A Compact Tri-Band Antenna Based on Inverted-L Stubs for Smart Devices

: We designed and constructed a novel, compact tri-band monopole antenna for intelligent devices. Multiband behavior was achieved by placing inverted-L shaped stubs of various lengths in a triangular monopole antenna fed by a coplanar waveguide. The resonance frequency of each band can be controlled by varying the length of the corresponding stub. Three bands, at 2.4 (2.37–2.51), 3.5 (3.34–3.71), and 5.5 (4.6–6.4) GHz, were easily obtained using three stubs of different lengths. For miniaturization, a portion of the longest stub (at 2.4 GHz) was printed on the opposite side of the substrate, and connected to the main stub via a shorting pin. To validate the concept, the antenna was fabricated on a low-cost 1.6-mm-thick FR-4 substrate with dimensions of 20 × 15 × 1.6 mm 3 . The antenna exhibited a moderate average gain of 2.9 dBi with an omnidirectional radiations over the bandwidths required for RFID, Bluetooth, ISM, WiMAX, and WLAN-band applications. These features make the antenna suitable for compact smart devices.


Introduction
The need for multiband, planar, low-profile compact antennas has increased given the incorporation of multiple communications systems within a single electronic device. The systems operate at various frequencies ranging from 2 to 7 GHz. Popular frequency bands include the Industrial Scientific and Medical (ISM-2.4 GHz), Bluetooth (2.4 GHz), and long term evolution (LTE-2.5) bands, the 5G sub-6 GHz band (3.5 GHz), and the radio frequency identification (RFID-2.54/5.8 GHz) and wireless local area network (WLAN-2.4/5.2/5.8 GHz) bands. No single antenna can operate in all of these frequency bands; multiple antennas are required. However, the devices must be compact and inexpensive. Therefore, multiband antennas are essential [1].
Many tri-band antennas operating in major frequency bands have been used to satisfy bandwidth requirements, and ensure stable gain, high radiation efficiency, and omnidirectional radiation reception [21][22][23][24][25][26][27][28][29][30]. A compact tri-band antenna with multiple metallic strips in a dual F-shaped monopole was presented in [21]. This antenna operates in the 1.9, 3.5, and 5.5 GHz bands and has good radiation characteristics. Another tri-band antenna, resonating at 2.4, 3.5, and 5.5 GHz and exhibiting omnidirectional radiation patterns, was proposed in [22]: a truncated rectangular patch, π -shaped slot, and inverted-L slot were used to achieve the required resonance. The antenna has a high radiation efficiency (flat gain of 3.39 dBi).
A uniplanar, ACS-fed tri-band antenna with extended rectangular strips was designed for portable system applications [23]. The antenna has a modified mouse and rectangular radiating strips; three different bands can operate simultaneously. Another ACS-fed F-shaped monopole with a rectangular split-ring resonator for tri-band operation was proposed in [24]. Although simple, all of the antennas described above have unique merits. However, their physical size [25][26][27][28][29][30] must be reduced for use in modern, multi-functional compact devices.
Simple, miniaturized, highly efficient and inexpensive multiband antennas are required for handheld devices. Here, we present a compact coplanar waveguide-fed tri-band antenna covering several frequency bands, including those of ISM, RFID, Bluetooth, WiMAX, WLAN, and other leading standards (e.g., 5G sub-6 GHz). The antenna has good impedance matching, omnidirectional gain, and moderate bandwidth.

Antenna Geometry and Design
In this section, we discuss the geometry of the tri-band antenna, parameter optimization, and design steps. Initially, the antenna geometry is presented with the design variables, and later design procedure is explained in detail.

Geometry of the Tri-Band Antenna
The antenna geometry is shown in Fig. 1. Three inverted-L shaped stubs varying in length, but with the same width (w), are connected to a triangular monopole antenna fed by a 50-ohm coplanar waveguide strip line. To minimize size, a portion of the longest stub is printed on the back of the substrate and connected to the other portion via a metallic pin. A low-cost FR-4 substrate with a dielectric constant ( r ) of 4.4 and loss tangent (tanδ) of 0.02 was used to print the antenna. The overall footprint of the antenna is given by A×W ×h. The optimized parameters of the antenna are as follows:

