) and the switch bias network (b) [13]

Figure 10. RF equivalent circuit of the PIN diode (a) and the switch bias network (b) [13]

of the fours PIN diodes allows choosing the operating frequency of the antenna (Figure 11). Finally, a reconfigurable slot antenna design (Figure 11) is presented in this paper. Four switches are used in order to tune the antenna over a range of 540–950 MHz. The integration of the fours PIN diodes allows choosing the operating frequency of the antenna (Figure 11).

(a) (b)

Figure 11. Reconfigurable slot antenna (a) and its measured |S11| parameters (b) [13]


**2.1.3. MicroElectroMechanical systems (MEMS)**

MEMS components can allow:

switches are used in order to tune the antenna over a range of 540–950 MHz. The integration

antenna performances. Indeed, to implement the electronic reconfigurability, the ideal shunt switches must be replaced by real PIN diodes. Therefore, the RF equivalent circuit of the diode has been studied (see Figure 10) for both the ON and OFF states. The reactive components Cp and Lp are modeling the packaging effect, while the others come from the electric properties of the diode junction in the ON and OFF positions. Then, the switch bias network was presented as an inductor of 470 nH and three 10 pF capacitors (Figure 10).

(a) (b)

switches are used in order to tune the antenna over a range of 540–950 MHz. The integration of the fours PIN diodes allows choosing the operating frequency of the antenna (Figure 11).

Figure 10. RF equivalent circuit of the PIN diode (a) and the switch bias network (b) [13]

#### MEMS components can allow: *2.1.3. MicroElectroMechanical systems (MEMS)*


**•** Continuous frequency tuning when they are used as a variable capacitance.

Figure 11. Reconfigurable slot antenna (a) and its measured |S11| parameters (b) [13]

**•** Discrete frequency tuning, when they are used as switches.

E. Erdil presents in [14] a reconfigurable microstrip patch antenna integrating RF MEMS capacitor for continuously tuning the resonant frequency (see Figure 12). The reconfigurability of the operating frequency is obtained by loading one of the radiating edges of the microstrip patch antenna with a CPW stub on which RF MEMS bridge type capacitors are periodically placed.

**Figure 13.** |S11|parameters for different actuation voltages and simulation results [14]

the outer and inner slots, to be shorted to RF ground.

integrating two double-arm MEMS actuators [15]

**Figure 13.** |S11|parameters for different actuation voltages and simulation results [14]

shorted to RF ground.

ble-arm MEMS actuators [15]

1.4 μm.

Discrete frequency tuning can be illustrated with a reconfigurable annular slot antenna with a monolithic integration of MEMS actuators presented by B.A. Cetiner in [15]. The architecture and a photograph of the microstrip-fed reconfigurable antenna annular slot are shown in Figure 14. The antenna has two concentric circular slots. According to MEMS switch S1 state, they can be individually excited in order to achieve frequency reconfigurability. S2 and S3 switches enable the metallic annular ring, which stays between the outer and inner slots, to be

 Discrete frequency tuning can be illustrated with a reconfigurable annular slot antenna with a monolithic integration of MEMS actuators presented by B.A. Cetiner in [15]. The architecture and a photograph of the microstrip-fed reconfigurable antenna annular slot are shown in Figure 14. The antenna has two concentric circular slots. According to MEMS switch S1 state, they can be individually excited in order to achieve frequency reconfigurability. S2 and S3 switches enable the metallic annular ring, which stays between

(a) (b)

**Figure 14.** Microstrip feeding line integrating a single-arm MEMS switch (a) and the annular slot

 The measured |S11| parameters (Figure 15) show that when MEMS switches are activated (down-state) by applying DC bias voltages, the antenna working band is around

**Figure 14.** Microstrip feeding line integrating a single-arm MEMS switch (a) and the annular slot integrating two dou‐

is increased from 0 to 11.9 V, where the height of the capacitive gap changes from 1.5 μm to

9

93

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838

**Figure 12.** Frequency tunable microstrip patch antenna integrating MEMS capacitors [14]

When a DC voltage is applied, the height of the MEMS bridges on the stub is varying, and thus the loading capacitance is also changing. Therefore, as showed Figure 13 the matching frequency around 16.05 GHz shifts down to 15.75 GHz as the actuation voltage is increased from 0 to 11.9 V, where the height of the capacitive gap changes from 1.5 µm to 1.4 µm.

