New Compact Wearable Metamaterials Circular Patch Antennas for IoT, Medical and 5G Applications

Abstract

The development of compact passive and active wearable circular patch metamaterials antennas for communication, Internet of Things (IoT) and biomedical systems is presented in this paper. Development of compact efficient low-cost wearable antennas are one of the most significant challenges in development of wearable communication, IoT and medical systems. Moreover, the advantage of an integrated compact low-cost feed network is attained by integrating the antenna feed network with the antennas on the same printed board. The efficiency of communication systems may be increased by using efficient passive and active antennas. The system dynamic range may be improved by connecting amplifiers to the printed antenna feed line. Design, design considerations, computed and measured results of wearable circular patch meta-materials antennas with high efficiency for 5G, IoT and biomedical applications are presented in this paper. The circular patch antennas electrical parameters on the human body were analyzed by using commercial full-wave software. The circular patch metamaterial wearable antennas are compact and flexible. The directivity and gain of the antennas with Circular Split-Ring Resonators (CSRR) is higher by 2.5dB to 3dB than the antennas without CSRR. The resonant frequency of the antennas without CSRR is higher by 6% to 9% than the antennas with CSRR. The computed and measured bandwidth of the stacked circular patch wearable antenna with CSRR for IoT and medical applications is around 12%, for S11 lover than −6dB. The gain of the circular patch wearable antenna with CSRR is around 8dBi.

1. Introduction

Small wearable sensors and antennas have been analyzed and presented in several papers and books in the last twenty years, see [1,2,3,4,5]. Patch, printed Slot, PIFA, Loop and other printed antennas are used in 5G communication, Internet of Things (IoT) and biomedical systems [2,3,4,5,6]. Meta material structures may be used to design small antennas with high efficiency for wearable medical and IoT systems, [6,7,8,9,10,11,12,13,14,15,16]. Meta material is a periodic artificial material that gain its electrical properties from the material structure rather than from its components. Periodic split ring resonators (SRRs) and metallic posts structures may be used to design materials with specific dielectric constant and permeability as presented in [7,8,9,10,11,12,13,14,15,16]. For example, artificial materials with negative dielectric permittivity were investigated in [7]. A quasi-analytical and self-consistent model to compute the polarizabilities of split ring resonators (SRR) is given in [8]. An experimental setup is also proposed for measuring the magnetic polarizability of SRR structures. Experimental data are compared with theoretical results by using the proposed model. The design of a compact microstrip patch antenna with a reduced size by using metamaterials technologies was presented in [11]. However, in [11] the antenna gain and bandwidth are the same as a standard patch antenna. A wideband transmission-line metamaterial antenna is developed in [12]. The antenna has two transmission line arms that resonate at two different frequencies. Each arm is a microstrip line loaded with five spiral inductors. The antenna radiation efficiency is 65.8% at 3.3 GHz. The antenna has 3% bandwidth. The antenna directivity is around 2.6dBi. The measured peak gain is around 0.79dBi. Small printed antennas such as printed dipole, PIFA and loop antennas have low efficiency [16,17,18,19,20,21,22,23,24,25,26,27,28].

Wearable systems were presented in [2,3,4] and in [29,30,31]. For example, a remote monitoring of patient in hospitals is presented in [29]. An adaptive thermal-aware routing protocol for wireless body area network is presented in [30]. A secure thermal-energy aware routing protocol in wireless body area network is presented in [31]. Active antennas and circular stacked patch antennas were presented in [32,33,34,35,36,37,38,39,40]. One of the main goals of wearable medical systems is to increase disease prevention. By using more wearable medical devices a person can handle and be aware of his private health. Sophistical analysis of continuously measured medical data of large number of medical centers patients may result in a better low-cost medical treatment.

Main Applications of Wearable Medical Systems

• Wearable Medical devices may help to monitor hospital activities;
• Wearable devices may help to operate and monitor home accessories to help diabetes patients, asthma patients, epilepsy patients and assist in solve cardiovascular diseases;
• Wearable devices may help to operate and monitor IoT devices.

