Design of Low-Profile Single- and Dual-Band Antennas for IoT Applications

Abstract

This paper presents novel low-cost single- and dual-band microstrip patch antennas. The proposed antennas are realized on a square microstrip patch etched symmetrically with four slots. The antenna is designed to have low cost and reduced size to use in Internet of things (IoT) applications. The antennas provide a reconfigurable architecture that allows operation in different wireless communication bands. The proposed structure can be adjusted to operate either in single band or in dual-band operation. Two prototypes are implemented and evaluated. The first structure works at a single resonance frequency (f₁ = 2.4 GHz); however, the second configuration works at two resonance frequencies (f₁ = 2.4 GHz and f₂ = 2.8 GHz) within the same size. These antennas use a low-cost FR-4 dielectric substrate. The 2.4 GHz is allotted for the industrial, scientific, and medical (ISM) band, and the 2.8 GHz is allocated to verify the concept and can be adjusted to meet the user’s requirements. The measurement of the fabricated antennas closely matches the simulated results.

1. Introduction

Lately, technology growth has been increasing rapidly. Most things in the world and human life will be affected by the Internet. Therefore, the Internet of things (IoT) received extensive consideration in observing and sensing applications [1,2]. The IoT consists of the observation region for sensing data, network region for transmitting data, and application region for connecting the objects and the cloud network. In the IoT, numeral devices, sensors, equipment, and things communicate with others through the cloud network [3]. Many antenna configuration types for IoT applications have been presented in recent years [4,5,6].

Single-band antennas can be used in some general equipment. However, modern systems need multi-functionally and smart antennas, which demonstrate higher efficiency and are compact. As technology rapidly grows, modern wireless systems (IoT devices) exhibit various functions and require operation at more than one frequency without increasing the antenna size [7,8,9,10]. In the literature, different shapes of the single-band and multi-band monopole antennas have been developed, such as H-shaped [11], circular-shaped [12], U-shaped open stub [13], M-shaped open stub [14], L-shape with coupled branch strip [9], L-shaped stub folded into U-shaped [4], D-shaped [15], U-shaped with rectangular patch inside [16], square spiral loop [17], tapered rectangular patch antenna [5], and symmetric bow-tie circular patch antenna [18]. Dual-band or multi-band operations can be obtained by etching some slots either in the radiating elements [3,18,19,20,21,22], in the ground plane of the antennas [23,24,25,26], or both [9,27,28]. A square microstrip patch etched with four symmetrical slots [29,30,31,32,33] has been proposed for nondegenerate dual-mode filter applications.

Table 1. Comparison of the proposed antenna with another state-of-the-art.

Ref.	Antenna Type	Substrate	No. of Ports	Resonance Frequencies (GHz)	Dimensions (mm2)	Maximum Gain (dB)	Bandwidth (MHz)
[1]	Slotted patch	FR-4	2	1.73 and 2.53	48 × 48	−3.8 and 1.9	30
[4]	U-shaped	Rogers RO4003	1	3.5	14 × 14	1.42	200
[5]	Slotted patch	FR-4	5	0.9, 1, 1.72, 2.18, 5 and 5.5	120 × 65	0.53, −0.96, 1.97, 3.75, 2.46, and 4.9	150
[6]	PIFA	FR-4	1	1.8 and 2.4	45 × 32	1.6 and 1.8	50
[11]	H-shaped	Rogers RT5880	1	1.8	72 × 17	5.5	40
[12]	Slotted circular patch	FR-4	1	1.8	24 × 40	1.9	180
[34]	Meander line	FR-4	1	2.4	40 × 10	1.35	146
[35]	Slotted	FR-4	1	2.4, 3.5, and 5.8	80 × 80	2.33, 3.14, and 2.89	140
[36]	C-shaped	FR-4	1	1.8, 2.4, and 5	209 × 260	4.40, 4.75, and 5.25	100
[15]	Monopole	FR-4	1	2.4 and 5	34 × 26	2.2 and 2	220
This work	Slotted square	Rogers RT5880	1	2.4	41.5 × 38.5	3.39	18
				2.4 and 2.8	38.5 × 38.5	3.45 and 3.2	20
	Wideband slotted square	FR-4		2.4	36.4 × 36.4	2.45	220

shows the performance comparison for previously published single-, dual-, and multi-band antennas. In this table, three of the proposed structures with good performance are included.

This study proposes a slotted square patch antenna for IoT applications. The antenna is designed to have low cost and reduced size to allow use in IoT applications. The antenna provides a reconfigurable architecture that allows operation in receive or transmit modes at the specified frequency. In addition, the antenna can be used in a full-duplex system transmitting and receiving at different frequencies. This study can reduce the interference with other nodes in the IoT network. The proposed antenna can operate at dual resonant frequencies. Two prototypes are designed and fabricated on low-cost FR-4 substrate (ε_r = 4.3, tan σ = 0.025, and h = 1.6 mm). The first configuration works at a single resonance frequency (f₁ = 2.4 GHz); however, the second configuration was developed to work at two resonance frequencies (f₁ = 2.4 GHz and f₂ = 2.8 GHz) within the same size. The two prototypes are fabricated, and the results are validated. The measured results correlate well with the simulated one.

