диафрагмированные волноводные фильтры / cebec00e-3d53-4cc6-8501-1eefe793b975

In this article, we present a new design concept for a multifunctional antenna providing single/dual frequency-notched ultrawideband (UWB) capability and its complementary multiple narrow-band functionality from a printed coplanar waveguide (CPW)-fed annular-ring monopole antenna (ARMA). We achieved the proposed multiple antenna configurations by loading square split-ring resonator (SRR) pairs of different dimensions beneath the substrate and above the superstrate, along with copper strips suitably positioned on the CPW feeding section of the antenna. The proposed antenna is compact, because of SRR loading in the substrate and superstrate, which provide seven different antenna functionalities/responses originating from a single radiating annular-ring monopole printed on

a dielectric substrate.

We experimentally verified the proposed multifunctionality of the antenna with measurements of the impedance and radiation characteristics of the fabricated prototypes with and without SRRs and copper strips. The present design concept can be exploited in obtaining the multiband/frequency reconfigurable characteristics to address the requirements of the softwaredefined radio (SDR) and cognitive radio (CR) environments using a single antenna element.

Digital Object Identifier 10.1109/MAP.2018.2796027

Date of publication: 27 February 2018

TECHNOLOGY ADVANCES TO DATE

A major technological challenge associated with wide-band and UWB antenna designs is the efficient rejection of interference from narrow-band services. Moreover, recent technologies, such as SDR in a CR environment, and the current surge in multipleinput, multiple-output (MIMO) methods for high-speed, high- data-rate applications requiring multiple UWB antennas on a common platform have further escalated the antenna design challenge [1]–[5].

Spectrum overlapping of various narrow-band services within the UWB spectrum, such as WiMAX (3.3–3.7 GHz), wireless local area network (WLAN) (5.15–5.85 GHz), and X-band satellite downlink (7.25–7.75 GHz), demands the mitigation of these interfering narrow bands for efficient communication utilizing the UWB spectrum. Various groups have addressed this by introducing intrinsic filtering properties at the desired notch frequencies of the UWB antenna. The majority of these narrow-band interference mitigation techniques, aimed at achieving single/dual/triple notches, use various slots [6]–[11], parasitic resonators, and complementary SRRs (CSRRs) [12] [13] on the radiator or feed section. This perturbs the physical structure of the antenna, which in turn results in compromised antenna performance. In addition, the design methodology is highly specific to the antenna shape and geometry, not easily adaptable to the radiators of other

geometries—nor are the notches easily scalable. A frequencynotching technique with minimal impact on the radiating aperture of the UWB antenna by inductively coupling a single pair of SRRs was demonstrated in [14].

Designing compatible antenna systems for CR applications is more complicated, as it requires the cohousing of multiple antennas to achieve the desired antenna functionality. Antennas for this system call for special attention, as they necessitate a UWB and a narrow-band capability on a common platform. The first antenna performs spectrum sensing of unused carrier frequencies and is called the sensing antenna. The second device needs to be reconfigurable, operating over a limited bandwidth, and is known as the communicating antenna or transmit/receive antenna. Hall et. al in [1] and Christodoulou et. al in [2] have highlighted the technology, challenges, and key design parameters of such CR systems and CR-based antennas.

Some novel antenna designs for CR applications involve band switching using a stepper motor-enabled rotational arrangement [15] and a pair of wideand narrow-band antenna structures, as proposed in [4] for a multistandard radio. In [4], a CPW-fed UWB monopole and a shorter reconfigurable microstrip patch on its opposite side are printed on the common substrate to invoke a simultaneous wideand narrow-band antenna response. However, isolation between the ports of several antennas housed in a circuit remains a challenge.

In this article, we propose a new concept of a multifunctional antenna design. We used a single ARMA with a wideband response to demonstrate the proof of concept. We fed the ARMA with a CPW-based transmission line and coupled the CPW feed with a pair of SRRs to obtain a frequencynotched wide-band response. We obtained dual notches using an additional SRR pair on a superstrate, suitably placed on the feeding section. The multilayered configuration, when further augmented with copper strips between the CPW signal line and the ground planes, yields a complementary response resulting in single/dual narrow-band characteristics. The single/dual notch frequencies are a function of the SRRs’ geometrical parameters and are independent of the antenna or the feed design. Thus, unlike the designs presented in [4] and [15], we use a single radiator to achieve multiple antenna responses. The novelties of the proposed multifunctional antenna are many.