Moreover, wearable antennas may be used in IoT devices. The internet of things (IoT) is a system of interrelated computing devices, mechanical and digital machines, personal devices, objects, animals, peoples, healthcare devices and wireless communication devices that are provided with unique identifiers (UIDs) and the ability to transfer data over a network without requiring human to human or human to computer interaction. The internet of things helps people everyday life, work smarter and get complete control over daily services and procedures. IoT provides smart devices to automate homes, companies and health care centers. IoT is essential to business. IoT provides businesses with a real time observation and inspection how their companies’ systems really work. IoT enables companies to automate processes and reduce labor costs. An IoT ecosystem consists of web-enabled smart devices that use embedded processors, sensors and communication hardware to collect, send and act on data they acquire from their environments. IoT devices share the sensor data they collect by connecting to an IoT gateway or other edge device where data is analyzed locally or sent to the cloud to be analyzed.

In this paper, meta-materials are employed to develop circular patch antennas with high efficiency for medical, IoT and 5G communication systems. Detail design of several types of metamaterial antennas with Circular Split-Ring Resonators (CSRRs) was presented by the author in [2,3,4]. Circular patch meta-material antennas were not presented in journals. However, circular patch meta-material antennas have several advantages over regular square and rectangular patches. Such as symmetry, ease to generate circular polarization and propagation mode. The computed and measured bandwidth of the stacked circular patch wearable antenna with SRR for IoT and medical applications is around 12%, for VSWR, Voltage Standing Wave Ratio, better than 3:1. The gain of the stacked circular patch with CSRR is around 8.5dB and with 95% efficiency.

The progress in development of small antennas is presented in Figure 1. Figure 1a presents the gain of several small antennas. Figure 1b presents the bandwidth of several small antennas. The values presented in Figure 1 are based on data and results presented in [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40].

2. Compact Circular Microstrip Antenna with Split Ring Resonators

A circular microstrip antenna with CSRRs is presented in this section. The antenna is printed on dielectric substrate with dielectric constant of 2.2, loss tangent of 0.002 and 1.6 mm thick. The radiating element consist of a circular patch with eighteen CSRRs, the maximum number of CSRR that can be inserted in the radiator area. The CSRRs improve the antenna effective area. The diameter of the circular antenna with CSRR, shown in Figure 2a, is 36 mm. The antenna center frequency is 2.6 GHz. The resonant frequency of the dominant mode TM11 of the circular microstrip antenna is given by Equation (1), see [1], where c is the light velocity in vacuum. Where $a_e$ is the radius of the circular microstrip antenna and it can be calculated by using Equation (2), [1]. ${\epsilon }_e$ is the effective dielectric constant. At 2.6GHz the diameter of the circular microstrip antenna should be 46.6 mm. The CSRR outer diameter ring is 5.2 mm, as shown in Figure 2b. The width of the CSRR strip is 0.15 mm. The CSRR dimensions was optimized by using 3D full analysis end commercial electromagnetic software. All the antennas presented in this paper were analyzed by using 3D full wave software, see [41].

$$f=\frac{1.8412c}{2\pi a_e\sqrt{{\epsilon }_e}}$$

(1)

$$a_e=\frac{1.8412c}{2\pi f\sqrt{{\epsilon }_e}}$$

(2)

The diameter of the circular antenna with CSRR is smaller by 23% than the diameter of the circular antenna without CSRR. The CSRRs design detail was presented in previous publications, see [2,3,4,5,16]. The CSRRs improve the effective area of the antenna. The maximum number of CSRRs is placed on the circular patch antenna. The circular antenna bandwidth is around 5% for S11 lower than −9 dB. The antenna bandwidth is around 8% for S11 lower than −6 dB as presented in Figure 3. The antenna beam width is around 82°. The antenna gain is around 7.6 dBi as presented in Figure 4. The antenna efficiency is around 83%. The gain and directivity of the circular patch antenna with CSRR is higher around to 2.5 dB to 3 dB than the circular patch antenna without CSRR. The antenna bandwidth may be improved by adding a second radiating layer.

