Introduction
With the tremendous increase in wireless communication, the demand for miniaturized radio frequency (RF) front-end systems has risen. Antennas and filters are the essential passive devices in the RF front end. These two passive devices are connected by transmission lines, leading to increased complexity, large size, impedance mismatch, and more loss [Reference Tamijani, Rizk and Rebeiz1, Reference Luo, Hong, Tang and Chen2]. Impedance mismatching circuits are introduced between the antenna and filter to reduce the loss [Reference Lee, Kidera, Pinel, Laskar and Tentzeris3–Reference Troubat, Bila, Th’evenot, Baillargeat, Monedilere, Verdeyme and Jecko5], but it reduces antenna gain and filter selectivity. Another method of filtering antenna design is introducing an impedance transformer between the radiating structure and the filter element. This method eliminates 50 Ω limitations between the filtering element and the antenna, but the selectivity and gain of the filter are reduced [Reference Zuo, Chen, Han, Li and Zhang6–Reference Quere, Quendo, El Hajj and Person8]. The method of implementing vias, stubs, slots, notches, and shorts on the radiating element of the filtering antenna is discussed in [Reference Chen, Zhao, Yan and Zhang9–Reference Ren, Xiong, Deng, Yin, Zhang and Guo13]. This approach reduced the size, but the selectivity of the filter deteriorated. Open loop filter resonators are coupled electrically to the radiator to filter the desired bands, as discussed in [Reference Tang, Chen and Ziolkowski14, Reference Deng, Tan, Hou, Sun and Guo15]. In this method, the structure is a little more complex, and the gain performance is less because of the loss during improper coupling. Recent research incorporates filters with radiators in two different techniques. One is the filter network loaded on the feed line of the main radiating structure, and the other is integrating the filtering element with an antenna using additional components like strips, switches, and biasing circuits. Integrating additional components with the existing planar structure is challenging and needs matching networks to switch to other operations bands.
A triangular ring stub-based multimode resonator is interdigitally coupled to the microstrip feed line to remove the out-of-band signal of an ultra-wideband (UWB) monopole antenna, which is explained in [Reference Sahoo, Gupta and Parihar16]. It is observed that the coupling between the multimode resonator and the feed line improved by removing ground under the coupling region. A multiple-mode resonator filter integrated with an elliptical patch monopole antenna with a 50 Ω microstrip feed line for cognitive radio application is described in [Reference Shome, Khan, Koul and Antar17]. This filtering antenna is tunable between the C band and UWB using PIN diode switches. The antenna gain is low and attributed to the power distribution over the wide frequency band. A bow tie–based UWB antenna is integrated with a fork-shaped resonator filter to the feed line using diodes tunable to UWB, 3.5, and 5.5 GHz with different switching states [Reference Potti, Balaji, Gulam Nabi Alsath, Savarimuthu, Selvam and Valavan18]. The antenna exhibits good radiation characteristics and gain, but the dimension is large, and the system complexity is high. A compact tunable filtering UWB antenna with an H- and T-shaped resonator filtering structures coupled with a feed line is described in [Reference Atallah, Abdel-Rahman, Yoshitomi and Pokharel19]. A varactor with the necessary biasing circuit is utilized to tune for different frequency bands. The antenna is constructed on a RO3003™ substrate. It is noticed that the defected ground structure is adopted to improve the insertion loss of the filter and the coupling among the feed line lines and the microstrip radiator. A compact tunable symmetrical ring resonator-based band-pass filter coupled with the microstrip feed line of a UWB antenna is discussed in [Reference Atallah, Abdel-Rahman, Yoshitomi and Pokharel20]. The antenna is built on a ROGERS 3003 substrate, and the gain of the antenna is higher than 1.5 dBi. This less gain is attributed to additional component losses. The varactor fed by the biasing circuit tunes continuously in the narrow band frequency region.
A planar absorptive-type quasi-Yagi Uda antenna with filtering capability is demonstrated in [Reference Wang, Fan, Gómez-García, Yang, Li, Wong and Zhang21]. A matching network is incorporated between the absorptive band stop filter and antenna to achieve good impedance matching. The antenna’s filtering capability and inband flat gain characteristics are the key aspects of the proposed filtering antenna. The researchers have exploited a second-order band stop filter with ground resistance for achieving the said characteristics in the quasi-Yagi antenna. A low-pass filter (LPF) is integrated at the SubMiniature version A (SMA) connector connecting to the feed line of a super wideband antenna without adopting any matching network. The gain and radiation efficiency of the antenna is good, but the filter structure is very complex [Reference Parthasarathy, Ramesh Venkatesan, Arumugam and Ponnusamy22].
