диафрагмированные волноводные фильтры / e2f04c5f-bd8e-489d-b35c-51b7fbfe89ec

Abstract—A new incorporated planar ultrawideband (UWB)/re- configurable-slot antenna is proposed for cognitive radio applications. A slot resonator is precisely embedded in the disc monopole radiator to achieve an individual narrowband antenna. A varactor diode is also deliberately inserted across the slot, providing a reconfigurable frequency function in the range of 5–6 GHz. The slot is fed by an off-centered microstrip line that creates the desired matching across the tunable frequency band. The measured antenna parameters are presented and discussed, confirming the simulation results.

Index Terms—Cognitive radio (CR), frequency-reconfigurable, slot antenna, ultrawideband (UWB) antenna, varactor diode.

I. INTRODUCTION

CURRENT communication networks operate by Fixed Spectrum Access (FSA) policy, in which some parts of the available spectrum have been assigned to one or more

users, and other users do not have the permission to use the dedicated band. With developing wireless communications and increasing demands for operating frequency bands, the radio spectrum has become scarce and congested. However, licensed users act sporadically within their assigned band such that, most of the time, operating frequency region can be idle. To efficiently use the available spectrum band and hence address the spectrum congestion, cognitive radio networks (CRNs) based on Dynamic Spectrum Access (DSA) and spectrum management technique have been proposed [1].

CRNs have two types of users: 1) primary users with licenses to act in certain spectrum bands; 2) CR users (secondary users) that have no allocated bands. CR users are considered as visitors to use idle frequency region portions of primary users in real time [2]. Hence, they should have comprehensive awareness from licensed and unlicensed bands in its operating environment. Through sensing and measurement of interference temperature and level of signal energy [2], CR can detect hole

Manuscript received September 28, 2011; revised November 24, 2011; accepted December 30, 2011. Date of publication January 04, 2012; date of current version March 19, 2012.

E. Erfani, J. Nourinia, and C. Ghobadi are with the Department of Electrical Engineering, Urmia University, Urmia, Iran (e-mail: el.erfani@yahoo. com; j.nourinia@urmia.ac.ir; ch.ghobadi@urmia.ac.ir).

M. Niroo-Jazi and T. A. Denidni are with INRS, Université de Québec, Montreal, QC H5A 1K6, Canada (e-mail: njazi@emt.inrs.ca; denidni@emt.inrs.ca).

Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2011.2182631

spectrums (white spectrums) in any time and select the best band region according to the quality-of-service (QoS) requirements. As soon as this portion of spectrum is requested to use by the primary user, the CR user should vacant this frequency band and reconfigure its transmission parameters to achieve a new suitable hole spectrum.

In the aspect of hardware, CR needs a specialized antenna for monitoring and communicating. In the recent years, different antenna designs for CR application have been reported in [3]–[7]. An ultrawideband (UWB) and a reconfigurable narrowband antenna can be chosen to handle sensing and communication functions, respectively. A technique based on integrating these two antennas into a same substrate has been proposed in [3] and [4]. In [3], as a narrowband antenna, a planar inverted-F resonator is printed on the reverse side of a coplanar waveguide (CPW)-fed UWB monopole antenna, and it utilizes the radiator of UWB as its own ground plane. In this structure, a matching circuit has been used to tune this antenna for three operating regions centered around 4, 8, and 10 GHz, leading to increase the antenna complexity and size. In [4], a UWB egg-shaped monopole antenna and five different narrowband patch radiators inside a circular section are printed on a same substrate. The operating frequency of this antenna can be adjusted by physically rotating the circular part via a stepper motor. At each rotation step, a certain frequency band is obtained by feeding an individual patch radiator. However, using a stepper motor requires more space and increases the complexity and cost of the antenna.

In another technique, sensing and communicating antennas are realized by switching between a narrowband antenna and a UWB resonator [5]–[7]. The antenna structure is fed by a single terminal in this case. This method is achieved by two ways:

1)incorporating a bandpass filter inside a UWB antenna [5], [6];

2)changing the structure of the antenna radiator or ground plane via switches [7]. A reconfigurable bandpass filter is integrated with a UWB antenna in [5]. The reconfigurability is based on incorporating nine switches within the defected microstrip structure (DMS) bandpass filter.