Antenna Design Procedure
The design process began with a simulation, performed using CST Microwave Studio (Dassault Systèmes, Vélizy-Villacoublay, France), of a triangular monopole fed by a coplanar waveguide (CPW). A CPW feed has several advantages, including a broad impedance bandwidth and simple design. The length of a monopole can be arbitrarily extracted using the following equation: where c is the velocity of light in free space, and f o is the central resonating frequency which is given by: Here, λ f is the guided wavelength at the central frequency and ε eff is the effective dielectric constant which is given by: where ε r is the dielectric constant of the substrate, p is the width of the monopole, and h is the thickness of the substrate. The radiating elements of the antenna are designed using a lossy material (cooper) with a standard thickness of 0.035 mm and are printed on top of an FR-4 substrate (tan δ = 0.02 and ε r = 4.4, thickness = 1.6 mm). To ensure accurate simulation, a sub-miniature version A (SMA) connector model was used to estimate the effect of the connector on antenna performance during actual measurements. The monopole (Antenna-1; dimensions of 15 mm × 20 mm) resonates at around 6 GHz. A quarter-wavelength-sized inverted-L-shaped stub (Antenna-2) was attached to the monopole for additional resonance at 2.4 GHz. To accommodate the longer stub, the substrate size was increased to 21 mm × 20 mm. To keep the antenna compact, the stub was folded and a portion was printed on the back of the substrate, connected to the main stub via a metallic pin (Antenna-3). Then, additional (similarly optimized) inverted-L shaped stubs were added (Antennas-4 and-5) to generate resonance at 2.4/3.5/6 GHz and 2.4/3.5/5.5/6 GHz, respectively. The design evolution is shown in Fig. 2. The stub positions and dimensions were carefully chosen. With the exception of Antenna-2, all antennas are 15 mm × 20 mm in size.
The reflection coefficient (|S 11 | values) of each antenna involved in the final design is also plotted for a clear understanding of the design procedure. Fig. 3a shows the |S 11 | plots of the initial design, i.e., of the triangular monopole alone (Antenna-1) and the monopole with straight (Antenna-2) and folded (Antenna-3) inverted-L-shaped stubs. The bandwidth of Antenna-1 ranges from 5.5 to 6.5 GHz because it is partially grounded [31]. Antennas-2 and-3 exhibit very similar performance because the stub sizes are the same, providing resonance at 2.4 GHz (range: 2.39-2.51 GHz) as well as 6 GHz. The stub length was optimized for resonance at 2.4 GHz. According to the fundamental monopole antenna theory, the length of the entire stub should be about a quarter-wavelength at the resonance frequency, i.e., 31.2 mm at 2.4 GHz. The total stub length of Antenna-3 is p + l 1 + w 1 + h + w 4 = 7 + 6 + 13 + 1.6 + 5.4 = 33 mm, which is very close to the theoretical requirement. In Antenna-4, a quarter-wavelength stub is added to the initial stub. This adds p + lp 2 + w 2 = 7 + 4 + 9.8 = 20.8 mm to Antenna-3, for additional resonance at 3.5 GHz (Fig. 3b). Thus, the stubs allow current to follow several paths, explaining the multiband behavior [32].  Similarly, another inverted-L stub was added to generate an additional band at 5 GHz [Antenna-5 (proposed antenna) in Fig. 4]. The 5 and 6 GHz bands deliberately overlap to provide the required ultrawide bandwidth (4.6-6.4 GHz) for the WLAN band. Importantly, we can tune the operating frequency by varying stub length. Resonance shifts to low frequencies when stub length is increased. Operating frequency can be increased according to user requirements by reducing the stub length. This can be understood by examining the |S 11 | characteristics of the final design with various values of w 4 and w 2 (Fig. 5). To explain the radiation mechanism, the surface current densities at the resonance frequencies are plotted in Fig. 6. At 2.4 GHz, the current is mainly in stub-1, but is distributed around stub-2 at 3.5 GHz. As predicted, the current is in stub-3 at 5 GHz. Thus, the compact antenna operating on demand at 2.4, 3.5, and 5.5 GHz enables RFID, Bluetooth, WiMAX, and WLAN band applications.

Results and Discussion
We used standard photolithography to fabricate a prototype of the antenna and measured its performance (Fig. 7). The antenna is very compact and can be easily embedded in small devices, such as USB dongles. The |S 11 | values were measured using an E5071C network analyzer (Agilent, Santa Clara, CA, USA), and far-field data were obtained with the antenna in an anechoic chamber. The simulated and measured gains plotted as well as the simulated radiation efficiency as a function of frequency, are shown in Fig. 9. The antenna exhibits a minimum measured gain of 1.8 dBi at the lowest resonance (2.4 GHz), 2.9 dBi in the middle band (3.5 GHz), and 4 dBi in the upper band (5.5 GHz). In contrast, the gain decreases to −3.9 dBi in the stop-bands, validating gain suppression at out-of-band frequencies. In general, the gain increases in the pass-band with the increasing frequency due to the increased effective size of the antenna [33,34]. Similarly, the antenna shows a high radiation efficiency (up to 88%) at resonating bands, while it is reduced to 40% at out-of-band frequencies. The radiation patterns at 2.4, 3.5, 5, and 6 GHz are shown in Fig. 10. The antenna exhibits omnidirectional radiation in the H-plane and bidirectional radiation in the E-plane. These patterns are stable at all frequencies; this is a particular strength of the design.

Performance Comparison
The performance of our antenna, in terms of size, operating bands, and average gain, is compared to those of state-of-the-art tri-band antennas [21][22][23][24][25][26][27][28][29][30] in Tab. 1. Our antenna outperforms the others in terms of size, gain, and radiation stability. The antennas in [22,27,30] have higher gains but are large in size. Our antenna is simple, and the operating frequency bands can be tuned as required. The small size and good radiation characteristics reflect the careful design of inverted-L stubs on a triangular monopole.

Conclusion
We designed and built a tri-band monopole antenna based on inverted-L shaped stubs. Multiband behavior was achieved by connecting stubs of various lengths to the upper side of a triangular monopole. The antenna was fabricated on a low-cost, commercial 1.6-mm-thick FR-4 substrate. The measurements validated the computed results. The antenna is small (20 × 15 × 1.6 mm 3 ), and exhibits good impedance matching and gain/radiation patterns. The bandwidths allow operation at 2.4 (2.37-2.51), 3.5 (3.34-3.71), and 5.5 (4.6-6.4) GHz.
Funding Statement: This work was supported by the ICT R&D program of MSIT/IITP, [2019-0-00102, A Study on Public Health and Safety in a Complex EMF Environment]. This work was also supported by the National Radio ResearchAgency, [Rapid measurement system for new technologyantenna].

Conflicts of Interest:
The authors declare that they have no conflicts of interest to report regarding the present study.