The research works in loading the filter in the microstrip feed line of the antenna are limited. Hence, this work is designed to load the filter directly in the antenna feed line with a simple filter structure. The design concept presented in this research is suitable for rejecting frequencies not interested in communication radio. Also, this concept extended to any monopole antenna or microstrip patch antenna that throws harmonics at high frequencies. The work is systematically organized to present the development of a UWB antenna, the design of the filter section, and the integration of both the antenna and filter. Since the proposed solution is scalable to any antenna, a UWB radiator is preferred to demonstrate the filtering characteristics. The reflection coefficient characteristics of the UWB antenna are modified to demonstrate the high-frequency rejection and harmonic suppression in antennas. As described earlier, this solution extended to any radiator.
Design of the UWB antenna
The proposed UWB antenna is constructed on an FR4 of dimension 43 × 30 mm with a thickness of 1.6 mm, a dielectric constant of 4.3, and a loss tangent of 0.023. The antenna simulations are conducted using Computer Simulation Technolog (CST) Studio Suite 2018, with the number of lines per wavelength being 15 and the accuracy being 10−4. The evolution stages and the geometry of the proposed UWB antenna are shown in Fig. 1. The reflection coefficient characteristics of the evolution stages are illustrated in Fig. 2. The antenna size is chosen relatively high to accommodate an LPF in the feed line and to get good gain performance. The partial ground plane of 22 × 30 mm is taken for good radiation efficiency.
The evolution stages of the proposed chamfered edge UWB antenna: (a) stage 1, (b) stage 2, and (C) stage 3—the proposed antenna.
Reflection coefficient characteristics of the evolution stages of the UWB antenna.
The evolution begins with a rectangular monopole antenna of patch size 19 × 28 mm, as shown in Fig. 1(a), and is excited using a simple microstrip line of 50 Ω characteristic impedance. The antenna shown in Fig. 1(a) delivers poor impedance matching in the UWB frequency region with reflection coefficient |S 11| > −10 dB, as evident from Fig. 2 stage 1. The impedance matching is further enhanced by chamfering the lowest edges of the rectangular patch to attain a beveled monopole antenna, as described in Fig. 1(b). The chamfering technique is a popular way to enhance the realized bandwidth of the antenna and impedance matching. The impedance matching can be controlled by the chamfering radius ‘r’. The radius value varies in steps for good impedance matching in the UWB frequency range. At the optimum value of 11 mm, better impedance matching is achieved from 2.5 to 3 GHz and 4 to 11.9 GHz, where the |S 11| ≤ −10 dB, as shown in Fig. 2 stage 2.
The antenna in Fig. 1(b) does not provide suitable impedance matching for the entire UWB range, and it can be attained by making a slot in the ground, as shown in Fig. 1(c). The geometry of the ground slot is selected to match the lower edge of the chamfered radiating patch of the antenna with a depth of 2 mm to improve the capacitance effect and thereby achieve the impedance matching over the wideband, as shown in Fig. 2 stage 3. The proposed chamfered edge UWB antenna shown in Fig. 1(c) offers a fractional bandwidth of 135% operating in the UWB region from 2.1 to 12 GHz with |S 11| ≤ −10 dB. The optimized dimensions of the monopole antenna are shown in Table 1.
Dimension details of the proposed UWB antenna
Design of low-pass filter
This section explains the design of an LPF to fit in the UWB antenna to make a filtering antenna. The current research aims to suppress the high-frequency components the antenna positioned in the transceiver system picks up. The UWB antenna deliberated formerly is assumed as the antenna that is integrated into a narrowband radio operating at 2.45 GHz. The UWB antenna can receive high-frequency components, which are considered noise by the 2.45 GHz radio. This noise reception is detrimental to the radio system due to the poor signal-to-noise performance of the transceiver. The UWB antenna is connected to a planar filter to eliminate this noise and improve the overall SNR before it is connected to the transceiver circuitry. The feed line–integrated filter is in addition to the existing filter in the transceiver architecture. Adding the feed line filter reduces the filter overhead and improves the out-of-band performance of the overall communication system.