9

**Figure 13.** |S11|parameters for different actuation voltages and simulation results [14]

the outer and inner slots, to be shorted to RF ground.

antenna performances. Indeed, to implement the electronic reconfigurability, the ideal shunt switches must be replaced by real PIN diodes. Therefore, the RF equivalent circuit of the diode has been studied (see Figure 10) for both the ON and OFF states. The reactive components Cp and Lp are modeling the packaging effect, while the others come from the electric properties of the diode junction in the ON and OFF positions. Then, the switch bias network was presented as an inductor of 470 nH and three 10 pF capacitors (Figure 10).

(a) (b)

Finally, a reconfigurable slot antenna design (Figure 11) is presented in this paper. Four switches are used in order to tune the antenna over a range of 540–950 MHz. The integration of the fours PIN diodes allows choosing the operating frequency of the antenna (Figure 11).

(a) (b)

Figure 11. Reconfigurable slot antenna (a) and its measured |S11| parameters (b) [13]


E. Erdil presents in [14] a reconfigurable microstrip patch antenna integrating RF MEMS capacitor for continuously tuning the resonant frequency (see Figure 12). The reconfigurability of the operating frequency is obtained by loading one of the radiating edges of the microstrip patch antenna with a CPW stub on which RF MEMS bridge type capacitors are periodically

When a DC voltage is applied, the height of the MEMS bridges on the stub is varying, and thus the loading capacitance is also changing. Therefore, as showed Figure 13 the matching frequency around 16.05 GHz shifts down to 15.75 GHz as the actuation voltage is increased from 0 to 11.9 V, where the height of the capacitive gap changes from 1.5 µm to 1.4 µm.

**•** Continuous frequency tuning when they are used as a variable capacitance.

**2.1.3. MicroElectroMechanical systems (MEMS)**

**Figure 11.** Reconfigurable slot antenna (a) and its measured |S11| parameters (b) [13]

**•** Discrete frequency tuning, when they are used as switches.

**Figure 12.** Frequency tunable microstrip patch antenna integrating MEMS capacitors [14]

MEMS components can allow:

MEMS components can allow:

92 Progress in Compact Antennas

placed.

*2.1.3. MicroElectroMechanical systems (MEMS)*

Figure 10. RF equivalent circuit of the PIN diode (a) and the switch bias network (b) [13]

Discrete frequency tuning can be illustrated with a reconfigurable annular slot antenna with a monolithic integration of MEMS actuators presented by B.A. Cetiner in [15]. The architecture and a photograph of the microstrip-fed reconfigurable antenna annular slot are shown in Figure 14. The antenna has two concentric circular slots. According to MEMS switch S1 state, they can be individually excited in order to achieve frequency reconfigurability. S2 and S3 switches enable the metallic annular ring, which stays between the outer and inner slots, to be shorted to RF ground. **Figure 13.** |S11|parameters for different actuation voltages and simulation results [14] Discrete frequency tuning can be illustrated with a reconfigurable annular slot antenna with a monolithic integration of MEMS actuators presented by B.A. Cetiner in [15]. The architecture and a photograph of the microstrip-fed reconfigurable antenna annular slot are shown in Figure 14. The antenna has two concentric circular slots. According to MEMS switch S1 state, they can be individually excited in order to achieve frequency reconfigurability. S2 and S3 switches enable the metallic annular ring, which stays between

integrating two double-arm MEMS actuators [15] The measured |S11| parameters (Figure 15) show that when MEMS switches are **Figure 14.** Microstrip feeding line integrating a single-arm MEMS switch (a) and the annular slot integrating two dou‐ ble-arm MEMS actuators [15]

activated (down-state) by applying DC bias voltages, the antenna working band is around

**Figure 14.** Microstrip feeding line integrating a single-arm MEMS switch (a) and the annular slot

The measured |S11| parameters (Figure 15) show that when MEMS switches are activated (down-state) by applying DC bias voltages, the antenna working band is around 5.2 GHz. Viceversa, when MEMS switches are in the up-state, the antenna is working at 2.4 GHz.