■■It achieves multiple functionality from the same antenna without altering the radiator.

■■The complementary narrow-band response is evolved from the same UWB antenna, without using the additional radiators or radiating elements that can increase the antenna dimensions and require additional means of electromagnetic isolation.

■■Scaling for any other combination of notches can be done by tailoring the SRR dimensions alone, without changing the radiator or the ground plane.

■■The radiation pattern of the narrow-band configurations is almost identical to that of the UWB monopole antenna and frequency-notched UWB monopole antenna.

■■The proposed concept is applicable to any other CPW/ microstrip-fed UWB antenna.

We validated the proposed concept using electromagnetic simulation [16], lumped element-based circuit simulation [17], and measurements of the fabricated prototypes.

ANTENNA CONFIGURATIONS

We evolve the proposed multiconfiguration antenna from an ARMA, fed by a CPW-based transmission line printed on a dielectric substrate, that offers the opposite side of the printed copper to accommodate passive components and active devices for various applications. The antenna’s proposed multifunctionality is achieved by exploiting the strong magnetic resonance of the SRRs loaded below the substrate and/or above the superstrate of the CPW feed region with and without the copper strip. The multiple configurations of the proposed antenna are illustrated in Figures 1 and 2. We load the basic CPW-fed ARMA (configuration A), shown in Figure 2(a), with various combinations of different-size SRRs and/or copper strips to derive six new antenna layouts [configurations B–G, shown in Figure 2(b)–(g), respectively] providing multiple antenna functionality.

UWB ANTENNA (CONFIGURATION A)

This is the fundamental building block of the proposed multiple antenna configurations. As shown in Figures 1 and 2(a), it consists of an annular ring of outer and inner radii R1 and R2, respectively, printed on an RT duroid substrate (fr1 = 2.33, tan d = 0.0012). It is excited by a CPW feed consisting of ground planes having widths W1 and W2 and length Ls, and a signal line of width S and length Ls1. We choose S and the width of the slots sg between the ground planes and signal line to ensure broadband (2–12 GHz) impedance (50 Ω) matching with the subminiaturized version-A connector (PE44614 from Pasternack). We optimize the feed gap t to provide a good impedance response for UWB characteristics. The dimensions of this configuration are indicated in Table 1.

FREQUENCY-NOTCHED UWB ANTENNA (CONFIGURATIONS B–D)

We derive these three layouts (B, C, and D) from the previous, fundamental configuration A by loading a pair of SRRs beneath the slot lines of the feeding CPW (configuration B), on the superstrate placed on the feed section of the CPW (configuration C), or by using both of these (configuration D), all of which is indicated in Figure 2(b)–(d). Figure 1(a) shows the geometrical layout of configuration B, where a pair of square SRRs with an outer-side dimension 2aext1, width c1, interring spacing d1, and split gaps g1 are precisely printed with the SRR centers matching with the center of the CPW slot lines at a distance Lc from the antenna’s feed port.

Configuration C is realized by a similar SRR loading on a superstrate of overall dimensions W # Ls2 # h2, as shown in Table 1, loaded above the feed section of basic antenna configuration A. We place the superstrate printed with the SRRs at the appropriate position and fix it with dielectric tape. Since the loaded superstrate has the same dielectric constant as that of the antenna substrate (fr2 = fr1 = 2.33) and it is located in the feed

(d)

region, it has very little impact on the impedance and radiation characteristics of the antenna. The SRR pair loaded on the superstrate, as shown in Figure 1(c), has an outer-side dimension 2 × aext2, width c2, interring spacing d2, and split gaps g2.

The SRR dimensions utilized for configurations B–D are summarized in Table 2. The different dimensions of the SRR pairs for configurations B and C excite different resonance frequencies f1 and f2, respectively [14], which yields different notch frequencies in the UWB spectrum. Configuration D is a multilayered structure in the antenna’s feed section, as it has SRRs printed beneath the substrate as well as on the superstrate. These multiple SRR pairs (their dimensions are summarized in Table 2) correspond to two different resonances within the UWB spectrum and thereby provide dual-notched UWB responses, with frequency notches at f1 and f2.