An LPF section is designed and included in the chamfered edge UWB antenna’s feed line to eliminate the high-frequency components. Since the proposed research intends to improve the SNR of the 2.45 GHz radio, the LPF is designed to offer a cutoff frequency of 3 GHz. The geometry of the LPF is illustrated in Fig. 3.
The geometry of the LPF section.
The filter is built on a 1.6-mm-thick FR4 substrate with a dielectric constant of 4.3 and a loss tangent of 0.023. The proposed LPF contains a central stepped impedance transmission line loaded with an H-unit cell. The stepped impedance structure provides low-pass filtering characteristics, whose roll-off rate is improvized using a resonator loaded onto the main transmission line. This research uses a three-element “H” unit cell resonator of two vertical strips of length “l”, a center horizontal strip of 3 mm length, and spacing g = 0.75 mm from the stepped impedance transmission line to improve the attenuation properties of the filter. The length of the vertical arms of the “H” resonator controls the stop-band roll-off rate. Further, the upper transmission zero can also be quickly altered by controlling the length “l” of the H-unit cell, keeping the width w 1 constant. The filter structure adopter, including stepped impedance structure and “H”-shaped unit cell, has been adopted in this research due to its symmetrical structure, which can give more symmetry in the omnidirectional radiation pattern.
The transmission coefficient characteristics of the proposed LPF have been described in Fig. 4. The figure inferred that sharp transition is realizable at the cutoff frequencies by increasing the length “l” of the H-unit cell. Furthermore, transmission zeros are obtained with an increase in l. An attenuation greater than −30 dB is brought near the cutoff frequency of the proposed LPF for the optimum value of l = 6.5 mm. The insertion loss of the proposed filter is close to 0 dB within the passband, resulting in good transmission properties while integrating the filter with the antenna in the future. The corresponding reflection coefficient characteristics are illustrated in Fig. 5. As evident from the figure, the transmission bandwidth is extended with an increase in l. The reflection coefficient is more than −30 dB in and around the desired operating frequency of 2.45 GHz. The optimized dimensions of the proposed LPF are described in Table 2.
Transmission coefficient characteristics of the LPF.
Reflection coefficient characteristics of the LPF.
Dimension details of the proposed LPF
Design of filtering antenna
This section describes the design of a passive filtering antenna, which is anticipated to form the transmission characteristics of the UWB antenna. As explained earlier, the UWB antenna is considered an antenna with multiple harmonics, which is inappropriate for a communication system that works at 2.45 GHz. The integration of the LPF (discussed in the section “Design of low-pass filter”) with the chamfered edge UWB antenna (discussed in the section “Design of the UWB antenna) is described in this section.
The LPF integrated with the proposed UWB antenna is shown in Fig. 6. The filter is loaded on the antenna’s feed line. The antenna has an overall footprint of 43 × 30 mm and is constructed on an FR4 substrate of a thickness of 1.6 mm, dielectric constant ε r = 4.3, and loss tangent δ = 0.023. The filter connects the 50 ohm characteristic impedance SMA connector and the edge-chamfered UWB antenna. The LPF eliminates the high-frequency components and makes the radiating patch receive excitation currents that fall within the passband of the LPF. Thus, the chamfered edge UWB antenna is transformed into a narrowband antenna that suits the requirements of the 2.45 GHz communication radio. The reflection coefficient characteristics of the proposed filtering antenna are shown in Fig. 7. The designed UWB antenna with a fractional bandwidth of 135% is transformed into a narrowband antenna with a fractional bandwidth of 29.5% ranging from 2.1 to 2.82 GHz centered at 2.45 GHz.
Passive filtering antenna: (a) front view and (b) rear view.
Reflection coefficient characteristics of the proposed filtering antenna.
The surface current distribution of the conduction and blocking state of the filtering antenna is illustrated in Fig. 8. During the current conduction state at 2.45 GHz, the current flows from the feed to the microstrip patch radiator, as described in Fig. 8(a). The current from the SMA line is absorbed in the LPF during the blocking state, and it does not reach the edges of the radiator, resulting in poor radiation efficiency. The surface current distribution under the blocking state at the sample frequency of 5.2 GHz is illustrated in Fig. 8(b), where a weak current flows from the SMA to the primary radiating patch.
Surface current distribution: (a) conduction state at 2.45 GHz and (b) blocking state at 5.2 GHz.