*2.2.1. Ferroelectric materials*

solid solutions BNT-BT are emerging.

Lead-based perovskite ceramics such as PbZrxTi1−xO3 (PZT) have been the leaders, for the past 50 years, on ferroelectric material research [17] for electronic devices, sensors, actuators, and medical ultrasonic transducers, owing to their good dielectric properties over a wide temper‐ ature range. Due to health care and environmental regulations, restriction of hazardous substances as lead has been required [18]. Since the last 10 years, many efforts have been mainly devoted in the field of microwave applications to BaxSr1-xTiO3 (BST) material which is one of the most attractive materials [19] because it presents high dielectric constant, relatively low dielectric loss, interesting tunability and small temperature dependence. In fact, in BST, the Curie temperature (Tc) which defines the ferroelectric/paraelectric transition is tuned by controlling the Ba/Sr ratio. More recently some other ferroelectric ceramics such as the tantalate niobate oxide KTaxNb1−xO3 (KTN) or the sodium bismuth titanate Na0.5Bi0.5TiO3 (BNT) and its

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 95

**Figure 16.** Measured |S11|parameters for both gate and drain tuning voltages [16]

Two different methods exist to polarize a ferroelectric material with a static electric field. A better tunability is obtained when the static electric field is perpendicular to the two electrodes. For antenna application point of view, antenna designs integrating ferroelectric materials have to move toward this kind of polarization in order to have a better reconfigurability. However, many efforts have to be devoted from realization and also simulation point of views. Therefore,

V. K. Palukuru et al. [20] present a tunable antenna using an integrated ferroelectric-thick film made of BST material. The antenna is depicted in Figure 17. It exhibits a folded slot antenna loading with a BST thin film varactor. In order to tune the dielectric permittivity of the BST film, a DC bias voltage is applied thanks to a bias-T component, which was attached to the Vector Network Analyzer. The BST varactor is placed over the radiating slot: the upper electrode (0.2 mm 0.2 mm) is part of the antenna's metallization and the lower electrode is the antenna's ground plane. In order to reduce the capacitance of the varactor, a slight horizontal

compact antenna community exhibits only few papers of this kind of antennas.

**Figure 15.** Measured and simulated |S11|parameters for MEMS switches activated (5.2 GHz) and deactivated (2.4 GHz) [15]

#### *2.1.4. Field Effect Transistor*

Continuous frequency tuning can be achieved by using Field Effect Transistor. In [16], S. Kawasaki presents a slot antenna loaded with two one-port reactive FET. The electrically length of the slot is changing according to the voltage bias applying on the FET. The measured |S11| parameters (see Figure 16) show a 10% frequency tuning range for a gate tuning voltage (Vgs) going from 0V to-0.6V while the drain voltage (Vds) is tuned from 0V to 0.4V.

#### **2.2. Agile antennas using tunable materials**

Changing the material characteristics in a part of antenna designs also promise the ability to tune them in frequency. The application of a static electric field can be used to change the relative permittivity of a ferroelectric material or a liquid crystal, respectively a static magnetic field can changed the relative permeability of a ferrite. In case of printed antennas, these changes modify the effective electrical length of antennas, and then resulting in shifts of their operating frequencies.

**Figure 16.** Measured |S11|parameters for both gate and drain tuning voltages [16]

#### *2.2.1. Ferroelectric materials*

The measured |S11| parameters (Figure 15) show that when MEMS switches are activated (down-state) by applying DC bias voltages, the antenna working band is around 5.2 GHz. Vice-

**Figure 15.** Measured and simulated |S11|parameters for MEMS switches activated (5.2 GHz) and deactivated (2.4 GHz)

Continuous frequency tuning can be achieved by using Field Effect Transistor. In [16], S. Kawasaki presents a slot antenna loaded with two one-port reactive FET. The electrically length of the slot is changing according to the voltage bias applying on the FET. The measured |S11| parameters (see Figure 16) show a 10% frequency tuning range for a gate tuning voltage

Changing the material characteristics in a part of antenna designs also promise the ability to tune them in frequency. The application of a static electric field can be used to change the relative permittivity of a ferroelectric material or a liquid crystal, respectively a static magnetic field can changed the relative permeability of a ferrite. In case of printed antennas, these changes modify the effective electrical length of antennas, and then resulting in shifts of their

(Vgs) going from 0V to-0.6V while the drain voltage (Vds) is tuned from 0V to 0.4V.