3. Active Receiving Compact Circular Patch Antennas

A general receiver block diagram with an active antenna is presented in Figure 5. A Mini-Circuits TAV541 PHEMT LNA, Low noise amplifier, is an integral part of the antenna as presented in Figure 5 and Figure 6. The circular patch is etched on a substrate with a dielectric constant of 2.2 and 1.6 mm thick. The circular patch antenna diameter is 40 mm. The active antenna configuration is shown in Figure 6. An integral input matching network, that is part of the antenna feed network, matches the circular patch to the LNA as shown in Figure 6. An output matching network matches the amplifier output port to the receiver. The dimensions of the input and output matching network are less than 20 × 30 mm and are printed on the same substrate. An integral DC bias network supplies the required voltages to the amplifiers. The amplifier specifications are given in

Table 1. Low noise amplifier (LNA) specification at S band.

Parameter	Specification	Remarks
Frequency range	0.4–3 GHz
Gain	18 dB @2 GHz	Vds = 3 V; Ids = 60 mA
N.F	0.5 dB @2 GHz
P1dB	19.1 dBm @2 GHz
OIP3	33.6 dBm @2 GHz
Max Input power	17dBm
Vgs	0.48 V
Vds	3 V
Ids	60 mA
Supply voltage	±5 V
Package	Surface Mount
Operating Temperature	−40 °C–80 °C

. The amplifier complex S parameters are listed in

Table 2. LNA S parameters and noise parameters at the S band.

F-GHz	S11		S11°	S21		S21°	S12		S12°	S22		S22°
1.04	0.74		−126.2	12.74		100.13	0.05		33.69	0.29		−94.96
1.21	0.71		−137.6	11.25		92.91	0.051		30.05	0.26		−104
1.53	0.687		−154.2	9.30		82.06	0.055		26.08	0.22		−119
1.75	0.67		−164.1	8.24		75.31	0.06		23.14	0.20		−128.4
2.02	0.67		−174.6	7.30		67.82	0.06		20.88	0.18		−138.8
Noise Parameters
F-GHz		N. FMIN			N11X			N11Y			rn
1		0.16			0.3424			52.98			0.042
1.9		0.30			0.368			100.93			0.03
2		0.32			0.371			106.01			0.03
2.4		0.39			0.383			125.79			0.03
3		0.48			0.400			153.93			0.036
3.9		0.63			0.430			−167.3			0.06
5		0.81			0.465			−125.53			0.11

. The amplifier noise parameters are listed in Table 2. The circular patch matched antenna S11 parameter on a human body is presented in Figure 7. The antenna S11 without the matching network is around -4dB in the frequencies from 2.4 GHz to 3.2 GHz. However, S11 of the antenna with the matching network is better than −10 dB in the frequencies from 2.4 GHz to 3.2 GHz. The active antenna bandwidth is around 25% for VSWR better than 2:1. Smith chart diagram of S11 with and without the output matching network is shown in Figure 8a. The antenna matching network was optimized, as shown in Figure 8b. The optimized matching network improved the antenna gain by 1.5 dB. The antenna bandwidth was improved to 44% for VSWR better than 3:1. The active antenna gain on human body is around 15dB at 3GHz and decreases to 11 dB at 3.6 GHz as presented in Figure 9. The antenna noise figure is better than two for frequencies from 2 GHz to 3.6 GHz. Measurement setups of similar antennas were presented in [2]. There is a good agreement between computed and measured results.

4. Active Transmitting Compact Circular Patch Antennas

A basic transmitter block diagram with an active antenna is presented in Figure 10. An active circular patch transmitting antenna is presented in Figure 11. The antenna diameter is 40 mm. In Figure 11, the high-power amplifier (HPA) is an integral part of the antenna. The HPA is an MMIC GaAs MESFET, VNA25. The HPA is connected to the transmitting circular patch. The circular patch is matched to the HPA by a matching output network. The HPA input matching network matches the amplifier port to the transmitter.

The dimensions of the input and output matching network are less than 22 × 30 mm. The amplifier specifications are listed in

Table 3. High-power amplifier (HPA) specification at S band.

