**3. Relevant parameter for frequency agile antenna studies – New approach for wireless applications.**

#### **3.1. How to integrate an active component – Challenges**

To implement the reconfigurability in an antenna, the knowledge of the active component or the tunable material is essential. Even in case of commercial components, as varactor or PIN diodes, users and particularly the microwave community do not have enough parameters and information at RF frequencies. Some papers detail the integration of the RF equivalent circuit of the used component [10],[13].

The varactor diodes integrated in an antenna for frequency reconfigurability is the most popular way. Thus, this section will focus on varactor diodes issues. However, arguments can be extended to other frequency tuning methods.

In [10], the paper completes the lack of information related to most of varactor diode data‐ sheets. Indeed, constructor only provide characteristics at low frequencies and do not give enough parameters for antenna application point of view, such as capacitance values, serial resistance and accepted power at RF frequencies. The chosen varactor in this paper has been characterized according to antenna designer criteria and its electromagnetic model has been deduced. This example is chosen in the framework of this chapter.

To correctly explain this example, the next subsection will investigate the place where the varactor can be integrated. Following the presentation of the varactor diode manufacturer's 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 simulation and realization:


#### *3.1.1. Varactor diode area in an antenna*

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

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

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)

#### **3.1.2. Varactor diode datasheet** *3.1.2. Varactor diode datasheet*

**<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**

**3. Relevant parameter for frequency agile antenna studies – New approach**

To implement the reconfigurability in an antenna, the knowledge of the active component or the tunable material is essential. Even in case of commercial components, as varactor or PIN diodes, users and particularly the microwave community do not have enough parameters and information at RF frequencies. Some papers detail the integration of the RF equivalent circuit

The varactor diodes integrated in an antenna for frequency reconfigurability is the most popular way. Thus, this section will focus on varactor diodes issues. However, arguments can

In [10], the paper completes the lack of information related to most of varactor diode data‐ sheets. Indeed, constructor only provide characteristics at low frequencies and do not give enough parameters for antenna application point of view, such as capacitance values, serial resistance and accepted power at RF frequencies. The chosen varactor in this paper has been characterized according to antenna designer criteria and its electromagnetic model has been

To correctly explain this example, the next subsection will investigate the place where the varactor can be integrated. Following the presentation of the varactor diode manufacturer's

**for wireless applications.**

100 Progress in Compact Antennas

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

of the used component [10],[13].

**3.1. How to integrate an active component – Challenges**

be extended to other frequency tuning methods.

deduced. This example is chosen in the framework of this chapter.

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 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

widely used varactor diode model with Lp and Cp the values of the package inductance and

(a) (b)

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

Port 1 50 Ω

Port 2 50 Ω

Cp

capacitance.

**3.1.2. Varactor diode datasheet**

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 capacitance. 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

enough parameters for high frequencies antenna application's point of view. Indeed, the

In [10], the chosen GaAs hyperabrupt varactor diode MGV125-22 (Aeroflex Metelics) [38]

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

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

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

**Figure 25.** Manufacturer's capacitance value (extracted at 1MHz) versus the DC bias voltage (a) and the Varactor di‐ ode model (b)

Lp=3.821nH, Cp=0.08pF and Rs=1.8Ω. The Cj

function of DC bias voltage value.

Lp=3.821 nH Cj Rs=1.8Ω Port 1

**Figure 27.** Electromagnetic model with the capacitance values Cj

Cp=0.08 pF

antenna electromagnetic simulation as illustrated in Figure 28.

**Figure 28.** Co-simulation of the antenna including measured S parameters of the diode

for 2 Volts and 10 Volts DC bias voltages.

**Microstrip line Varactor diode** *TRL calibration kit*

the measurement on S11 parameter on the [400 MHz – 1 GHz] frequency band (b)

**-0.2**

**0.2**

**-0.4**

of DC bias voltage value.