Today, in mobile systems, a compact antenna with high performance features, low cost, and less complexity is demanded. In this letter, a new antenna configuration for CRNs is presented. This antenna is constructed by incorporating a reconfigurable slot resonator into an UWB antenna in a unique substrate, while the antenna size is kept small. Simulated and experimental results of the fabricated prototype are presented and compared.

Fig. 1. Configuration of the proposed UWB/reconfigurable narrowband antenna. (a) Top view. (b) Bottom view.

II. ANTENNA CONFIGURATION AND DESIGN

To sense channels, identify “white spectrums,” and communicate in a CR system, an antenna consists of two parts: a UWB radiator as a spectrum sensor and a reconfigurable narrowband resonator for communicating. Our target is to integrate these two individual radiators in a same substrate. Therefore, an elliptical disc monopole is chosen as an omnidirectional radiation pattern UWB radiator, covering the entire band from 3 to 11 GHz, and a reconfigurable narrow slot resonator is used for communicating in the operating range of 5–6 GHz. By accurate examination of disc monopole current distribution, it is observed that the center part of the disc has low current density. Therefore, the narrowband antenna can be incorporated into the resonator disc without degrading the sensing antenna performance.

The proposed antenna is shown in Fig. 1, in which an elliptical disc fed by a microstrip line is printed on a 40 36 mm RO4350B substrate (, ) with thickness of 0.662 mm. A partial ellipse, with major and minor radii of mm and mm, is etched on the bottom layer as a ground plane. By controlling the distance between the ground plane and UWB radiator and also shaping the ground plane, the antenna impedance matching is optimized. Furthermore, a step-fed matching technique is used in the feeding line to control the input impedance across the desired band.

According to the required communication band, the narrow slot antenna is designed for the first slot dominant mode. To match the radiation resistance of this mode to 50- input impedance, an offset fed technique is used for a stub-loaded microstrip feed line. In addition, a symmetric stub is used inside the slot to reduce the effective length of the resonant slot by folding the slot current distribution. This leads to better isolation between two antennas by increasing the distance between current distributions of resonators. A variable capacitor is precisely placed across the slot to reconfigure its operating frequency. An isolated pad is created inside the slot to accommodate the varactor diode. This pad is connected to another pad in the opposite side of the substrate through a via, preparing the dc-biasing line of the variable capacitor through a resistor. Both pads’ dimensions and the via diameter are optimized to adjust the resonant frequency of the slot. When the varactor diode is biased (5 V ), it resonates at 6 GHz. The capacitor value is 0.5 pF in this case. By reducing the biasing voltage, the capacitance increases, and hence the resonant frequency decreases. The results show that the elliptical shape of the

Fig. 2. Measured and simulated reflection coefficient of sensing antenna.

Fig. 3. Measured and simulated reflection coefficient of communicating antenna for various biasing voltages of varactor diode.

UWB disc monopole and the H-shape of slot resonator both enhance the slot antenna bandwidth.

III. SIMULATION AND EXPRIMENTAL RESULTS

The proposed antenna configuration shown in Fig. 1 is simulated by Computer Simulation Technology (CST) software, and its dimensions are optimized according to the required radiation parameters for both UWB and reconfigurable narrowband antennas. To model the capacitor, according to its data sheet, a capacitor of pF is considered as an equivalent for MA4ST2200 variable capacitor biased with 5 V . The tuning range of capacitance is between 0.5–7.64 pF for the biasing voltage of V. A chip-resistor of k is used in the dc-feed line path to limit the transient current of the source. By changing the capacitance, the resonant frequency of slot antenna is tuned from 5 to 6 GHz.

Then, the antenna is fabricated with LPKF printed circuit board (PCB) prototyping technology. Its input reflection coefficient for two ports and coupling between them by using a calibrated vector network analyzer were measured. The simulated and measured reflection coefficients for the sensing antenna are depicted in Fig. 2, validating the expected operating band ranging from 3.3 to 11 GHz. Fig. 3 compares the simulated and measured reflection coefficient results of the communicating antenna for various biasing voltages of varactor diode. When the bias voltage is raised from 0 to 5 V , a frequency

Fig. 4. Measured and simulated transmission coefficient for various biasing voltages of varactor diode.

