диафрагмированные волноводные фильтры / 8df55f39-c0fe-48ca-837b-3e9b61044d59

Abstract— A compact frequency agile multiband filtenna is presented in this paper. The proposed reconfigurable filtering antenna performs independent switching between four operating bands viz. 1.8 GHz (GSM), 2.4 GHz (Bluetooth), 3.5 GHz (WiMax) and 5.2 GHz (WLAN). This switching allows the unused bands to be utilized by the secondary user making the antenna suitable for cognitive radio applications. An elliptical wideband monopole antenna is excited through a reconfigurable band pass filter (BPF) integrated onto the 50 Ω transmission line. The BPF comprises of four distinct structures to tune to four different frequencies using PIN diodes. The proposed filtenna has the reflection coefficient lesser than -20 dB for all the desired operating bands. Furthermore, this multiband filtenna offers gain of about 1.1 dBi, 2.6 dBi, 3 dBi and 3.4 dBi at the four frequency bands. The prototype filtenna is fabricated and the simulation results are verified experimentally.

Index Terms— cognitive radio, filtenna, independent switching, multiband, reconfigurable filter, resonator

I. INTRODUCTION

In recent years, the researchers have been focusing more on the suppression of interference and noise at the receiver front end [1]. It can be controlled by suppressing the undesired signals. This is facilitated by integrating filter into the 50 Ω feed line of the antenna at the receiver front end without increasing the circuit complexity. The integrated filter makes the antenna to discriminate the desired signals from the noisy environment

by rejecting the unwanted signals.

Several papers are available in the literature to realize this multifunctional module. A planar filtering antenna operating at Ku-band is reported in which unwanted out of band signals are suppressed using Substrate Integrated Waveguide (SIW) technique [2]. An electrically small monopole filtenna is demonstrated in [3] using Capacitive Loaded Loop (CLL) based filter to improve the out of band rejection level. Hence from the study it is evident that integrating filter on the feed line suppresses higher order harmonics and provides interference rejection while maintaining the radiation properties of the primary antenna.

Reconfigurable filtenna is reported for dynamic spectrum access with improved SNR to support multiple applications

with interference pre-filtering. In literature, several frequency reconfigurable filtennas are proposed in [4-5] to continuously tune to different frequency bands. Wideband to narrow band reconfiguration is reported in the literature using PIN diodes. A slot based resonator is used as a filter to achieve wideband to narrow band switching [6]. A loop based band pass filter and a three pole hairpin band pass filter is reported in [7] to obtain switching between wideband and narrow band frequencies. Thus, from the literature it is observed that there is a requirement for compact multiband filtenna to achieve frequency selectivity and spectrum efficiency.

The frequency selectivity of the device depends on the reconfigurable RF band pass filter integrated onto the feed line of the filtenna. So to design the filter extensive study on the state of art has been carried out. A few referred papers to bring up reconfigurability in the microstrip filter are discussed in [8- 10]. In [8] reconfiguration has been done using a dual band BPF with independent switching between pass bands. To adapt between Wi-Fi and UMTS reception two reconfigurable BPF is presented in [9] to attain accurate operating frequencies using two PIN diodes. Hence a compact, independently switched, easy to reproduce, frequency reconfigurable band pass filter compatible with the designed antenna is necessary to meet the requirement for the objective taken.

In this paper, a novel multiband frequency agile filtenna is presented by integrating a switchable filter on the feed line of a wideband antenna. The BPF design consists of a C- Shaped Resonator (CSR), Inverted Pulse Shaped Resonator (IPSR), Meandered Loop Resonator (MLR) and Open Circuited Stub (OCS) for operating at four desired frequencies. For switching, PIN diodes are deployed at suitable positions in the resonators. The proposed filter is able to reconfigure between four different bands by enabling the antenna to radiate at four frequencies such as 1.8 GHz (GSM), 2.4 GHz (Bluetooth), 3.5 GHz (WiMax), and 5.2 GHz (Wi-Fi).

II. DESIGN PROCEDURE

The layout of the proposed reconfigurable filtenna is presented in Fig. 1. The prototype is developed on a 1.6 mm thick FR4 substrate with dielectric constant 4.3 and loss tangent 0.023.

(a) (b)

Fig.:1 The proposed quad-band filtenna: (a) Front view (b) Rear view: L = 30 mm, W = 60 mm, Xr = 10.5, Yr = 14 mm, t1= 1.5 mm, s1 = 3.2 mm, s2 = 9 mm, s3 = 10.5 mm h = 29 mm.