The study is scheduled as follows. The non-slotted and slotted square patch antennas are discussed in Section 2 and Section 3, respectively. In Section 4, the single-band antenna is designed and explained. The dual-band antenna is discussed in Section 5. In Section 6, the bandwidth improvement is discussed. Section 7 displays the simulated and measured results for both antennas. The conclusion and discussions are presented in Section 8.

2. Non-Slotted Square Patch

The square patch resonator can be considered as a square cavity with magnetic walls. The field inside the square cavity matches those of ${\mathrm{TM}}_{\mathrm{mn}0}^{\mathrm{z}}$ modes [37]. The consequential modes of the square patch type (${\mathrm{TM}}_{\mathrm{mn}0}^{\mathrm{z}}$) can be calculated using the same method as described in [29]. The resonance frequencies of the resultant modes for the non-slotted square patch can be obtained using [38]:

$$f_{m, n}=\frac{150}{\alpha \sqrt{{\epsilon }_{eff}}}\sqrt{m^2+n^2} \mathrm{GHz},$$

(1)

where m and n are the nonnegative integer numbers and the effective relative permittivity $\left({\mathsf{\epsilon }}_{\mathrm{eff}}\right)$ is given using

$${\epsilon }_{eff}=\{\begin{array}{c}{\epsilon }_{eff}\left(w\right) if m=0 \\ {\epsilon }_{eff}\left(w\right) if n=0 \\ \frac{{\epsilon }^2{}_{eff}\left(w\right)}{{\epsilon }_r} otherwise\end{array}$$

(2)

$${\epsilon }_{eff}\left(w\right)=\frac{1}{2}\left({\epsilon }_r+1+\frac{\left({\epsilon }_r-1\right)}{\sqrt{\left(1+\frac{10h}{w}\right)}}\right)$$

(3)

where w is the square width in mm, and α is the effective width of the square, given as:

$$\alpha =w+2\left|\Delta w\right| \mathrm{mm}$$

(4)

$$\Delta w=0.412h\frac{{\epsilon }_{eff}\left(w\right)+\left(\frac{0.3w}{h}\right)+0.264}{{\epsilon }_{eff}\left(w\right)-\left(\frac{0.258w}{h}\right)+0.8},$$

(5)

where h is the substrate thickness in mm.

The performance of a microstrip antenna is affected by fringing fields. The strength of the fringing field is mainly dependent on the patch’s dimensions and the substrate’s height. Due to the fringing field, the physical dimensions of the square patch appear smaller than its effective electrical dimensions [39]. Because of the fringing field, the effective width (α) increased by 2∆w.

In our design, the 20 mm long non-slotted square patch is designed on the top of the low-cost FR-4 substrate with relative permittivity (ε_r = 4.3) and thickness of h = 1.6 mm. Here, the resonance frequencies of the first two fundamental modes (degenerate modes), TM₁₀₀ and TM₀₁₀, are equal (f₁₀₀ = f₀₁₀ = 2.805 GHz). For the TM₁₁₀ mode, the resonance frequency is f₁₁₀ = 3.966 GHz. Modes’ resonance frequencies depend on the current distribution of each mode. As an example, the current parallel to the X-direction excites only the TM₁₀₀ mode, the current parallel to Y-direction excites only the TM₀₁₀ mode, and the current parallel to the diagonal excites the three modes. The feeding mechanism mainly determines the current in the patch. Therefore, appropriate feeding can control the excited modes and thus the resonance frequencies of the patch. The symmetric feed lines are used to excite the single dominant mode; however, asymmetrical feed lines are used to excite the two degenerate modes [29]. In addition to that, differential excitation is used to excite the modes, such as the TM₁₀ and TM₃₀ modes for rectangular patch [40,41], and TM₁₀ and TM₁₁ modes for an equilateral triangular microstrip patch antenna [42].

3. Slotted Square Patch

Figure 1 illustrates the geometry of the proposed slotted square patch antenna. The main building block of the proposed antenna is a microstrip square patch with length L. The square patch is etched with four symmetrical slots along X- and Y-axes, as shown in the figure. All slots have the same length (L_S) and width (W_S).

