A low profile pentagonal shaped monopole antenna is designed and presented for wearable applications. The main objective of this paper is to design a miniaturized ultrawide band monopole planar antenna which can work efficiently in free space but also on the surface of the human body. The impact of human tissues on antenna performance is explained using the proposed pentagonal monopole antenna. The antenna is designed with a pentagonal radiator and a matched feed line of 50 ohm and square slots are integrated on defected ground of FR4 substrate with a size of 15 mm × 25 mm to achieve ultrawide band (UWB) performance in free space and human proximity. This overall design will enhance the antenna performance with wide bandwidth ranging from 2.9 GHz to 11 GHz. Specific absorption rate (SAR) of the proposed antenna on dispersive phantom model is also measured to observe the exposure of electromagnetic energy on human tissues. The simulated and measured results of the proposed antenna exhibit wide bandwidth and radiation characteristics in both free space and human proximity.
Body centric communications (BCC), a subpart of Body Area Networks (BAN), is one of its most active and potential research fields. Preeminence of health care and entertainment has made body centric communication an active research topic [
In this paper, a low profile pentagonal shaped planar monopole antenna is presented with wide bandwidth from 2.9 GHz to 11 GHz. This antenna exhibits good frequency and time domain characteristics appropriate for BCC. This antenna can be easily integrated because of its compact dimensions for wearable applications. In the present study, antenna performance characteristics are investigated (i) in free space and (ii) on three-layered dispersive tissue equivalent phantom model. The proposed pentagonal monopole (PM) antenna design and simulations are carried out in HFSS and CST commercial electromagnetic software to evaluate frequency and time domain characteristics.
The proposed PM antenna is designed and presented for UWB wearable applications as shown in Figure
Geometry of the antenna: (a) front view and (b) bottom view.
Fabricated prototype of the antenna with front and bottom views.
Bandwidth of the proposed antenna depends upon the feed line width, edge of the pentagonal radiator, and partially defected ground with rectangular slots. The width of the feed line “
The proposed antenna bandwidth can be increased with variations made in the ground plane and feed structure. The lower limit of the antenna bandwidth can be improved by varying the feed line width and feed gap space. Upper limit of this antenna structure can be improved by inserting the slots in ground plane and inserting a flared edge at the feed line connecting to the pentagonal radiator. In the proposed antenna design, the shape of the patch, feed line with flared edge structure, and partial ground with rectangular slots enhance the bandwidth and inflate radiation pattern normal to the patch surface. The dimensions of the proposed antenna structure are
Parametric analysis of the PM antenna design with various design parameters are simulated using HFSS software. Here, to improve the PM antenna performance, the following parameters are investigated: (i) feed line with flaring structure, (ii) gap between ground plane and radiator, (iii) feed line width, and (iv) effect of edge and center slots inserted in the ground plane. Elaborated analyses of the abovementioned parameters are given below.
Introduction of flaring has improved the impedance bandwidth by increasing the upper edge of the frequency spectrum. This is due to the increase in surface current distribution along the patch, which is shown in Figure
Surface current distributions of PM antenna: without flared feed at (a) 3.1 GHz and (b) 10.5 GHz and with flared feed at (c) 3.1 GHz and (d) 10.5 GHz.
Effect of flaring structure on impedance bandwidth.
Feed gap is a crucial parameter, which can be used to reduce the coupling currents between the top edge of ground plane and bottom edge of the patch radiator. This gap has shown a significant change in impedance bandwidth. Bandwidth variation with different feed gaps (0.95 mm, 1.95 mm, 2.95 mm, and 3.95 mm) has been investigated and an optimum value is chosen from the analysis shown in Figure
Effect of feed gap variation on impedance bandwidth.
Variation in the feed line width (
Effect of feed width on impedance bandwidth.
In this subsection, clear analyses of the effect of ground plane slot widths and locations are presented. Optimum slot widths and locations are taken from the analysis and are presented here along with the justification.
The high frequency response has been improved with inclusion of rectangular slots in the ground plane. This has also increased the current distribution around the slots in the ground plane as shown in Figure
Effect of edge slots width on impedance bandwidth.
To further enhance the bandwidth, a center slot has been introduced in the ground plane. It has shown an optimistic performance. With increment in width of the center slot, upper edge of the frequency spectrum has been affected. These effects are presented in Figure
Effect of center slot width on impedance bandwidth.