A. Design of the wideband antenna

An elliptical monopole antenna of foot print 30 × 60 mm is considered for the design of the proposed filtenna. The curved surface of the ellipse provides wide impedance bandwidth and hence elliptical monopole is considered as the primary radiator. The circumference of the ellipse determines the lowest operating frequency of the monopole antenna. A partial ground plane with an etched rectangular slot is developed on the rear side of the antenna to obtain wideband impedance matching [14]. The reflection coefficient of wideband antenna is shown in Fig 2. This wideband antenna provides 116% impedance bandwidth.

corresponding to quarter wavelength at the fourth operating frequency is enclosed between IPSR and transmission line for obtaining the resonance at 5.2 GHz. The designed filter provides complete flexibility in selecting the center frequency and bandwidth of the four bands independently by altering the electrical length and gap between the resonator and the transmission line respectively.

The coupling diagram [12] of the designed filter is depicted in Fig. 4 (a). From the coupling diagram it can be understood that the current flow from source to load occurs only through the resonators. There is no direct connection between source and load and hence high suppression of spurious signals in the stop bands is achieved. There is an inter resonator coupling occurring between resonators 1 and 3 and also resonators 2 and 4. The resonators 3 and 4 are separated by the center split transmission line and so there is no inter resonator coupling between the two resonators. Fig .4 (b) shows the step by step evolution of the quad-band filter with respective return loss response. Four PIN diodes are used for switching between the four operating bands. BAR64-03W PIN diode is selected for switching which has 2.1Ω series resistance. During forward bias, the inductance offered by the diode is 1.8 nH. The RF signal and DC lines are isolated using 47 nH RF choke inductor and 18 pF DC blocking capacitor [16-18]. The S-parameter response of the filter when all the diodes in ON state is shown in Fig. 5 (a).

Fig. 2: Simulated reflection coefficient of the wide band elliptical monopole

B. Reconfigurable filter design

The geometry of the proposed reconfigurable filter is shown in Fig. 3. It consists of four independent resonators coupled to the stepped impedance transmission line split at the center. A slot is introduced at the center of the 50 Ω lines to attain all stop characteristics. The four resonators designed at discrete frequencies are coupled to the split transmission line. The impedance of the transmission line is increased at the center to accommodate for active energy coupling between the transmission line and the resonators. Since the transmission line is split, current reflects and so, energy coupling between two ports is acquired through these distinct resonators. The designed CSR is fixed to have the physical length equivalent to quarter wavelength at 1.8 GHz. The resonance at 2.4 GHz is attained by designing the IPSR for a length equivalent to half wavelength at the designated frequency. A meandered loop resonator is enclosed by the transmission line and CSR for transmission of the electromagnetic signals at 3.5 GHz. The energy is coupled to the MLR through CSR from the transmission line. The loop is meandered to increase the electrical length of the resonator within the available space similar to fractal structures [15]. An OCS with the length

Fig. 3: Proposed reconfigurable filter (a) Front view (b) Side view:f1=5.5 mm, f2=4.75 mm, f3=3 mm, f4=1.35 mm, f5=0.88 mm, g1=w1=w7=0.75mm, g2=w2=l14=l9=1 mm, l1=7.25 mm, l2=l3=4.2 mm, l4=5.74 mm, l5=2 mm, l6=2.3 mm, l7=3.28 mm, l8=6.2 mm, l10=1.25 mm, l11=0.25 mm, l12=5.4 mm, l13=3.7 mm, l15=6 mm, l16=0.55 mm, L=29 mm, w3=0.5 mm, w4=0.625 mm, w5=1.45 mm,w6=0.4mm, w8=1.36 mm,W=30 mm, t=0.035 mm, h=1.635 mm.

Fig. 4: (a) Coupling diagram of the proposed reconfigurable filter. (b). Evolution of the proposed reconfigurable filter

C. Design of Reconfigurable multiband filtenna

The characterized filter is integrated in the feed of the wideband antenna to realize the fine selectivity between different pass-bands using four PIN diodes. The input given to antenna reaches the radiating part through the four designed

radiators. The distance between the filter and the radiator is maintained as 5 mm hence it does not disturb the radiating properties of the radiator. Fig. 5 (b) shows the comparison between reflection coefficient response of the designed antenna, filter and filtenna. For improving the efficiency of cognitive radios the sensing and communication functions should be performed by separate antennas [13]. Therefore the wideband antenna described in section II (A) can be implanted for sensing the channel and the designed filtenna is used for communication through the spectrum holes. This filtenna is also suitable for switching on/off of a particular frequency band at different time slots for different users, (i.e.) TDMA system

(a) (b)

Fig. 5: (a). S-Parameter response of the quad-band filter, (b) Comparison on reflection coefficient response of antenna, filter and filtenna

III. WORKING PRINCIPLE

The proposed reconfigurable multiband filtenna offers quadband, tri-band, dual-band or single band operation by switching the PIN diodes. Table I depicts the diode ON/OFF condition for the 5 switching states.