50 Ω

**•** Second method: Co-simulation

value presented in Figure 27 decreases as a function

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

versus the DC bias voltage (b)

**Voltage (V)**

**‐1.4**

**-1.4 -1**

=1.81 pF **V=10V**

=1.81 pF **V=10V**

**1.4**

**V=2V Cj=1.81 pF**

**V=2V Cj=1.81 pF**

**3**

**3**

=0.37 pF

=0.37 pF

**0.60.8 1 2 3**

**2**

**2**

**1.4**

**1.4**

**0.8**

**0.8**

**1**

Simulated for Cj

**1**

Simulated for Cj

**0.2 0.4 0.8 1.4**

Measured for V=10V Measured for V=2V Simulated for Cj

Measured for V=10V Measured for V=2V Simulated for Cj

**0.2 0.4 0.8 1.4**

**0.2 0.4 0.8 1.4**

**0.6**

**0.6**

**0.4**

**0.4**

**0.2**

**0.2**

**‐0.2**

**-0.2**

**-0.6**

**‐0.4**

**-0.4**

**‐3**

**-3**

**‐2**

**-2**

**‐1.4 ‐1**

**-1.4 -1**

**0.60.8 1 2 3**

**0.60.81 2 3**

**‐0.8**

**-0.8**

**-0.8**

**‐0.6**

**-0.6**

**‐3**

**-3**

**Cj=0.37 pF**

**Cj=0.37 pF**

**3**

103

**2**

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

**‐2**

**-2**

The varactor diode is soldered on a 50 Ω impedance microstrip line as shown Figure 26. A dedicated TRL (Through-Reflect-Line) calibration kit is manufactured (Figure 26) in order to de-embed both connectors and lines. Thus S parameters of the single varactor diode can be deduced. Considering the varactor diode model previously presented in Figure 26, Cj, Lp, Cp and Rs values can be deduced for each voltage and for a constant injected power of -10 dBm. Therefore, model component values are adjusted (see Figure 26) in order their S parameters to correspond with the measured ones. The Figure 26 shows the comparison between S parameters of the determined electromagnetic model (Agilent ADS) and the measured ones

**0.6**

**0.4**

**0.8**

**1**

(a) (b) Figure 26. Characterization of the varactor diode (a) and the Comparison between the electromagnetic model and the measurement on S11 parameter on the [400 MHz – 1 GHz] frequency band (b)

**Figure 26.** Characterization of the varactor diode (a) and the Comparison between the electromagnetic model and

(a) (b)

These results are given as an example and the same work has been done for varactor reverse bias voltages varying from 2V to 22V with a 2V step. As expected, the corresponding electromagnetic model presents constant values according to the DC bias voltage (Figure 27): Lp=3.821nH, Cp=0.08pF and Rs=1.8Ω. The Cj value presented in Figure 27 decreases as a

(a) (b)

Another way is to directly insert the S parameters touchstone file of the varactor diode in the

**0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2**

**Capacitance value (pF)**

Port 2 50 Ω

In the MGV125-22 case, junction capacitance values are specified at 1 MHz and the Q factor equals 3000 at 50 MHz for a DC bias voltage of –4 Volts. Q is defined by Q=1/(ωC<sup>j</sup> Rs). This formula can be used to calculate the series resistance Rs of the varactor model at the measured frequency, its value is assumed to be constant with reverse voltage. Thus, at 50 MHz Rs=1.06Ω. It is important to note that Rs impacts directly the antenna total efficiency. A too high value (from 3Ω) is basically penalizing for antenna performances. This enhances the need to assess its value at microwave frequencies. For this purpose, the hyperabrupt varactor diode has to be characterized close to operating conditions (here between 470 MHz and 862 MHz).