Parameter	Specification	Remarks
Frequency range	0.4–2.5 GHz
Gain	17.8 dB @2 GHz	Vds = 5 V; Ids = 85 mA
N.F	5.5 dB @2 GHz
P1dB	18.0 dBm @2 GHz
OIP3	29 dBm @2 GHz
Max. Input power	10dBm
Vgs	0.48 V
Vds	5 V
Ids	85 mA
Supply voltage	±5 V
Package	Surface Mount
Operating Temperature	−40 °C–80 °C

. In

Table 4. High-power amplifier S parameters at the S band.

F-GHz	S11 dB	S11°	S21 dB	S21°	S12 dB	S12°	S22 dB	S22°
1.6	−12.8	134.3	18.3	123.3	−44.2	−93.4	−18.9	113.7
1.8	−14.3	101.2	17.9	83	−43	−86.3	−22	69.5
2	−16.5	61.8	17.3	43.5	−40.4	94.6	−27	6.42
2.16	−18.5	22.1	16.8	12.9	−38	−105.5	−27.8	−70.2
2.4	−19.4	−53.9	15.7	−31.8	−36	−128	−22.2	−147.2
2.56	−17.7	99.7	15	−60	−34.6	−145.6	−19.3	−179.4
2.7	−15.7	131	14.3	−84.3	−33.8	−160.3	−17.5	158.1
2.86	−13.7	159	13.5	−111.1	−33	−177.7	−16	134.7
3	−12.2	179.1	12.7	−134.1	−32.4	167.4	15.2	116.3

the HPA complex S parameters are listed. The active transmitting circular patch S11 parameters is better than 2:1 in the frequency range from 2 to 3GHz as presented in Figure 12. The antenna matching network, presented in Figure 13a, was optimized to improve the active antenna bandwidth up to 50% for VSWR better than 3:1. A Smith chart diagram of S11 with and without the output matching network is shown in Figure 13b. The optimized active transmitting antenna S21 parameter, gain on human body, is presented in Figure 14. The active circular patch antenna gain, computed and measured, is 11.0 ± 2dB for frequencies ranging from 1.9 to 3.3 GHz. The active transmitting circular patch antenna output power is around 18 dBm.

5. Active Receiving Compact Double Layer Circular Patch Antennas

A compact double layer active circular patch antenna is presented in Figure 15. The commercial PHEMT LNA, the same LNA as presented in Section 3, is an integral part of the antenna as presented in Figure 15. The circular patch and the antenna feed network are etched on a substrate with dielectric constant of 2.2 and 1.6 mm thick. The circular resonator diameter is 40 mm. The circular radiator is etched on a substrate with dielectric constant of 2.2 and 1.6 mm thick. The circular radiator diameter is 42 mm. The active antenna configuration is shown in Figure 15. An input matching network matches the antenna to the LNA. An output matching network matches the amplifier output port to the receiver. An integral DC bias network supplies the required voltages to the amplifiers. The circular patch antenna S11 parameter on a human body is presented in Figure 16. The active antenna bandwidth is around 30% for VSWR, which is better than 2:1.

The active antenna gain is around 14 dB at 2.58 GHz and decreases to 10.5 dB at 3.2 GHz. The active antenna S21 parameters, gain on human body, are shown in Figure 17. The active antenna gain is 12 ± 2 dB for frequencies from 2.4 to 3.2 GHz. The active circular patch antenna noise figure is 0.6 ± 0.2 dB for frequencies from 2.4 to 3.4 GHz, see Figure 18. The antenna matching network, presented in Figure 19a, was optimized to improve the active antenna bandwidth up to 70% for VSWR better than 3:1. Smith chart diagram of S11 with and without the output matching network is shown in Figure 19b. The optimized active receiving antenna S21 parameter, gain on human body, is presented in Figure 20. The active circular patch antenna gain is 13.0 ± 3 dB for frequencies ranging from 2 GHz to 4 GHz. There is a good agreement between computed and measured results.