For the slotted square patch in Figure 1, the vertical slots (along the Y-axis) cut the current parallel to the X-axis and increase the effective length of the TM₁₀₀ mode and decrease its resonance frequency. Similarly, the horizontal slots cut the contemporary parallel to the Y-axis and increase the effective length of the TM₀₁₀ mode that reduces its resonance frequency. For symmetrical slots, both the TM₁₀₀ and TM₀₁₀ are affected simultaneously in the same manner, and therefore they have the same resonance frequency denoted f₁. The resonance frequencies of these two modes decrease as the lengths of the slots Ls increase. These two modes can be excited simultaneously, or only one of them can be excited according to the feeding mechanism. The vertical and horizontal slots cut the current of the TM₁₁₀ mode, and thus, its resonance frequency, denoted as f₂, decreases faster with the lengths of the slots Ls. The slots’ resultant modes can be considered distorted types of those described in the above section. The effect of the length of the slot Ls and width Ws on the resonance frequencies of the modes TM₁₀₀, TM₀₁₀, and TM₁₁₀, is studied using the patch of side L = 20 mm on low-cost FR-4 substrate with ε_r = 4.3, thickness h = 1.6 mm, and loss tangent of 0.025. The resonance frequencies f₁ and f₂ generated by computer simulation technology (CST) simulators using the same technique presented in [29] are shown in Figure 2. The resonance frequencies f₁ and f₂ decrease as the slots’ Ls lengths increase, but f₂ decreases faster since it is affected by both vertical and horizontal slots. It has to be shown that these frequencies are also affected by the width of the slots, and this can be used to add more fine control of the resonance frequencies of each mode, as will be seen in the next section. Figure 2 can be generated for any other patch length L and any other dielectric material.

4. Single-Band Antenna Design

The same patch size of length L = 20 mm is considered in this design. Only one of the two modes, TM₁₀₀ and TM₀₁₀, can be excited by the inset feed. The TM₁₀₀ mode is the only exciting mode if the feeder is positioned along the X-axis as shown in Figure 3. The width of the feed line can be optimized for matching purposes. The single band operation at the specific frequency of this antenna can be adjusted by the appropriate selection of the slot’s length and width. The characteristics of the designed antenna were simulated and calculated using a CST simulator. Figure 4a,b shows the effect of the dimensions of the slot on S₁₁ performance of the antenna. First, Figure 4a shows the result of the L_S variation on the resonance frequency f₁ when the slot width is 1 mm.

As slot length increases, the resonance frequency f₁ decreases. The effect of slots width Ws on the resonance frequency f₁ is observed in Figure 4b for slot length = 8.5 mm. The slot’s width can be used for fine adjustment of f₁. As slot width increases, the resonance frequency f₁ decreases.

The single band antenna design is selected to operate at 2.4 GHz, corresponding to slot length 8.5 mm and width 1 mm. The simulated S₁₁ is shown in Figure 5.

Figure 6 shows the surface current distribution for the developed single-band antenna at 2.4 GHz. It is observed from this figure that the current along the X-axis turned around the vertical slots and increased the effective length of the patch.

In addition, the simulated 2D and 3D radiation patterns at 2.4 GHz are shown in Figure 7. It has been noted that the proposed antenna gain with FR-4 substrate is less than 0 dB, while the directivity = 4.72 dB. This gain reduction is due to the low performance of the low-cost FR-4 substrate. The low gain can be overcome using high-quality dielectric material such as RT Duroid 5880. Another antenna is designed on this substrate with the parameters: ε_r = 2.2, thickness = 1.6 mm, and loss tangent = 0.0009 to prove the concept. The patch dimensions are optimized to operate at the same 2.4 GHz frequency as: patch length = 24 mm, slot length = 9.4 mm, and slot width = 4.5 mm. A little increase in the patch length is observed due to the decrease in the dielectric constant. The simulated S₁₁ and 2D radiation patterns of this antenna are shown in Figure 8. A significant increase of 3.95 dB in the gain is observed.

5. Dual-Band Antenna Design

Figure 9 shows the dual-mode slotted patch antenna with a coaxial probe feeder. By using this feeding technique, the three modes are excited. The degenerate modes TM₁₀₀ and TM₀₁₀ have the same resonance frequency f_1, and the TM₁₁₀ mode has resonance frequency f₂. The probe position is selected by trials to match all modes. The dual-frequency of operation can be chosen directly from Figure 2 at a specific slot length when the width of the slot is 1 mm. More analysis can also be added for more design flexibility. Figure 10a,b shows the effect of the dimensions of the slot on S₁₁ performance of the dual-band antenna. Figure 10a shows the effect of Ls for constant Ws = 1 mm, and Figure 10b illustrates the effect of Ws for Ls = 8.5 mm. The slot’s width can be used for fine adjustment of f₁ and f₂. As the slot width increases, the resonance frequencies f₁ and f₂ decrease.

The dual-band antenna design is selected to operate at 2.4 GHz and 2.8 GHz, corresponding to slot length 8.5 mm and width 1 mm. Figure 11 shows the simulated S₁₁ of the proposed dual-band antenna.