[15]

*2.1.4. Field Effect Transistor*

94 Progress in Compact Antennas

operating frequencies.

**2.2. Agile antennas using tunable materials**

versa, when MEMS switches are in the up-state, the antenna is working at 2.4 GHz.

Lead-based perovskite ceramics such as PbZrxTi1−xO3 (PZT) have been the leaders, for the past 50 years, on ferroelectric material research [17] for electronic devices, sensors, actuators, and medical ultrasonic transducers, owing to their good dielectric properties over a wide temper‐ ature range. Due to health care and environmental regulations, restriction of hazardous substances as lead has been required [18]. Since the last 10 years, many efforts have been mainly devoted in the field of microwave applications to BaxSr1-xTiO3 (BST) material which is one of the most attractive materials [19] because it presents high dielectric constant, relatively low dielectric loss, interesting tunability and small temperature dependence. In fact, in BST, the Curie temperature (Tc) which defines the ferroelectric/paraelectric transition is tuned by controlling the Ba/Sr ratio. More recently some other ferroelectric ceramics such as the tantalate niobate oxide KTaxNb1−xO3 (KTN) or the sodium bismuth titanate Na0.5Bi0.5TiO3 (BNT) and its solid solutions BNT-BT are emerging.

Two different methods exist to polarize a ferroelectric material with a static electric field. A better tunability is obtained when the static electric field is perpendicular to the two electrodes. For antenna application point of view, antenna designs integrating ferroelectric materials have to move toward this kind of polarization in order to have a better reconfigurability. However, many efforts have to be devoted from realization and also simulation point of views. Therefore, compact antenna community exhibits only few papers of this kind of antennas.

V. K. Palukuru et al. [20] present a tunable antenna using an integrated ferroelectric-thick film made of BST material. The antenna is depicted in Figure 17. It exhibits a folded slot antenna loading with a BST thin film varactor. In order to tune the dielectric permittivity of the BST film, a DC bias voltage is applied thanks to a bias-T component, which was attached to the Vector Network Analyzer. The BST varactor is placed over the radiating slot: the upper electrode (0.2 mm 0.2 mm) is part of the antenna's metallization and the lower electrode is the antenna's ground plane. In order to reduce the capacitance of the varactor, a slight horizontal offset is used between the electrodes. Therefore, the electric field for biasing the material is both in the vertical and the horizontal directions.

H. Jiang presents a coplanar waveguide (CPW) square-ring slot antenna as showed Figure 19 [21]. Nine shunt ferroelectric BST thin film varactors are integrated with the CPW antenna

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 97

structure achieving both antenna miniaturization and reconfiguration.

**Figure 19.** Coplanar waveguide square-ring slot antenna integrating BST material [21]

*2.2.2. Liquid crystal*

Figure 20 shows the measured |S11|parameter with DC bias voltages from 0 V to 7 V. Therefore

Another technological approach for designing an agile antenna is the use of liquid crystal as a tunable dielectric. Indeed, the characterizations of liquid crystals [22-24] have shown that they are promising tunable materials for microwave applications, especially for operating frequencies above 10 GHz. The material features low dielectric loss and continuous tunability with low bias power consumption. The literature show that some microwave applications are using liquid crystals, e.g. for polarization agile antenna [25], tunable patch antennas [26],

In this framework, L. Liu presents in [34] a tunable patch antenna using a liquid crystal. Its operating frequency is around 5 GHz with a tuning range around 4% in measurement. The Figure 21 presents the antenna geometry composed of three layers of Taconic substrate. The liquid crystal is injected in the middle layer just under the microstrip patch and between the ground plane and patch top hat. A DC bias voltage was applied between the patch and ground

the antenna working band is continuously tuned from 5.28 GHz up to 5.77 GHz.

reflectarrays [27-28], filters [29], resonators [30] and variable delay lines [31-33].

across the liquid crystal using a bias tee at the feed input.

**Figure 17.** Folded slot antenna with the BST varactor (a), side cross-section (b) and top view (c) of the BST varactor [20]

The |S11| parameters (Figure 18) show that a frequency tunability of 3.5% can be obtained with a change of the bias voltage from 0V to 200V.