6. Metamaterial Wearable Double Layer Circular Patch Antennas

A Metamaterial Wearable Double Layer Circular Patch Antenna is shown in Figure 21. The circular patch and the antenna feed network are etched on a substrate with dielectric constant of 2.2 and 1.6 mm thick, see Figure 21a. The circular resonator diameter is 36.3 mm. The circular radiator is etched on a substrate with a dielectric constant of 2.2 and 1.6 mm thick, see Figure 21b. The circular radiator diameter is 39.3 mm. The radiating element consist of a circular patch with eighteen CSRRs, the maximum number of CSRRs. The Double Layer Circular Patch Antenna is shown in Figure 21c. The spacing between the resonator may be varied from 0 mm to 8 mm to get wider bandwidth from 5% to 10%, The diameter of the circular antenna with CSRR is smaller by 25% than the diameter of the circular antenna without CSRR. The CSRR outer diameter ring is 5.2 mm, see Figure 21d. The width of the CSRR strip is 0.15 mm. The antenna S11 parameter is presented in Figure 22. The antenna beam width is 82°. The antenna gain is around 8.5 dBi as presented in Figure 23a. The antenna efficiency is around 92%. The gain and directivity of the circular patch antenna with CSRR is higher by 2.5 dB than the circular patch antenna without CSRR. The 3D radiation pattern of the Double Layer Circular patch antenna with CSRR is shown in Figure 23b.

7. Wearable Metamaterial Antennas for 5G, IoT and Medical Applications

The antennas presented in this paper may be used in 5G, IoT and medical applications. The metamaterial antennas S11 variation near the patient body were computed by using the structure presented in Figure 24a. Several dielectric constant and conductivity of human body tissues are given in

Table 5. Electrical properties of body tissues [16,17].

Tissue	Property	430 MHz	600 MHz	1 GHz	1.2 GHz
Small intestine	σ	1.74	1.74	1.74	1.74
	ε	128.1	128.1	128.1	128.1
Fat tissues	σ	0.045	0.05	0.054	0.058
	ε	5.00	5	4.72	4.57
Stomach tissues	σ	0.67	0.75	0.96	0.97
	ε	42.9	41.40	39.66	39.05
Blood	σ	1.72	1.78	1.91	1.98
	ε	57.3	56.5	55.40	55.00
Colon	σ	1.00	1.05	1.30	1.44
	ε	63.6	61.9	60.00	59.40
Skin	σ	0.57	0.6	0.63	0.76
	ε	41.6	40.45	40.25	39.65
Lung tissues	σ	0.27	0.27	0.27	0.27
	ε	38.4	38.4	38.4	38.4
Kidney tissues	σ	0.90	0.90	0.90	0.90
	ε	117.45	117.45	117.45	117.45

[16]. The antenna location on the human body is considered by computing S11 for different dielectric constant and conductivity of the body tissues. The dielectric constant of the body tissues varies from 5 at fat tissues to 63 at the colon area, and to 117 at the kidney tissues. The dielectric constant of the body tissue shifts the wearable antenna resonant frequency by 2% to 5%. Wearable antennas may be inserted inside a belt as shown in Figure 24b. The belt thickness and dielectric constant shift the antenna resonant frequency. The belt thickness and dielectric constant was optimized to get the best electrical performance. The antennas impedance and radiation characteristics were computed and measured for air spacing, between the antennas and patient body, of 0 to 10 mm.

A comparison between computed and measured results of antennas with and without CSRR is given in

Table 6. Comparison between printed antennas with and without CSRR.

Antenna	Frequency (MHz)	BW %	Computed Gain dBi	Measured Gain dBi	Length. (cm)	Efficiency %
Circular patch with CSRR	2630	8	7.5	7.8	3.6	85
Circular patch without CSRR	2630	1.5	4.5	4.3	4.8	85
Printed dipole with CSRR	350	10	5.5	5.7	19.8	95
Dipole without CSRR	400	10	2.5	2.5	21	90
Stacked circular patch with CSRR	2700	8	8.5	8.4	4	95
Stacked circular patch without CSRR	2700	8	5.5	5.3	4.8	90

. The measured results agree with the computed results. Results listed in Table 6 and

Table 7. Comparison of electrical characteristics of wearable antennas [1,2].