TABLE I

VARIOUS OPERATING STATES OF DIODE

In the first state, all the diodes are forward biased. The surface current distributions at the four operating bands are presented in the Fig 6 (a) (i-iv). Low resistance offered by the diodes allows the RF signal to get transmitted to the radiator through the four resonators for quad-band operation. When the diode D1 is alone reverse biased, the signal propagates through the coupled IPRS, MLR and OCS. Since the diode offers infinite resistance in OFF state, it restricts the signal transmission through CSR. For state 3, the diode D2 is turned OFF and so the RF signal gets transmitted through CSR, MLR and OCS. Signal transmission through IPSR is incomplete since the path is open circuited. Thus the filtenna does not radiate at 2.4 GHz. At the fourth state, the diode D3 present in the MLR is turned OFF. The signal transmission path through the MLR is cut off. But current flow through CSR, IPSR and OCS are not affected. Hence the resonance at 1.8 GHz, 2.4 GHz and 5.2 GHz remains unaltered. The diode D4 embedded in the OCS is reverse biased. The diodes existing in the other resonators are forward biased. Hence the current through these resonators travels the complete path and excites the radiator. This allows the filtenna to radiate at 1.8 GHz, 2.4 GHz and 3.5 GHz. The surface current distribution of the filtenna at states 2-5 is interpreted in Fig. 6(b) (i-iv).

Fig. 7 shows the S-parameter response of the proposed filtenna in all five operating states. The proposed prototype is well suited for cognitive radio applications. The discrete

switching between GSM, Bluetooth, WiMAX and Wi-Fi applications are obtained through the filter placed at the feed of the primary antenna. Therefore, whenever the primary user is unavailable, the proposed filtenna facilitates the secondary user to make use of the available spectrum thus improving the spectrum efficiency and frequency selectivity at the RF front end without increasing the device complexity.

Fig. 6: Surface current distributions at the four operating frequencies when PIN diodes (a) turned ON (b) turned OFF; (i) 1.8 GHz, (ii) 2.4 GHz, (iii) 3.5 GHz, (iv) 5.2 GHz

Fig. 7: Simulated S-parameter for different operating states of the diode

IV. RESULTS AND DISCUSSION

The proposed filtenna is fabricated and tested. Photograph of the fabricated prototype is presented in Fig. 8(f).

A. Reflection coefficient

The simulated and measured reflection coefficient of the proposed filtenna at the five operating states are compared in Fig 8. It is verified that the proposed filtenna has the return loss above 15 dB in all the states. Filter response is also measured at all the states and good agreement is found between the simulated and measured results. It is also depicted that the insertion loss is less than 3 dB in all operating bands. Percentage bandwidth of 2.5% at 1.8 GHz, 3.9% at 2.4 GHz, 4.5% at 3.5 GHz and 6.7% at 5.2 GHz is realized in all the four bands.

B. Radiation characteristics

The radiation pattern at the four operating frequency is investigated in both X-Z plane and X-Y plane. Since it is a monopole antenna the E- plane and H-plane has approximate dumb-bell shape and circular pattern respectively. From Fig. 9 it is observed that the measured radiation pattern has good agreement with the simulations. The radiation pattern of the filtenna is found similar to that of the primary antenna. The gain and efficiency plot of the proposed design is given in Fig 10(a). The gain obtained at the four frequencies in the wideband antenna is 1.4 dBi, 2.7 dBi, 3.254 dBi, 3.628 dBi. The developed filtenna has the realized gain of 1.1 dBi, 2.6 dBi, 3 dBi and 3.4 dBi at the four operating bands respectively with low variation of 0.4 dBi from primary antenna and the radiation efficiency of above 60%. If needed, gain of the filtenna can be improved using EBG [19] or FSS characterized appropriately.

Fig. 10 (b) illustrates the group delay characteristics of the filter. It can be seen that the obtained group delay is flat with maximum variation of 0.7 ns.

Table II shows the comparison of the proposed reconfigurable filtenna with the existing similar prototypes. It can be observed that the designed filtenna is compact and discrete switching between four applications is obtained instead of band limited frequency tuning or narrow band to wide band reconfiguration.

25% compared to [4-11]. The filtenna can be scaled to operate at any desired frequency by altering the length of the resonators.

6.The prototype offers gain of 1.1 dBi at the lowest operating band which is higher than the gain obtained in [3], [5] and [7]. A minimum gain variation of 0.4 dBi is achieved between primary antenna and filtenna compared to [2], [4] and [6] where the variation is above 1 dBi.