Fig. 6. Measured and simulated sensing antenna radiation pattern at 6 GHz.

(a) -plane. (b) -plane.

Fig. 5. Measured and simulated sensing antenna radiation pattern at 3 GHz.

(a) -plane. (b) -plane.

Fig. 7. Measured and simulated sensing antenna radiation pattern at 9 GHz.

(a) -plane. (b) -plane.

tuning range of 1 GHz is achieved. The antenna bandwidth is about 220 MHz at each biasing voltage without losing the desired matching. It is believed that the discrepancies between the simulation and measurement results are attributed to the fabrication accuracy and soldering effect. Nevertheless, the obtained results support the integrity of designed integrated antenna.

In CR application, achieving good isolation between two antennas is important issue. Fig. 4 shows the measured and simulated mutual coupling between sensing and reconfigurable antenna for different biasing voltages of the varactor diode. As it can be noticed, isolation of better than 20 dB was achieved across the UWB operating band except in the range of 4–6 GHz, which is 16 dB for its peak value at the resonant frequency of the reconfigurable slot. However, by slightly increasing the size of the UWB monopole antenna, the isolation between sensing and communication antenna can be improved to better than

20 dB across the tuning range as well.

For more investigations of the antenna performances, the measured radiation patterns are compared to the simulations in Figs. 5–10. As it is clear, both the UWB and reconfigurable narrowband antennas well confirm the desired beams. Because of its resonant nature at the lower part of achieved ultra operating bandwidth (up to 8 GHz), the pattern of monopole disc is completely omnidirectional. However, by getting closer to the higher part of the frequency band, the radiation mechanism

Fig. 8. Measured and simulated reconfigurable slot antenna radiation pattern at 5 GHz. (a) -plane. (b) -plane.

tends to be similar to a tapered slot antenna. Therefore, the antenna radiation beam changes to a nearly omnidirectional shape at higher frequencies.

The H- and E-plane radiation patterns of the reconfigurable antenna in Figs. 8–10 demonstrate the performance of the slot resonator by changing the capacitor value for three different frequencies of 5, 5.5, and 6 GHz. Some degradation is noticed in one side of the H-plane pattern, especially at the lower part of the tuning range; this is because of the dc-feed line effect of the varactor diode.

Fig. 9. Measured and simulated reconfigurable slot antenna radiation pattern at 5.5 GHz. (a) -plane. (b) -plane.

Fig. 10. Measured and simulated reconfigurable slot antenna radiation pattern at 6 GHz. (a) -plane. (b) -plane.

The measured and simulated gains of UWB antenna in the -plane are shown in Fig. 11. For comparison, the simulated peak gain of the antenna is depicted in this figure as well. The antenna gain in the -plane is measured using comparison method, confirming the predicted result. Since different parts of the UWB antenna are radiated across the bandwidth, its radiation pattern is changing, and therefore some gain reduction is observed in the ranges of 5–6 and 9–12 GHz.

However, as it can be noticed in this figure, the calculated peak gain of the antenna demonstrates smother response with better gain compared to the ones obtained in -plane. The measured peak gains of the reconfigurable antenna for the frequencies of 5, 5.5, and 6 GHz are 1.87, 1.36, and 1.73 dB, respectively.

Fig. 11. Measured and simulated peak gain of UWB antenna.

IV. CONCLUSION

In this letter, an integrated elliptical monopole antenna with reconfigurable slot radiator on a same substrate has been successfully introduced for cognitive radio applications. This antenna can offer sensing and communicating functions with a reasonable size. The achieved results have shown that the isolation between the narrow and UWB antenna is reduced to better than

16 dB by folding the slot resonator current distribution using a balanced stub inside the slot. Moreover, by an offset-fed configuration, the first dominant mode of the slot can easily be excited, providing the desired wireless communication operating frequency bandwidth.

REFERENCES

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