#### *3.1.3. Varactor diode characterization*

#### **•** First method: Electromagnetic model

The varactor diode is soldered on a 50 Ω impedance microstrip line as shown Figure 26. A dedicated TRL (Through-Reflect-Line) calibration kit is manufactured (Figure 26) in order to de-embed both connectors and lines. Thus S parameters of the single varactor diode can be deduced. Considering the varactor diode model previously presented in Figure 26, Cj , Lp, Cp and Rs values can be deduced for each voltage and for a constant injected power of-10 dBm. Therefore, model component values are adjusted (see Figure 26) in order their S parameters to correspond with the measured ones. The Figure 26 shows the comparison between S parameters of the determined electromagnetic model (Agilent ADS) and the measured ones for 2 Volts and 10 Volts DC bias voltages.

These results are given as an example and the same work has been done for varactor reverse bias voltages varying from 2V to 22V with a 2V step. As expected, the corresponding electro‐ magnetic model presents constant values according to the DC bias voltage (Figure 27):

**2**

**1.4**

**3**

The varactor diode is soldered on a 50 Ω impedance microstrip line as shown Figure 26. A dedicated TRL (Through-Reflect-Line) calibration kit is manufactured (Figure 26) in order to de-embed both connectors and lines. Thus S parameters of the single varactor diode can be deduced. Considering the varactor diode model previously presented in Figure 26, Cj, Lp, Cp

**0.6**

**0.4**

**0.2**

**0.8**

**1**

parameters of the determined electromagnetic model (Agilent ADS) and the measured ones

Figure 26. Characterization of the varactor diode (a) and the Comparison between the electromagnetic model and the measurement on S11 parameter on the [400 MHz – 1 GHz] frequency band (b) **Figure 26.** Characterization of the varactor diode (a) and the Comparison between the electromagnetic model and the measurement on S11 parameter on the [400 MHz – 1 GHz] frequency band (b)

These results are given as an example and the same work has been done for varactor reverse

Lp=3.821nH, Cp=0.08pF and Rs=1.8Ω. The Cj value presented in Figure 27 decreases as a function of DC bias voltage value. bias voltages varying from 2V to 22V with a 2V step. As expected, the corresponding electromagnetic model presents constant values according to the DC bias voltage (Figure 27): Lp=3.821nH, Cp=0.08pF and Rs=1.8Ω. The Cj value presented in Figure 27 decreases as a

**Figure 27.** Electromagnetic model with the capacitance values Cj versus the DC bias voltage (b)

**•** Second method: Co-simulation

function of DC bias voltage value.

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 capacitance.

(a) (b)

**Figure 25.** Manufacturer's capacitance value (extracted at 1MHz) versus the DC bias voltage (a) and the Varactor di‐

In the MGV125-22 case, junction capacitance values are specified at 1 MHz and the Q factor

formula can be used to calculate the series resistance Rs of the varactor model at the measured frequency, its value is assumed to be constant with reverse voltage. Thus, at 50 MHz Rs=1.06Ω. It is important to note that Rs impacts directly the antenna total efficiency. A too high value (from 3Ω) is basically penalizing for antenna performances. This enhances the need to assess its value at microwave frequencies. For this purpose, the hyperabrupt varactor diode has to

The varactor diode is soldered on a 50 Ω impedance microstrip line as shown Figure 26. A dedicated TRL (Through-Reflect-Line) calibration kit is manufactured (Figure 26) in order to de-embed both connectors and lines. Thus S parameters of the single varactor diode can be

and Rs values can be deduced for each voltage and for a constant injected power of-10 dBm. Therefore, model component values are adjusted (see Figure 26) in order their S parameters to correspond with the measured ones. The Figure 26 shows the comparison between S parameters of the determined electromagnetic model (Agilent ADS) and the measured ones

These results are given as an example and the same work has been done for varactor reverse bias voltages varying from 2V to 22V with a 2V step. As expected, the corresponding electro‐ magnetic model presents constant values according to the DC bias voltage (Figure 27):

equals 3000 at 50 MHz for a DC bias voltage of –4 Volts. Q is defined by Q=1/(ωC<sup>j</sup>

be characterized close to operating conditions (here between 470 MHz and 862 MHz).

deduced. Considering the varactor diode model previously presented in Figure 26, Cj

Port 1 50 Ω

Lp Cj Rs

Cp

Port 2 50 Ω

Rs). This

, Lp, Cp

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

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

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

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

**3.1.2. Varactor diode datasheet**

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

*3.1.3. Varactor diode characterization*

**•** First method: Electromagnetic model

for 2 Volts and 10 Volts DC bias voltages.