**Figure 18.** Measured |S11|parameters for different DC-bias voltages [20]

H. Jiang presents a coplanar waveguide (CPW) square-ring slot antenna as showed Figure 19 [21]. Nine shunt ferroelectric BST thin film varactors are integrated with the CPW antenna structure achieving both antenna miniaturization and reconfiguration.

**Figure 19.** Coplanar waveguide square-ring slot antenna integrating BST material [21]

Figure 20 shows the measured |S11|parameter with DC bias voltages from 0 V to 7 V. Therefore the antenna working band is continuously tuned from 5.28 GHz up to 5.77 GHz.

#### *2.2.2. Liquid crystal*

offset is used between the electrodes. Therefore, the electric field for biasing the material is

**Figure 17.** Folded slot antenna with the BST varactor (a), side cross-section (b) and top view (c) of the BST varactor

The |S11| parameters (Figure 18) show that a frequency tunability of 3.5% can be obtained with

both in the vertical and the horizontal directions.

96 Progress in Compact Antennas

a change of the bias voltage from 0V to 200V.

**Figure 18.** Measured |S11|parameters for different DC-bias voltages [20]

[20]

Another technological approach for designing an agile antenna is the use of liquid crystal as a tunable dielectric. Indeed, the characterizations of liquid crystals [22-24] have shown that they are promising tunable materials for microwave applications, especially for operating frequencies above 10 GHz. The material features low dielectric loss and continuous tunability with low bias power consumption. The literature show that some microwave applications are using liquid crystals, e.g. for polarization agile antenna [25], tunable patch antennas [26], reflectarrays [27-28], filters [29], resonators [30] and variable delay lines [31-33].

In this framework, L. Liu presents in [34] a tunable patch antenna using a liquid crystal. Its operating frequency is around 5 GHz with a tuning range around 4% in measurement. The Figure 21 presents the antenna geometry composed of three layers of Taconic substrate. The liquid crystal is injected in the middle layer just under the microstrip patch and between the ground plane and patch top hat. A DC bias voltage was applied between the patch and ground across the liquid crystal using a bias tee at the feed input.

can be achieved with relatively poor radiation efficiencies, i.e. 14% at least (at 0V) to 40% (at

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 99

Frequency-tuned ferrite-based antennas are rarely presented in the literature. In [35] and [36], authors study patch antennas on ferrite substrates (Figure 23) whereas A. Petosa presents in [37] a ferrite resonator antenna. In this latter, biasing of the ferrite with a static magnetic field is achieved using a permanent magnet. The magnet was located under the ferrite antenna beneath the ground plane. For a parallel magnetic-bias orientation, the resonance frequency can be tuned on 8% of bandwidth. That scales to 9% for a perpendicular magnetic-bias

All results presented in the literature and investigated the properties of ferrite-based microstrip antennas indicate that factors including non-uniform bias fields and the multiple modal field distributions excited in a bulk ferrite substrate may preclude their use in practical applications.

Now that an overview of the most common used techniques for compact antennas to become active has been presented, the next part will address both challenges and how to integrate

active components in order to maximize the antenna performances and efficiency.

**Figure 22.** Measured |S11|parameters for different DC-bias voltages [34]

*2.2.3. Ferrite materials*

orientation.

10V).

**Figure 20.** Measured |S11|parameters for different DC-bias voltages [21]

**Figure 21.** Tunable patch antenna using a liquid crystal [34]

The Figure 22 shows the measured return losses for the patch antenna for three states of liquid crystal bias: 0V, 5V and 10V. The 0V state reveals that the used liquid crystal without DC bias presents somewhat high losses around 0.12 for the loss tangent. Thus a 4% frequency tuning can be achieved with relatively poor radiation efficiencies, i.e. 14% at least (at 0V) to 40% (at 10V).