Equivalent circuit analysis
The filtering antenna performance is analyzed using the equivalent circuit, and the simulations are performed with the help of the advanced design system (ADS). The approximate equivalent circuit of the proposed filter-integrated antenna is shown in Fig. 9. The UWB antenna is represented as a series-connected RLC circuit, where each RLC represents the narrowband antennas. The combination of multiple RLC circuits provides overlapping frequency bandwidths, resulting in the realization of UWB frequency response. The frequency response of the antenna is tailored using an LPF. The LPF of the proposed antenna is represented by the inductor in the series arm and a series LC section in the shunt arm of the equivalent circuit. The inductor L s in the series arm and the capacitor C f1 in the shunt arm represent the low-pass filtering action, wherein the strong attenuation near the cutoff frequency is realized by adding an inductor L f1 in series with C f1. The shunt L f1C f1 forms a series resonant circuit, which provides infinite attenuation near the cutoff frequency.
Equivalent circuit of the proposed filtering antenna.
The frequency of infinite attenuation is located at 3 GHz, closer to the operating frequency of 2.45 GHz. The L s, L f1, and C f1 component values are calculated using Ryder 1961 [Reference Ryder23]. The complete equivalent circuit of the designed filtering antenna is shown in Fig. 9. The S-parameter characteristics of the LPF section before integrating with the antenna are analyzed using ADS. For 50 ohm termination, the L s and C f1 values of the LPF with Chebeshev characteristics are 1.47 nH and 586 fF. The S-parameters in Fig. 10 show that the roll-off rate is poor for the LPF. The roll-off rate is enhanced by adding an inductance, L f1, in the shunt arm. The addition of L f1 alters the characteristic impedance of the filter network. Hence, the newly calculated component values for the proposed filter are L s = 5.47 nH, L f1 = 34.30 nH, and C f = 114.3 fF. The resultant S-parameters closely match the narrow band results obtained in the three-dimensional (3D) electromagnetic solver, as evident in Fig. 10. The insertion loss is close to 0 dB and the reflection coefficient is less than −10 dB in the narrow band region.
S-parameter characteristics of the LPF (dashed line: LPF with capacitive shunt arm; solid line: LPF with LC shunt arm).
The equivalent circuit simulation of the proposed filtering antenna described in Fig. 9 is evaluated for S-parameter characteristics using ADS. The filter-integrated UWB antenna provides a narrow operating band centered around 2.45 GHz. The results obtained using ADS are compared against the CST simulation results and are shown in Fig. 11. The equivalent circuit modeling is close to the results obtained in the 3D electromagnetic tool. The difference in bandwidth is attributed to the utilization of dielectric material in CST, which is not considered in the case of ADS simulation.
S-parameter characteristics of the proposed filtering antenna using CST and ADS.
Fabrication and measurement
The conventional photolithography process is adopted to fabricate the prototype of the filtering antenna. A 50 ohm edge mount receptacle excites the passive filtering antenna. The antenna measurements are performed using Fieldfox microwave analyzer N9917A operating from 30 kHz to 18 GHz, as illustrated in Fig. 12(a). The proposed antenna simulated and measured reflection coefficient characteristics as shown in Fig. 12(b). The measured fractional bandwidth of the proposed antenna is 30%. From the figure, it is evident that the simulation results agree with the measurement results.
(a) Measurement setup and (b) simulated and measured S-parameter characteristics of the proposed filtering antenna.
The radiation performance of the proposed filtering antenna is tested in an anechoic chamber. A standard JR12 model double-ridged waveguide horn can work from 1 to 12 GHz and is used as the transmitter antenna to estimate the radiation performance of the proposed filtering antenna. The photograph of the measurement setup is shown in Fig. 13. The simulated radiation pattern of the reference UWB antenna in Fig. 1(c) is described in Fig. 14(a). The antenna has an omnidirectional radiation pattern with a figure “8”-like radiation pattern along the elevation plane. The filter-integrated antenna’s simulated and measured radiation pattern is described in Fig. 14(b). The radiation patterns of the symmetrical filter structure integrated antenna in the azimuth and elevation plane are symmetrical a bit more. Integrating the filter into the feed line does not alter the original characteristics of the monopole antenna since it does not disturb the radiating aperture. The slight deviation between the simulated and the measured results are attributed to the cable losses and transmitting antenna. The gain and efficiency of the proposed monopole antenna are greater than 75% and 4.2 dBi, respectively, during the simulation and measurement at 2.45 GHz, as shown in Fig. 15. The proposed filter-integrated antenna has a larger aperture and radiating edges, which leads to more gain and efficiency. According to antenna theory, the antenna’s gain is directly proportional to the radiator’s electrical size.