NARROW-BAND ANTENNAS (CONFIGURATIONS E–G)

The narrow-band configurations are aimed at obtaining complementary antenna functionality compared to the frequencynotched UWB configurations of B–D. Combinational loading of the SRRs and the copper strips, which shorts the CPW signal with the ground planes, forms a narrow bandpass filter [18] around the SRR resonance frequency that, in turn, excites the ARMA over the corresponding passband frequency. This transforms the previous frequency-notched UWB antennas (configurations B–D) into narrow-band antennas [configurations E–G, demonstrated in Figure 2(e)–(g)], providing a complementary impedance profile. This particular complementary antenna response evolves from our previous article [18], and we exploit it to achieve multiple narrow-band antenna functionality at f1, f2, and (f1, f2). The narrow-band operation is highly sensitive to the position of the copper strips and provides the best excitation when the center of the strips is aligned with the center of the SRRs on the other side. We indicate this in Figure 1(a), with the center of the shunt strips located at a distance Lc from the antenna’s feed port.

The antenna configurations derived from the same ARMA, as summarized in Table 3, provide similar radiation patterns, highly correlated with the monopole type, even for narrow-band configurations. This is because the SRR/strips loading in the antenna’s feed section that purposes to change the impedance characteristics does not impact the radiation aperture of the antenna. We confirmed this using reflection coefficients and radiation pattern measurements in a fully calibrated anechoic chamber.

LUMPED ELEMENT EQUIVALENT CIRCUIT

The complementary response of the proposed antenna configurations can be interpreted from the antennas’ lumped element equivalent circuit models, with SRRs loaded in the substrate or superstrate layer or in both layers with and without strips loading. The SRR resonance is contributed by the time-varying magnetic field of the propagating electromagnetic signal of the host CPW medium, and, therefore, the configurations can be modeled as LC resonator tank circuits [19], [20]. Because of the SRRs’ subwavelength resonance phenomenon, the host CPW medium can also be represented as a cascade of lumped element equivalent circuits. The inclusion of copper strips between

TABLE 1. THE DESIGN PARAMETERS

OF THE SQUARE SRRs-LOADED

MULTILAYERED ARMA PRINTED ON A DIELECTRIC SUBSTRATE HAVING fr1 = 2.33 AND tan d = 0.0012. THE SUPERSTRATE LAYER MATERIAL IS THE SAME AS THE SUBSTRATE (PARAMETRIC VARIABLES AS SHOWN IN FIGURE 1).

TABLE 2. THE DESIGN PARAMETERS OF THE SQUARE SRRs LOADED BENEATH THE SUBSTRATE (fr1 = 2.33, tan d = 0.0012)

AND/OR ABOVE THE SUPERSTRATE (fr2 = 2.33, tan d = 0.0012) (PARAMETRIC VARIABLES AS SHOWN IN FIGURE 1).

the signal line and the ground planes (for configuration G) is modeled by introducing an additional shunt inductor Lp. Thus, using the lumped element equivalent circuits of the SRRs in two layers, the host CPW medium, and the shorting copper strips (for configuration G), we obtain the resultant lumped element equivalent circuit of antenna configurations D and G, as shown in Figure 3.

It should be noted that the equivalent circuit model of configuration D is exactly the same as that of Figure 3 minus the

TABLE 3. A FUNCTIONAL BOOLEAN TABLE OF THE SQUARE SRRs-LOADED, CPW-FED ARMA DEMONSTRATING SEVEN ANTENNA CONFIGURATIONS.

shunt inductors indicating the copper strips (shown in the dotted line). Applying symmetry along the longitudinal axis through the center of the signal line, we model the CPW as a series inductor L and shunt capacitor C. We terminate the input port with 2R0 = 100 X, while the CPW’s output port is terminated with 2ZA (f ) where, ZA (f ) is the input impedance of the standalone ARMA obtained from electromagnetic simulation. We obtain the CPW inductance and capacitance as L = p)Lpul and C = p)Cpul, where Lpul and Cpul are the per unit length inductance and capacitance of the CPW, and p is the unit cell size obtained from the SRR dimensions. The SRRs loaded beneath the substrate and on the superstrate are electromagnetically coupled with the host CPW medium, with a coupling coefficient k calculated as in [19], and are represented by two separate LC

circuits. Two different SRR pairs (for configurations D and G) are modeled as two tank circuits: LSRR1 and CSRR1 represent the self-inductance and capacitance of the bottom SRR, while LSRR2 and CSRR2 represent the top SRR pair, loaded on the superstrate.