**DC bias voltage (V)**

**Manufacturer's capacitance values @ 1 MHz**

capacitance.

102 Progress in Compact Antennas

**0.5**

**1**

**Junction capacitance value Cj (pF)**

ode model (b)

**1.5**

**2**

Another way is to directly insert the S parameters touchstone file of the varactor diode in the antenna electromagnetic simulation as illustrated in Figure 28.

**Figure 28.** Co-simulation of the antenna including measured S parameters of the diode

Thanks to the previous TRL calibration, only the varactor diode S parameters are inserted in the simulator. By this way, both antenna and varactor diode are combined and the S parameters of the global device can be directly simulated. An example (presented paragraph 3.3) will confirm that the two previous methods exhibit similar antenna performances. **2 1.4 1 0.8 0.6 0.4**

#### **3.2. Limitations of currently varactor diode method – Power characterization 3**

Figure 29 provides some information regarding the accepted power by the varactor diode: high injected power involves some varactor diode distortions. **0.2**

ing the antenna total efficiency, it equals 50% in the real case whereas it reaches 60% when the

It has been measured for a -10 dBm RF power and its performances compared with three simulations: with both presented methods and with the varactor diode's datasheet (Figure 31). |S11| parameters show that both investigated methods and measurement present a good agreement. Moreover, they are different from |S11| parameters determined with the varactor diode datasheet. That underlines the relevance of the varactor diode characterization. Regarding the antenna total efficiency, it equals 50% in the real case whereas it reaches 60%

**Vias**

**RF + DC**

**DC block capacitor**

**Varactor**

Miniature Antenna with Frequency Agility http://dx.doi.org/10.5772/58838 105

Power characterization is illustrated on Figure 31. This figure reminds the measured |S11| parameter for a power of-10 dBm. For a 10 dBm injected power, the measured |S11| parameter

Power characterization is illustrated on Figure 31. This figure reminds the measured |S11| parameter for a power of -10 dBm. For a 10 dBm injected power, the measured |S11|

(a) (b)

There is a good agreement between the measurement and the simulation. This figure shows that the non-linearity of the varactor diode disturbs the |S11| parameter of the antenna.

There is a good agreement between the measurement and the simulation. This figure shows that the non-linearity of the varactor diode disturbs the |S11| parameter of the antenna. Thus,

Therefore, this section has presented how to integrate and characterize a varactor diode. It reveals the importance of the diode characterization in a design flow dedicated to antenna

Therefore, this section has presented how to integrate and characterize a varactor diode. It reveals the importance of the diode characterization in a design flow dedicated to antenna

Figure 31. |S11| parameters for the different methods (a) and according to the injected power (b)

Thus, this kind of varactor diode has to be used only for reception devices.

this kind of varactor diode has to be used only for reception devices.

**Figure 31.** |S11| parameters for the different methods (a) and according to the injected power (b)

varactor datasheet is used in electromagnetic simulations.

Figure 30. Basic IFA prototype

**Figure 30.** Basic IFA prototype

structure.

structure.

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

**|S11| (dB)**

is compared with the simulated one with the second method.

**0.6 0.62 0.64 0.66 0.68 0.7 -35**

**Frequency (GHz)**

when the varactor datasheet is used in electromagnetic simulations.

parameter is compared with the simulated one with the second method.

**First method Second method Measurement Datasheet**

**Figure 29.** Measured S parameters according to the injected power

**-0.2**

This figure presents S parameters of the diode for only three values of injected power:-10 dBm, 0 dBm and 10 dBm. The non-linear distortion of the diode has been studied. It reveals that the varactor diode model well fits measurements for an injected RF power lower than-5 dBm. Beyond this injected power (see for 0 dBm), no varactor model can fit the measurement. Regarding antenna's parameters, the following example will show that a large RF power (upper than-5 dBm) involves a mismatched antenna. As far as the DVB-H application, the system is only working in receiving mode, the diode distortion will never appear and the linear electromagnetic model can be used and integrated in the electromagnetic simulator. Indeed, for receiver devices, the antenna accepted power is far lower than – 5 dBm.

