диафрагмированные волноводные фильтры / a4754ca2-af1b-4285-9732-f2173929e7cb

Abstract—A compact ultrawideband (UWB)-reconfigurable antenna for cognitive radio platforms is reported in this paper. The UWB antenna is a semicircular radiator with elliptical ground plane and is fed by coplanar waveguide (CPW) transmission line. A hairpin resonator is integrated on the ground plane of UWB radiator and can be treated as a reconfigurable band pass filter by changing its position and length. In addition, two PIN diodes are incorporated on both the arms of resonator as tuning element to switch to different communication band. The UWB antenna covers wide impedance bandwidth ranges from 1.9 GHz to 29.5 GHz, suitable for spectrum sensing application. On reconfigurable band state the antenna switching continuously from 9.2 GHz to 10.4 GHz with different state of operation of PIN diodes for the sake of communication. The simulated impedance bandwidth and radiation behavior for both the sensing and communication band demonstrates the design concept as a single port reconfigurable antenna for cognitive radio (CR) environment.

Index Terms—Ultrawideband, Hairpin resonator, Coplanar waveguide, cognitive radio.

I. INTRODUCTION

Due to the explosive growth of wireless standards and application, the electromagnetic radio spectrum has reflected in a very crowded manner. On the other hand it has been noticed that the current assigned spectrum is highly underutilized. Owing to such facts intelligent communication system for efficient utilization of radio spectrum has attracted considerable attention in research. Therefore cognitive radio (CR) [1] has become most promising solution in wireless system for efficient communication. The requirement and limitations of integrated antenna in CR environment is discussed by Hall et al. in [2]. Most of these proposed configuration [3–5] comprises of two port antenna system as wideband/narrowband with reasonable isolation between the ports in order to avoid coupling. In such cases isolation enhancement with limited space is difficult and challenging. Hence the requirement of single port reconfigurable antenna to achieve wideband and narrowband operation provides an additional challenge for CR environment. Such single port antenna design requires suitable integration of different switching elements into the antenna structure. Many researchers utilize PIN diode, varactor diode, RF MEMS as switches into the radiating part of the antenna [6–8]. But this includes some extensive care during design as the radiation behavior of antenna is affected due to the placing of biasing lines. This leads to a solution of placing switching elements on the feeding line or ground plane rather than radiating structure [9]. In the said arrangement the

radiating edges are not disturbed ensuring very less fluctuation in radiation behavior.

The current proposed work utilizes a single port planar reconfigurable antenna system with a very compact dimension. Here the UWB antenna is integrated with a hairpin resonator on its ground plane. The resonator producing shunting effect on metallic ground plane and behaves as a reconfigurable band pass filter without any switching element. Further, by incorporating two pin diodes on each arm of resonator adjacent to feed line different frequency bands are obtained for different states of diode. The position of diodes and width and length of resonator are tuned by using finite element based high frequency structure simulator ANSYS HFSS version 14 [10]. A series of parametric analysis have been carried out in order to obtain the optimized dimension of proposed antenna. Here Section II demonstrates antenna design and configuration, Section III describes all simulated results and Section IV concludes major findings.

II. ANTENNA DESIGN AND CONFIGURATION

The complete dimension of proposed antenna is 20 × 25.32 mm2. The antenna is printed on FR4 substrate with dielectric constant 4.4, loss tangent 0.02 and thickness 1.6 mm. The overall design and analysis of proposed antenna have been explained by three different configurations as depicted in Fig. 1. The configuration ‘A’ comprises of a semicircular monopole radiator with elliptical ground plane as UWB sensing antenna. The variation of elliptical ratio in the ground plane plays a vital role for good impedance matching. The configuration ‘B’ consists of a hairpin resonator on the ground plane of UWB radiator, connecting to both the sides of feed line. This helps to realize as a reconfigurable band pass filter by varying its position, length and width as depicted in Fig. 3. The configuration ‘C’ incorporates two PIN diodes on each arm of resonator adjacent to feed line. This helps to realize switching to different bands at different states of diode. Here, the length (L) and width (W ) of the proposed antenna can be calculated in terms of guided wavelength as

2Wg + 2S + g ≈ 0.30λg

Lg + T + R ≈ 0.39λg

where

εe ≈ εr + 1 2

(c)

Fig. 1. Different design configuration of proposed antenna (a) configuration ‘A’ (b) configuration ‘B’ (c) configuration ‘C’.

TABLE I

OPTIMIZED DIMENSIONS OF PROPOSED ANTENNA (MM).

Fig. 2. S11 of UWB antenna (configuration A).

where, εr is the dielectric constant of substrate. The radius of semicircular radiator is estimated using the relation [11], considering the lower cutoff frequency of UWB as

87.94

R = fr√εr

where “fr” is the resonant frequency in GHz (see Table I).