We simulated the lumped element-based equivalent circuit in Figure 3, corresponding to configurations D and G, using the ADS circuit simulator [17]. The equivalent circuit parameters for these configurations are LSRR1 = 8.63 nH, LSRR2 = 6.58 nH,

CSRR1 = 68.91 fF, CSRR2 = 69.96 fF, a nd Lp = 118 nH

(the latter for configuration G). The circuit parameters for the host CPW medium for a unit cell size of p = 5 mm are L = p)Lpul and C = p)Cpul, where Lpul = 261.16 nH/mm and Cpul = 63.29 pF/mm. The coupling coefficients between the host CPW and the two SRR pairs are k1 = 0.25 and k2 = 0.2. Figure 4 shows the magnitude of the simulated reflection coefficient S11 in decibels for configurations D and G. As observed from circuit simulation, configuration D provides two frequency notches, at 6.52 GHz and 7.42 GHz, while configuration G corresponds to the complementary impedance profile, with narrow-band operation at 6.55 GHz and 7.44 GHz. We can also observe a similar complementary response for configurations B/E and C/F, where only one tank circuit, corresponding to either the bottom or top SRR, is to be considered. Thus, we validated the proposed design concept with lumped elementbased equivalent circuit simulation even before rigorous electromagnetic simulations and measurements of the fabricated prototypes.

MEASUREMENTS AND RESULTS

We thoroughly investigated the fabricated prototypes of the CPW-fed ARMA with square SRR loadings beneath the substrate and/or on the superstrate, with and without copper strips

Transmission Line

2ZA(f )

Antenna

Impedance

FIGURE 4. The S11 characteristics of the proposed multifunctional antenna for configurations D and G derived from the lumped element equivalent model.

loading, as shown in Figure 5, measuring for both impedance and radiation characteristics. We designed and simulated three prototype antennas—an ARMA without any loading, an ARMA with a pair of SRRs printed on the back side of the CPW feed section, and an ARMA with a pair of SRRs printed on the back side of the CPW feed section and copper strips shorting the signal line of the CPW with the ground planes, all having

an overall size of 50 × 50 × 1.575 mm3—using a commercial electromagnetic simulator [16]. We loaded these antennas, with a superstrate layer having a pair of SRRs printed on it, above the feed section of the CPW signal line, producing various antenna configurations with multiple antenna functionalities. Each of these combinations provides a unique reflection coefficient response, which we measured using an Agilent PNA-X N5224A network analyzer. For all of the antenna prototypes, we carried out the radiation pattern measurement in a fully calibrated nearfield anechoic chamber, as shown in Figure 6. We used a broadband preamplifier (Agilent 83051 A)-coupled broadband horn as the transmitting antenna, while the SRR/copper-strips-loaded ARMA with and without superstrate loading was in receive mode for various configurations.

UWB ANTENNA (CONFIGURATION A)

The measured and simulated plots for the S11 versus frequency of the proposed ARMA are shown in Figure 7(a). Figure 7(b) compares the measured maximum realized gain in the xy plane with that of the simulation. As revealed in Figure 7, the simple CPW-fed ARMA operates over the entire UWB spectrum (3.1– 10.6 GHz) with a resonance dip around 3.6 GHz (corresponding to the quarter-wavelength resonance of the disk diameter), with a consistent flat gain profile over the band. The measured radiation pattern exhibits monopole-type omnidirectionality with axial null (along the y axis) in the E-plane and a circularly

symmetric H-plane (xz plane), with acceptable coto cross-polar separation over the entire bandwidth. (For brevity, the patterns are not shown.) This antenna, which we call configuration A, serves as the basic prototype for all of the other six configurations, as detailed in the following sections.

AUT

FIGURE 6. The setup for radiation pattern measurement for the fabricated CPW-fed ARMA for various configurations in the anechoic chamber, with cables and transmitting antenna. AUT: antenna under test.

0

FIGURE 7. (a) The simulated and measured S11 and (b) the maximum realized gain characteristics of the proposed CPWfed ARMA for UWB response (configuration A).