#### **3.3. Example of a basic tunable DVB-H antenna**

This subsection investigates an example to show the interest of the previous varactor diode characterization. This was briefly presented in [4], it is completed here by adding the first method (electromagnetic model) and the power characterization. A basic IFA prototype loaded by the same varactor diode (see Figure 30) has been manufactured.

It has been measured for a-10 dBm RF power and its performances compared with three simulations: with both presented methods and with the varactor diode's datasheet (Figure 31). |S11| parameters show that both investigated methods and measurement present a good agreement. Moreover, they are different from |S11| parameters determined with the varactor diode datasheet. That underlines the relevance of the varactor diode characterization. Regard‐

simulations: with both presented methods and with the varactor diode's datasheet (Figure 31). |S11| parameters show that both investigated methods and measurement present a good **Figure 30.** Basic IFA prototype

Thanks to the previous TRL calibration, only the varactor diode S parameters are inserted in the simulator. By this way, both antenna and varactor diode are combined and the S parameters of the global device can be directly simulated. An example (presented paragraph 3.3) will

**1**

Figure 29 provides some information regarding the accepted power by the varactor diode:

**3**

**2**

**1.4**

**-3**

**-2**

**-1.4**

confirm that the two previous methods exhibit similar antenna performances.

**0.8**

**3.2. Limitations of currently varactor diode method – Power characterization**

**3**

**2**

**1.4**

**-0.20.2 0.4 0.60.8 <sup>1</sup> 1.4 <sup>2</sup> <sup>3</sup>**

Injected power = -10 dBm Injected power = 10 dBm Injected power = 0 dBm

**1**

**0.8**

**0.6**

**0.4**

**0.2**

**0.4**

**0.2**

104 Progress in Compact Antennas

**-0.2**

**-0.2**

**-0.4**

**-0.4**

**Figure 29.** Measured S parameters according to the injected power

**3.3. Example of a basic tunable DVB-H antenna**

**-3**

**-1**

This figure presents S parameters of the diode for only three values of injected power:-10 dBm, 0 dBm and 10 dBm. The non-linear distortion of the diode has been studied. It reveals that the varactor diode model well fits measurements for an injected RF power lower than-5 dBm. Beyond this injected power (see for 0 dBm), no varactor model can fit the measurement. Regarding antenna's parameters, the following example will show that a large RF power (upper than-5 dBm) involves a mismatched antenna. As far as the DVB-H application, the system is only working in receiving mode, the diode distortion will never appear and the linear electromagnetic model can be used and integrated in the electromagnetic simulator. Indeed,

This subsection investigates an example to show the interest of the previous varactor diode characterization. This was briefly presented in [4], it is completed here by adding the first method (electromagnetic model) and the power characterization. A basic IFA prototype loaded

It has been measured for a-10 dBm RF power and its performances compared with three simulations: with both presented methods and with the varactor diode's datasheet (Figure 31). |S11| parameters show that both investigated methods and measurement present a good agreement. Moreover, they are different from |S11| parameters determined with the varactor diode datasheet. That underlines the relevance of the varactor diode characterization. Regard‐

**-2**

**-1.4**

for receiver devices, the antenna accepted power is far lower than – 5 dBm.

by the same varactor diode (see Figure 30) has been manufactured.

**-0.8**

**-1**

**0.2 0.4 0.60.8 <sup>1</sup> 1.4 <sup>2</sup> <sup>3</sup>**

**-0.8**

**-0.6**

**-0.6**

high injected power involves some varactor diode distortions.