Figure 12a,b shows the current distribution at these two resonance frequencies, respectively. It is observed from the distributions that the vertical and horizontal slots will affect the resonance frequency of the TM₁₀₀ and TM₀₁₀ modes, respectively. However, the TM₁₁₀ mode has currents in both X and Y directions. Thus, its resonance frequency will be affected by both vertical and horizontal slots. Figure 13 shows the 3D radiation pattern at f₁ and f₂. The gain of the proposed dual-band antenna with FR-4 substrate is still less than 0 dB. This issue is due to the low performance of the low-cost FR-4 substrate. However, the gain can be improved by using any other high-performance substrate. For example, the simulation of the same dual-band antenna with RT Duroid 5880 substrate shows that the gain can be increased to about 3.45 dB and 3.2 dB at 2.4 GHz and 2.8 GHz, respectively. The radiation patterns of the dual-band slotted square patch antenna are shown in Figure 14. It is observed that a maximum gain of 3.45 dB (at 2.4 GHz) and 3.2 dB (at 2.8 GHz) are in the broadside direction.

6. Bandwidth Enhancement

Conventional microstrip antenna structures are narrowband and their impedance bandwidth is typically 1–2%. However, the bandwidth can be improved by either increasing the substrate thickness, decreasing the dielectric constant (ε_r) of the substrate [43,44,45,46,47], or both. Other techniques, such as proximity coupled feed [48], aperture-coupled feed [49,50], and L-shaped feeding probe [51,52], can also be used. Some other configurations are suggested, such as slotted antenna [53,54], log-periodic array [55], E-shaped patch [56], circularly polarized patches [57,58], and defected ground method [59,60].

In this paper, bandwidth enhancement is attained for the same proposed structure by increasing the substrate thickness and decreasing the dielectric constant (ε_r). This arrangement is developed by adding foam substrate between the ground and the upper FR-4 substrate. For the matching purpose, a slot of size 6 × 6 mm² is etched at the center of the radiating patch. The modified structure is shown in Figure 15. Figure 16 shows the simulated S₁₁ of the modified structure. As observed from this figure, significant bandwidth improvements with –10 dB bandwidth of 220 MHz (2.33–2.54 GHz) are achieved.

The simulation of this structure shows a significant increase in the gain of the antenna. The maximum gain of the antenna with FR-4 is less than 0 dB, while this structure gain is increased to 1.87 dB.

7. Experimental Results

Two prototypes for this antenna are fabricated and evaluated: one for a single-fed single-band antenna and the other for a single-fed dual-band antenna. Figure 17 shows photographs of the fabricated slotted square patch antennas. Both antennas are designed and simulated using CST simulators. However, experimental validation was conducted using the Anritsu Vector Network Analyzer (Anritsu VNA 37369C). Figure 18 shows the simulated and measured S₁₁ for the fabricated single-band antenna. Further, Figure 19 shows the comparison between the simulated and measured S₁₁ for the dual-band antenna. The figures show that the simulation and measured results closely match.

In addition, a time-domain antenna measurement system GEOZONDAS (GEOZONDAS-TDAMS) [61] is used to measure the radiation patterns for the proposed antenna. A measurement setup was established to validate the far-field radiation pattern of the antenna, as shown in Figure 20. The antenna under test was placed on the top of the rotor, as shown in Figure 21. Figure 22 and Figure 23 illustrate the simulated and measured radiation patterns of the single band antenna on xz-plane with FR-4 substrate and RT Duroid 5880 substrate, respectively.

8. Conclusions

This study demonstrated a novel single-fed single-band and dual-band microstrip patch antenna. This antenna was realized on a square microstrip patch etched symmetrically with four slots. The antenna was designed to have low cost and reduced size to allow use in IoT applications. This antenna provides a reconfigurable architecture that allows operation in receive or transmit modes at the specified frequency. In addition, the antenna can be used in a full-duplex system sending and receiving at different frequencies, which can reduce the interference with other nodes in the IoT network. The proposed antenna operates at dual operating frequency bands of 2.4 GHz and 2.8 GHz that have been specified for the upcoming IoT technology. Two prototypes were fabricated with low-cost FR-4 substrate and tested. The first antenna works at a single resonance frequency (f₁ = 2.4 GHz); however, the second structure works at two resonance frequencies (f₁ = 2.4 GHz and f₂ = 2.8 GHz) within the same size. The 2.4 GHz is allotted for the industrial, scientific, and medical (ISM) band, and the 2.8 GHz is allocated to verify the concept and can be adjusted to meet the user’s requirements. The results were successfully demonstrated regarding return loss, radiation, and gain. The maximum gain achieved by the dual-band antenna is 3.45 dB and 3.2 dB at 2.4 GHz and 2.8 GHz, respectively. The measurement of the fabricated antennas closely matched the simulated results.