Antenna	Frequency (GHz)	Bandwidth %	VSWR	Computed Gain dBi	Measured Gain dBi
Circular patch with CSRR	2.63	8	2:1	75	7.8
Active Circular Receiving Patch	2.5	25	2:1	13.5	14.0
Stacked circular patch with CSRR	2.7	8	2:1	8.5	8.4
Printed dipole [2]	0.43	5–10	2:1	2–3	2–3
Dipole with CSRR	0.4	8–12	2:1	5–7	5–7
Dipole (CSRR and strips) [2]	0.35	50	2.5:1	5–7.5	5–7.5
Loop [2]	0.43	5–10	4:1	0	0
Patch [2]	2.2	1–3	2:1	2–3	2–3
Stacked Patch [2]	2.2	10–15	2:1	4–5	4–5
Slot [2]	2.5	50	2:1	3	3
T shape slot [2]	2.5	60	2:1	3	3
Active slot	2.5	40	3:1	12–20	12–21
Active T slot	2.5	50	3:1	12–20	12–21
Active slot with CSRR [2]	2.5	50	2.5:1	10–16	11–16
Active Stacked Circular Patch	2.5	25	2:1	12–14	11–15

show that the gain of printed antennas with CSRR is 2dB to 3 dB higher than the antennas without CSRR. Wideband passive and active slot antennas, loop and parch antennas were presented in [1]. Printed dipoles with and without CSRR were presented in [2]. A comparison of computed and measured results of compact wearable antennas for medical, 5G and IoT systems is listed in Table 7.

A photo of a prototype of the Active Stacked Circular Patch is shown in Figure 25a. A block diagram and layout of the antenna components are presented in Figure 25b. Global medical monitoring health system with WBAN and BAN Systems is presented in Figure 26. This global medical monitoring health system may be accessed online at any time from every place in the world.

Wearable medical systems and sensors can measure body temperature, heartbeat, blood pressure, sweat rate, perform gait analysis and other physiological parameters of the person wearing the medical device. Gait analysis is a useful tool both in clinical practice and biomechanical research. Gait analysis using wearable sensors provides quantitative and repeatable results over extended time periods with low cost and good portability, showing better prospects and making great progress in recent years. In sport and commercial wearable sensors are used in several applications of gait analysis.

8. Conclusions

Passive and active circular patch wearable antennas may be used in receiving or transmitting WBAN, 5G, IoT and medical systems. In transmitters, an HPA power amplifier is connected to the antenna. In receiving channels, a low noise amplifier (LNA) is connected to the antenna.

Wideband passive and active circular patch antennas may be employed in wideband wearable 5G communication systems for commercial and medical applications. Passive and active printed antennas bandwidth, efficiency and gain and noise figure are presented and summarized in this paper. The directivity and gain of the antennas with split-ring resonators is higher by 2.5 to 3dB than the antennas without CSRR. The resonant frequency of the antennas without CSRR is higher by 6% to 9% than the antennas with CSRR.

The active and passive antennas presented in this paper are compact, low-cost, wideband passive and active antennas for receiving and transmitting wearable IoT, medical and 5G communication systems.

Wearable body area networks seem to be a significant option for healthcare organizations, hospitals and patients. Wearable technology provides a convenient service that may improve the long-term health and physiological response of patients. Wearable systems support the development of personal health-care networks with online immediate response to treat and improve patients’ health.

The active transmitting circular patch output power is around 18dBm. A summary and comparison of the electrical characteristics of the wearable antennas with and without CSRRs are presented in this paper.

This paper presents wideband active and passive circular patch antennas with high efficiency for commercial and medical applications. The active circular patch antennas bandwidth is around 25% for a reflection coefficient lower than −10dB. The antenna bandwidth and gain were improved by optimizing the active antenna matching network. The active receiving circular patch antenna gain is around 13.5dB. The wideband active and passive circular patch antennas may be designed as dual polarized and circular polarized antennas.

In future work circular polarized circular patch metamaterial antennas with high efficiency for medical, IoT and 5G communication systems will be presented. Moreover, in future work fractal circular patch antennas, with and without CSRRs, with high efficiency for medical, IoT and 5G communication systems will be presented.