**0.6**

ing the antenna total efficiency, it equals 50% in the real case whereas it reaches 60% when the varactor datasheet is used in electromagnetic simulations. agreement. Moreover, they are different from |S11| parameters determined with the varactor diode datasheet. That underlines the relevance of the varactor diode characterization. Regarding the antenna total efficiency, it equals 50% in the real case whereas it reaches 60%

Power characterization is illustrated on Figure 31. This figure reminds the measured |S11| parameter for a power of-10 dBm. For a 10 dBm injected power, the measured |S11| parameter is compared with the simulated one with the second method. when the varactor datasheet is used in electromagnetic simulations. Power characterization is illustrated on Figure 31. This figure reminds the measured |S11|

parameter for a power of -10 dBm. For a 10 dBm injected power, the measured |S11|

There is a good agreement between the measurement and the simulation. This figure shows **Figure 31.** |S11| parameters for the different methods (a) and according to the injected power (b)

parameter is compared with the simulated one with the second method.

Thus, this kind of varactor diode has to be used only for reception devices. Therefore, this section has presented how to integrate and characterize a varactor diode. It There is a good agreement between the measurement and the simulation. This figure shows that the non-linearity of the varactor diode disturbs the |S11| parameter of the antenna. Thus, this kind of varactor diode has to be used only for reception devices.

that the non-linearity of the varactor diode disturbs the |S11| parameter of the antenna.

Figure 31. |S11| parameters for the different methods (a) and according to the injected power (b)

reveals the importance of the diode characterization in a design flow dedicated to antenna structure. Therefore, this section has presented how to integrate and characterize a varactor diode. It reveals the importance of the diode characterization in a design flow dedicated to antenna structure.

#### **3.4. Discussion and trade-offs between agility techniques and physical limitations**

According to the aimed application, trade-offs are necessary to design a frequency tunable antenna.

**References**

1789

2789-2797, Dec. 2008

pp.1,3, 12-16 April 2010

77,80, 2012

Feb. 2006

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[3] Li, Y., Zhang, Z., Chen, W., Feng, Z. and Iskander, M. F. (2010), A compact DVB-H antenna with varactor-tuned matching circuit. Microw. Opt. Technol. Lett., 52: 1786–

[4] Huitema, L.; Reveyrand, T. et al., "A compact and reconfigurable DVB-H antenna for mobile handheld devices," *Antennas and Propagation (EUCAP), Proceedings of the 5th*

[5] Canneva, F.; Ribero, J.; Staraj, R., "Tunable antenna for DVB-H band," *Antennas and Propagation (EuCAP), 2010 Proceedings of the Fourth European Conference on*, vol., no.,

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[8] Behdad, N.; Sarabandi, K., "A varactor-tuned dual-band slot antenna," *Antennas and*

[9] Behdad, N.; Sarabandi, K., "Dual-band reconfigurable antenna with a very wide tun‐ ability range," *Antennas and Propagation, IEEE Transactions on*, vol.54, no.2, pp.409,416,

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[11] Laheurte, J.-M., "Switchable CPW-fed slot antenna for multifrequency operation,"

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2002). Nice, France, 12-14 November 2002, volume 2, pages 53-56

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*European Conference on*, vol., no., pp.1314-1317, 11-15 April 2011


However, previous paragraphs have revealed that varactor diodes are not usable for trans‐ mitter devices because of their non-linearity for considering power levels.

The RF characterization of ferroelectric films shows high power handling capability [39]. Good permittivity tunability may be obtained if both materials properties and variable capacitor sizes have been properly dimensioned. The conclusion is the same for MEMS variable capacitor.

Studies on both ferroelectric material and MEMS capacitor merit extended investigations because these solutions seem to be the best and most promising alternatives faced with varactor diodes.

Performing rigorous full-wave analysis of these new components is the new challenge to extract their accurate electromagnetic models. Antennas would be optimized by considering the real response of these components. These investigations would enable the co-development of antennas integrating components' electromagnetic models.