Experimental setup inside the anechoic chamber.
Radiation pattern at 2.45 GHz: (a) the reference antenna and (b) the proposed filtering antenna.
Gain and efficiency characteristics of the proposed filtering antenna.
The proposed passive filtering antenna performance is compared with the existing research and illustrated in Table 3. The followings are inferences made from the table:
1. The antenna geometry is simple and small compared to that of Tang et al. [Reference Tang, Chen and Ziolkowski14], Jian et al. [Reference Ren, Xiong, Deng, Yin, Zhang and Guo13], and Wang et al. [Reference Wang, Fan, Gómez-García, Yang, Li, Wong and Zhang21].
2. The proposed antenna has a stable omnidirectional radiation pattern, in contrast to conventional notch band antennas that have significant radiation pattern distortion.
3. When compared to Tang et al. [Reference Tang, Chen and Ziolkowski14], Atallah et al. [Reference Atallah, Abdel-Rahman, Yoshitomi and Pokharel19, Reference Atallah, Abdel-Rahman, Yoshitomi and Pokharel20], and Jian et al. [Reference Ren, Xiong, Deng, Yin, Zhang and Guo13], the proposed filtering antenna has high gain characteristics.
4. The simple H-unit cell filter is loaded in the feed line itself without adding additional components like diodes, biasing circuits, and matching networks when compared to Atallah et al. [Reference Atallah, Abdel-Rahman, Yoshitomi and Pokharel19, Reference Atallah, Abdel-Rahman, Yoshitomi and Pokharel20] and Wang et al. [Reference Wang, Fan, Gómez-García, Yang, Li, Wong and Zhang21].
5. The complexity level of the proposed filtering antenna is less when compared to the existing works stated.
6. The filter-integrated UWB antenna provides a good harmonic rejection at high frequencies, where the rejection is boosted by integrating an LPF in the feed line.
Comparison of the proposed filtering antenna with the existing research works
NR, Not Reported.
Conclusion
The design of a passive filtering antenna containing an LPF constructed with a central stepped impedance transmission line loaded with an H-unit cell in the microstrip feed line of the antenna is discussed in this article. The passive filtering antenna shows good filtering performance and radiation characteristics with good harmonic suppression till 12 GHz. The edges of the antenna are champed to operate in the UWB frequency range and obtain fractional bandwidth of 135%. The UWB antenna is converted into a narrowband antenna operating at 2.45 GHz with a bandwidth of 29.5% (ranging from 2.1 to 2.82 GHz) by modifying the microstrip feed line of the UWB antenna. The design of the LPF elaborated with necessary parametric analysis by changing the length of the H-unit cell. The novel H-unit cell-loaded LPF offered flexibility in adjusting the transmission bandwidth and the upper cutoff frequency of the LPF. The filter-integrated UWB antenna is fabricated and tested, and the results are presented. The measured and simulated characteristics are on good terms. The proposed passive filtering antenna offered a gain of more than 4.2 dBi. The results show that the proposed filtering antenna is suitable for out-of-band rejection in communication radio.
Funding statement
This research received no specific grant from any funding agency, commercial, or not-for-profit sectors.
Competing interests
The authors report no conflict of interest.
Ms. Bhakkiyalakshmi Ramakrishnan is working as an Assistant Professor in the Department of Electronics and Communication Engineering (ECE) at SRM Institute of Science and Technology (SRMIST). She is currently pursuing her PhD under the guidance of Dr. M. S. Vasanthi, Associate Professor, Department of ECE, SRMIST. Her research interests include antenna design and synthesis.
Dr. Vasathi Murugiah Sivashanmugham is an Associate Professor in the Department of Electronics and Communication Engineering, SRM Institute of Science and Technology, Kattankulathur. She earned her PhD in the year 2014 for her research on wireless sensor communications. She has a rich academic experience from 1995 and has been instrumental in organizing several research-oriented workshops and conferences. She is a life member of ISTE & IETE and Member of IEEE. Her research interests include wireless sensor communications and the development of integrated antenna systems.