III. SIMULATED RESULTS AND DISCUSSION

On the basis of above optimization parameter the proposed antenna for three different configuration have been analyzed

Fig. 3. S11 of narrowband due to hair pin resonator (configuration B).

Fig. 4. S11 for different state of diodes (configuration C).

TABLE II

STATES OF SWITCHES FOR DIFFERENT BANDS.

using ANSYS HFSS [10]. The reflection coefficient for configuration ‘A’ covers wide impedance bandwidth of 1.9 GHz to 29.5 GHz as depicted in Fig. 2. Such a wide impedance bandwidth is obtained due to elliptical ground plane as well as smoothed semicircular radiator fed by CPW transmission line. The S parameter of configuration ‘B’ in the presence of hairpin resonator is depicted in Fig. 3. It is seen that the structure behaves as a band pass filter at 9.6 GHz frequency. In addition, the band pass filter modifies its operating frequency by varying the length and width of resonator. In configuration ‘C’ two PIN diodes are introduced on each arm of resonator adjacent to the transmission line using MA4FCP300 [12] data sheet. According to the MA-COM data sheet the PIN diode can be modeled as a series resistance of 4 Ω during ON state and a parallel combination of a 20 kΩ resistor with 0.04 pF capacitor for its OFF state, as presented on Table III. The simulated reflection coefficient for different states of diode are demonstrated in Fig. 4 and different conditions are depicted in Table II.

From the simulated result it is clear that the antenna switched continuously in the range of 9.2 GHz to 10.4 GHz for the different tuning operation of diode. When both the diodes

Fig. 5. Radiation pattern of UWB antenna (a) and (b) at 8.3 GHz, (c) and

(d) at 17.7 GHz. and communication antenna (e) and (f) at 9.6 GHz.

are turned ON current flow directly through the diode due to short circuit. Hence the variation of current path becomes shorter and a wideband resonance is obtained under this conditions as in state 1. When both the diodes are turned OFF current flow through the whole arms of resonator. So the resonance obtained in such case is quite similar to the presence of hairpin resonator only as presented in state 2. In both of these cases the current path is symmetrical and impedance matching is around −25 dB. On the other hand when diode D1 (left arm) ON and D2 (right arm) off and vice versa then one arm of resonator is shorter than the other arm and current path becomes asymmetrical on both sides of feed line. Due to asymmetrical current path impedance matching is better in both the states (state 3 and 4) and is around −50 dB with narrow band response.

The simulated radiation patterns of UWB sensing antenna at 8.3 GHz and 17.7 GHz and reconfigurable communication antenna at 9.6 GHz are depicted in Fig. 5.

The radiation patterns at both the frequencies of UWB antenna shows omni directional pattern at H plane and bidirectional pattern at E plane. Hence the antenna is well suited for spectrum sensing application in CR environment. For reconfigurable band the pattern shows good radiation behavior making it useful for communication purpose.

The simulated surface current distribution at 9.6 GHz when both the diodes are ON and OFF condition respectively are depicted in Fig. 6.

The simulated surface current demonstrates that during OFF state condition of both the diodes maximum current couples at the edges of hairpin resonator rather than the diodes. On the other hand during ON state condition of both the diodes maximum current couples in both the diodes rather than the edges of resonator. The simulated gains at different states of diode are presented in Fig. 7. For the state 1(D1 D2 ON) simulated gain is almost maximum about 1.4 dB and at state 2 (D1 D2 OFF) gain is about 1 dB. For the rest two states i.e. D1 ON D2 OFF and D1 OFF D2 ON the gain variation is almost 1.2 dB. However for all the states the gain variation is almost constant within the desired pass band.

The transient response of UWB antenna with Gaussian pulse is shown in Fig. 8. The waveform responses show almost minimum dispersion for both input and output voltage.

IV. CONCLUSION

In this paper a compact CPW fed UWB semicircular radiator with frequency reconfigurable characteristic is designed, simulated and analyzed. The UWB is responsible candidate

Fig. 8. Simulated input/output pulses of proposed antenna.

for spectrum sensing with good radiation behavior and wide impedance bandwidth. The presence of hair pin resonator on the feed line without any switching element performing the task of band pass filter. Additionally, continuous frequency tuning is realized by utilizing two PIN diodes on each arm of resonator basically from 9.2 GHz to 10.4 GHz which is well suited for X band satellite link. With these features the proposed antenna can be considered as a suitable candidate for cognitive radio application.

REFERENCES

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[9]Y. Tawk, J. Costantine, and C. G. Christodoulou, “A varactor based reconfigurable filtenna,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 716–719, 2012.

[10]Ansys High Frequency Structure Simulator (HFSS) Version 14.0.

[11]C. A. Balanis, Antenna theory analysis and design, 3rd ed., Wiley India Pvt. Ltd., 2011.

[12]M/A-COM Data Sheet for MA4PBL027 beam lead PIN diode.