### **4. Conclusion**

To conclude, an overview of compact and frequency agile antenna has been presented and detailed in this chapter while mentioning a lot of literature references. A special part has been dedicated to the presentation of the most common method to achieve a frequency tuning: the use of varactor diodes. Their integration within an antenna to be the most efficient has been shown. The study exhibits varactor diodes characterization and also reveals their limitation. A summary of the presented methods according to the intended application has been pre‐ sented. Eventually, some ideas on varator diodes alternatives have been proposed in order to make antenna tunability viable for transmitter devices.

### **Author details**

L. Huitema and T. Monediere

University of Limoges, Xlim Laboratory, France

#### **References**

**3.4. Discussion and trade-offs between agility techniques and physical limitations**

**•** For discrete frequency tuning, PIN diodes or MEMS switches can be planed.

diodes, MEMS variable capacitor and tunable materials can be used.

mitter devices because of their non-linearity for considering power levels.

of antennas integrating components' electromagnetic models.

make antenna tunability viable for transmitter devices.

University of Limoges, Xlim Laboratory, France

antenna.

106 Progress in Compact Antennas

capacitor.

diodes.

**4. Conclusion**

**Author details**

L. Huitema and T. Monediere

According to the aimed application, trade-offs are necessary to design a frequency tunable

**•** For continuous frequency tuning, which is often aiming for compact antennas, varactor

However, previous paragraphs have revealed that varactor diodes are not usable for trans‐

The RF characterization of ferroelectric films shows high power handling capability [39]. Good permittivity tunability may be obtained if both materials properties and variable capacitor sizes have been properly dimensioned. The conclusion is the same for MEMS variable

Studies on both ferroelectric material and MEMS capacitor merit extended investigations because these solutions seem to be the best and most promising alternatives faced with varactor

Performing rigorous full-wave analysis of these new components is the new challenge to extract their accurate electromagnetic models. Antennas would be optimized by considering the real response of these components. These investigations would enable the co-development

To conclude, an overview of compact and frequency agile antenna has been presented and detailed in this chapter while mentioning a lot of literature references. A special part has been dedicated to the presentation of the most common method to achieve a frequency tuning: the use of varactor diodes. Their integration within an antenna to be the most efficient has been shown. The study exhibits varactor diodes characterization and also reveals their limitation. A summary of the presented methods according to the intended application has been pre‐ sented. Eventually, some ideas on varator diodes alternatives have been proposed in order to


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**Chapter 5**

**Active Compact Antenna for Broadband Applications**

The recent development of wireless communication technology and the miniaturization of electronics components increase the demand for compact systems applications including small antennas. One of the major challenges is the integration of antennas inside devices in a limited area. The main characteristics of these antennas are large frequency bandwidth and small size. Many passive antennas such as monopole, dipole and printed antenna have been largely studied to yield small size relatively to the wavelength or broadband behavior. Previous studies have shown that antenna miniaturization impacts negatively antenna bandwidth and

The active antennas have found a wide interest for industrial applications in last years. The terminology of the active antenna indicates that the passive antenna elements are combined with an active device on the same substrate to provide a non-separated device and to improve antenna performances, especially in the field of size reduction and frequency bandwidth. The ability to adjust the size reduction and the frequency bandwidth of an active antenna is also very suitable when the antenna is included inside devices with many components located in

In this chapter, we are interested in the improvements brought by the active antennas towards size reduction and the covered bandwidth. We present our work on compacts actives antennas in which we develop new techniques to reduce the size of antennas with good performances. Our main works is carried on two solutions; the first one is a broadband antenna with a very important size reduction. This solution corresponds to an active printed monopole integrating a bipolar transistor directly on the antenna structure without matching circuit. The second solution is a tunable narrow frequency band antenna operating on a wide band based on a

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Y. Taachouche, M. Abdallah, F. Colombel,

Additional information is available at the end of the chapter

printed loop antenna associated to a varactor diode.

G. Le Ray and M. Himdi

http://dx.doi.org/10.5772/58839

**1. Introduction**

impedance matching [1].

a limited area.