**Figure 22.** Measured |S11|parameters for different DC-bias voltages [34]

#### *2.2.3. Ferrite materials*

**Figure 21.** Tunable patch antenna using a liquid crystal [34]

**Figure 20.** Measured |S11|parameters for different DC-bias voltages [21]

98 Progress in Compact Antennas

The Figure 22 shows the measured return losses for the patch antenna for three states of liquid crystal bias: 0V, 5V and 10V. The 0V state reveals that the used liquid crystal without DC bias presents somewhat high losses around 0.12 for the loss tangent. Thus a 4% frequency tuning Frequency-tuned ferrite-based antennas are rarely presented in the literature. In [35] and [36], authors study patch antennas on ferrite substrates (Figure 23) whereas A. Petosa presents in [37] a ferrite resonator antenna. In this latter, biasing of the ferrite with a static magnetic field is achieved using a permanent magnet. The magnet was located under the ferrite antenna beneath the ground plane. For a parallel magnetic-bias orientation, the resonance frequency can be tuned on 8% of bandwidth. That scales to 9% for a perpendicular magnetic-bias orientation.

All results presented in the literature and investigated the properties of ferrite-based microstrip antennas indicate that factors including non-uniform bias fields and the multiple modal field distributions excited in a bulk ferrite substrate may preclude their use in practical applications.

Now that an overview of the most common used techniques for compact antennas to become active has been presented, the next part will address both challenges and how to integrate active components in order to maximize the antenna performances and efficiency.

datasheet, a complete characterization meeting antenna designer's criteria will be explained. That will lead to the determination of the varactor diode S parameters. With the knowledge of the latter, two methods will be explained and described to reach an accurate antenna

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 101

**•** The electromagnetic equivalent circuit of the varactor diode can be deduced from the S

**•** The S parameters of the varactor diode can be directly injected in the electromagnetic simulator CST Microwave Studio® and the antenna performances deduced thanks to a co-

The varactor diode is a tunable capacitor which loads the antenna in order to artificially increase its electrical length. To be the most efficient, it must be placed where the electrical field is maximum. Take a folded Inverted F Antenna for example presented Figure 24. The maximum of the electrical field is at the end of the radiating element (see Figure 24). To be integrated at this place and joined the ground plane at the same time, the varactor diode can be soldered between the ribbon and the ground. In most cases, DC-block capacitors have to

integrated at this place and joined the ground plane at the same time, the varactor diode can be soldered between the ribbon and the ground. In most cases, DC-block capacitors have to

(a) (b) Figure 24. Inverted F Antenna Measured (a) and the total electric field on the radiating element (b)

**Figure 24.** Inverted F Antenna Measured (a) and the total electric field on the radiating element (b)

In [10], the chosen GaAs hyperabrupt varactor diode MGV125-22 (Aeroflex Metelics) [38] has a capacity range between 0.2 pF and 2 pF for 0 to 22 Volts tuning voltage as shown Figure 25. However, these values are given as a rough line and the datasheet does not give enough parameters for high frequencies antenna application's point of view. Indeed, the values for junction capacitance Cj (Figure 25) and the quality factor Q are supplied by the manufacturer and are almost always specified at a low frequency. The Figure 25 shows the widely used varactor diode model with Lp and Cp the values of the package inductance and

In [10], the chosen GaAs hyperabrupt varactor diode MGV125-22 (Aeroflex Metelics) [38] has a capacity range between 0.2 pF and 2 pF for 0 to 22 Volts tuning voltage as shown Figure 25. However, these values are given as a rough line and the datasheet does not give enough parameters for high frequencies antenna application's point of view. Indeed, the values for

(a) (b)

**Manufacturer's capacitance values @ 1 MHz** Lp Cj Rs

Port 1 50 Ω

(Figure 25) and the quality factor Q are supplied by the manufacturer

Port 2 50 Ω

Cp

simulation and realization:

simulation.

parameters thanks to Agilent ADS.

*3.1.1. Varactor diode area in an antenna*

be added for the varactor's DC-bias not to be shunt.

be added for the varactor's DC-bias not to be shunt.

**3.1.2. Varactor diode datasheet**

*3.1.2. Varactor diode datasheet*

junction capacitance Cj

**<sup>0</sup> <sup>5</sup> <sup>10</sup> <sup>15</sup> <sup>20</sup> <sup>0</sup>**

**DC bias voltage (V)**

capacitance.

**0.5**

**1**

**Junction capacitance value Cj (pF)**

**1.5**

**2**

**Figure 23.** Patch antenna on a ferrite substrate [37]