FREQUENCY-NOTCHED UWB CONFIGURATIONS (CONFIGURATIONS B–D)

Simple SRR loading in the feed section of configuration A transforms the UWB antenna into the frequency-notched UWB antennas of configurations B–D. Figure 8 plots the simulated and measured S11 versus frequency of proposed antenna configuration B. As discussed previously, the SRRs loaded beneath the CPW slots, which thereby become magnetically coupled with the propagating electromagnetic signal, inhibit propagation around the antenna’s resonance frequency. This contributes to the measured frequency notch at 6.61 GHz against 6.51 GHz for the simulated notch. Figure 9 shows the E- and H-planes’ measured normalized radiation patterns for proposed antenna configuration B at 3.8 GHz, 6.2 GHz, and 9.4 GHz. There is a monopole-type radiation pattern with axial null for the E-plane and a nearly omnidirectional pattern for the H-plane. The selected frequencies span the entire UWB spectrum, excluding the notch frequency, where there is no effective radiation from the antenna.

The measured and simulated maximum realized gain values versus frequency of the proposed SRR-loaded antenna of configuration B, plotted in Figure 10, exhibits a drastically reduced gain of −3.65 dBi at 6.61 GHz, with an acceptable gain in the range of 1 to 3 dBi over the rest of the UWB spectrum, reconfirming the presence of a strong notch contributed by the SRRs’ resonance. Figure 11 shows the measured and simulated S11 versus frequency response of antenna configuration C, with the SRRs loaded separately by printing them on a superstrate placed on the ARMA’s CPW feed region.

As illustrated in Figure 11, a measured frequency notch at 7.46 GHz is contributed because of the superstrate-loaded SRRs. This configuration exhibits a consistent monopole-type radiation pattern over the entire UWB spectrum, as with configuration B, and with a similar maximum gain profile. The notch frequency of this configuration can be easily controlled by judiciously selecting the SRRs’ geometrical parameters printed on the superstrate. This enables the same antenna to be used for

FIGURE 8. The simulated and measured S11 characteristics of the proposed CPW-fed ARMA loaded with square SRRs beneath the substrate for a single-notched UWB response (configuration B).

FIGURE 12. The simulated S11 characteristics of the proposed CPW-fed ARMA loaded with square SRRs on the superstrate for varying SRR dimensions. c = 0.5 mm, d = 0.3 mm, and

g = 0.5 mm.

FIGURE 15. The simulated and measured maximum realized gain (xy plane) characteristics of the proposed CPW-fed ARMA loaded with square SRRs beneath the substrate

and on the superstrate for a dual-notched UWB response (configuration D).

FIGURE 13. The simulated and measured S11 characteristics of the proposed CPW-fed ARMA loaded with square SRRs beneath the substrate and on the superstrate for a dualnotched UWB response (configuration D).

The frequency notch in configuration C and the second notch of configuration D can be reconfigured to some other frequency by loading a different superstrate layer with the proper selection of the SRRs’ geometrical parameters. The measured gain reduction at the notch frequency for all of the configurations is in the range of 7 to 5 dB over the entire UWB spectrum. For a complete rejection of the unwanted band, a proper design of the receiving stage with the desired sensitivity is crucial. On the other hand, for applications requiring a better gain separation at the notch frequency, additional SRRs of the same size can be employed beneath the substrate or above the superstrate. Alternatively, the use of a thinner high-dielectric-constant superstrate (for configurations C and D) provides a stronger resonance because of the better coupling of the field, which, in turn, helps to obtain a reduced gain at the notch frequency.

As with basic UWB configuration A, the SRR-loaded con-

FIGURE 14. The measured normalized (a) E-plane (xy) and (b) H-plane (xz) copolar radiation pattern of the dual-notched UWB antenna of configuration D.

Loading copper strips between the CPW signal line and the ground planes in the previous configurations of frequency-notched UWB antennas (configurations B–D) leads to complementary impedance characteristics that produce narrow-band antennas (configurations E, F, and

Frequency (GHz)

Frequency (GHz)

12

A

10

8

6

4

Distance (mm)

(a)

Distance (mm)

(c)

Frequency (GHz)

Distance (mm)

(b)

Poynting_Ma

2.1000e+005

1.9456e+005

1.7912e+005

1.6368e+005

1.4823e+005

1.3279e+005

1.1735e+005

1.0191e+005

8.6470e+004

7.1028e+004