The impedance bandwidth variations with respect to the location of slots in the ground plane are investigated and presented in Figure
Effect of slots location on impedance bandwidth.
The results of PM antenna over the bandwidth range of 2.9 GHz to 11 GHz are analysed and discussed in this section. The PM antenna exhibits wide impedance matching performance with good radiation characteristics. In order to observe the PM antenna performance in free space as well as on phantom model, various performance characteristics are evaluated and presented below.
The return loss characteristics are measured with the help of programmable network analyser (PNA). The measured reflection coefficient (
Simulated and measured reflection coefficients of PM antenna.
Time domain analysis of UWB antennas represents the phase linearity and distortion in the received pulses. In UWB pulse transmission, linear phase variation provides low pulse distortion in received signals. The pulse distortion can be analysed with the help of group delay and fidelity factor. Hence, linear phase variation in
Measured group delay.
Human tissues are lossy and dispersive in nature at higher frequencies. Due to the electromagnetic properties of tissues, antenna characteristics like resonant frequency, bandwidth, and radiation pattern are drastically affected. In the proximity of human body, antenna performance degrades. This is not suitable for the required applications. In general, increase in frequency causes decrease in tissue dielectric constant and increase in conductivity and loss tangent [
A tissue equivalent phantom model is constructed with three layers (skin, fat, and muscle) in HFSS and CST software as shown in Figure
Dielectric properties of human tissue at 5 GHz frequency.
Tissue type | Permittivity ( |
Loss tangent | Conductivity ( |
Mass density ( |
---|---|---|---|---|
Skin (dry) | 36.6 | 0.288 | 2.35 | 1109 |
Fat | 5.13 | 0.16 | 0.18 | 911 |
Muscle | 50.8 | 0.267 | 3.03 | 1090 |
Dispersive phantom model with three layers.
Return loss characteristics are simulated for the proposed PM antenna in HFSS software for the following considerations: (i) antenna in free space and (ii) antenna placed at a distance of 1 mm and 2 mm above the tissue layer. These results, presented in Figure
PM antenna return loss characteristics on dispersive phantom model.
Experimental setup for reflection coefficient measurement.
On shoulder
On hand
Measured reflection coefficients comparison of the PM antenna.
Peak gain variations of the proposed PM antenna are evaluated in CST MW STUDIO as a function of distance from the tissue model and are presented in Figure
Peak gain of the proposed PM antenna.
In close proximity of human tissues, antennas exhibit very poor radiation efficiencies due to high power absorption in the tissues. Its efficiency increases with increase in separation distance [
Radiation efficiency.
Radiation patterns for the proposed PM antenna in free space and dispersive tissue model with 1 mm separation are shown in Figure
Radiations patterns without and with phantom model.
SAR is used to investigate the electromagnetic energy absorption in human body tissues under reactive near fields. SAR is calculated as
SAR values depend on the antenna distance from the phantom as well as on the tissues dielectric constant [
1 gram averaged peak SAR values with different antenna distances from the phantom.
Frequency (GHz) | SAR mw/kg | ||||||||
---|---|---|---|---|---|---|---|---|---|
1 mm | 5 mm | 10 mm | |||||||
Skin | Fat | Muscle | Skin | Fat | Muscle | Skin | Fat | Muscle | |
3.1 | 129 | 31.2 | 111.9 | 52 | 8.35 | 38.8 | 25.5 | 2.71 | 14.7 |
6 | 386 | 39.6 | 118.2 | 220 | 1.7 | 56.9 | 99.6 | 6.46 | 19.1 |
10.5 | 480 | 46.7 | 37.5 | 110 | 8.3 | 8.6 | 36.5 | 2.1 | 1.63 |
A low profile PM antenna is designed and demonstrated for body centric communications. This antenna exhibits good frequency domain and time domain characteristics. The impedance matching characteristics of this antenna in free space and on dispersive phantom model show excellent agreement with measured reflection coefficient on human shoulder and hand in UWB frequency range. This antenna also meets the low SAR value specifications prescribed by FCC. Hence, this antenna can be an excellent choice for wearable and UWB localization applications.
All authors declare that there are no conflicts of interest regarding this manuscript.