Volume 14, Issue 14 p. 1791-1794
Research Article
Free Access

Reconfigurable log-periodic dipole array on textile

Alexander D Johnson

Corresponding Author

Alexander D Johnson

Department of Electrical and Computer Engineering, College of Engineering and Computing, Florida International University, Miami, FL, 33174 USA

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Matthew W. Nichols

Matthew W. Nichols

Department of Electrical and Computer Engineering, College of Engineering and Computing, Florida International University, Miami, FL, 33174 USA

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Satheesh Bojja Venkatakrishnan

Satheesh Bojja Venkatakrishnan

Department of Electrical and Computer Engineering, College of Engineering and Computing, Florida International University, Miami, FL, 33174 USA

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John L. Volakis

John L. Volakis

Department of Electrical and Computer Engineering, College of Engineering and Computing, Florida International University, Miami, FL, 33174 USA

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First published: 26 October 2020
Citations: 3

Abstract

A microwave planar log-periodic dipole array (LPDA) antenna embroidered on textiles is presented for the first time. This antenna gives a 3.33:1 impedance bandwidth with 5 dBi average gain across the 0.9–3GHz band. This multi-layer textile antenna employs a new denim stack-up for repeatable measurements. The presented LPDA is tested under conformal conditions, with a demonstrated ability of frequency-dependent beam-steering through physical manipulation. Further, the antenna is considered for body-worn applications due to its conformal and low profile. A 2cm thick ground beef slab was used to emulate the human tissue for measurements. Overall, the presented LPDA brings forward new opportunities for several long-distance wideband and wearable applications.

1 Introduction

There is a need for wideband directive antennas in applications such as Global System for Mobile Communications (GSM) (900 MHz, 1.8 GHz), mobile (2.1 GHz), or WiFi (2.4 GHz) band receivers. Planar log-periodic dipole array (LPDA) antennas [[1], [2]] have been extensively used for a variety of communication applications due to its wide bandwidth, high gain, and planar profile. However, rigid substrates, such as printed circuit boards (PCBs), are often not practical for integration onto curved surfaces. To remedy this, many flexible substrates have been investigated [[3]-[6]].

Notably, textile-based electronics have been of great interest in recent years due to improved embroidery resolution [[7]]. Yet, the majority of works focus on textile substrates for narrowband antenna elements, such as patch antennas [[8]]. Others have also looked into wideband spiral antennas [[7]]. However, previous embroidered textile antenna uses only single-sided radiators, sometimes with copper taped ground planes. In this paper, a planar UWB LPDA antenna embroidered on a double-sided denim textile substrate is presented for the first time. This builds on the previous literature [[9]], of narrowband two-element log-periodic folded dipole array (LPFDA). In contrast to the improved textile metallisations methods employed here, the previous antenna designs relied on unreliable adhesive-backed metal tapes and stainless steel fibres with low conductivity.

2 Design of textile LPDA

The design of the LPDA depicted in Fig. 1 follows [[10]-[13]]. This describes the LPDA as a frequency-independent antenna with a periodic characteristic impedance (Z0) that varies logarithmically with respect to the frequency. Specifically, this LPDA antenna is designed for operation across 0.9–3 GHz, with a chosen directivity of 6.5 dBi [[14]] for the ideal design. Hence, the corresponding values (σ and τ) for the design were found using the Carrel table [[10]]. The total length (L), designed bandwidth (BS), and the number of elements (N) were determined from [[15]]
urn:x-wiley:17518725:media:mia2bf02186:mia2bf02186-math-0002(1)
urn:x-wiley:17518725:media:mia2bf02186:mia2bf02186-math-0004(2)
urn:x-wiley:17518725:media:mia2bf02186:mia2bf02186-math-0006(3)
The final antenna parameters are σ = 0.14; τ = 0.77; BS = 7. As in [[14]], the number of dipole elements was chosen to be N = 12, for optimal performance over the band. The largest dipole length is designed as half the wavelength (λ/2) of the lowest operational frequency. The remaining spaces and dipole dimensions were determined by
urn:x-wiley:17518725:media:mia2bf02186:mia2bf02186-math-0008(4)
From this starting point, the final design parameters in Table 1 were optimised for impedance matching and high gain with the FR4 substrate (ɛr = 4.4). The denim LPDA was modelled with a 1 mm uniform substrate of ɛr = 1.6, according to [[16]]. A tapered line was used on the denim LPDA to aid in impedance matching. The simulated return loss of the designed LPDA is given in Fig. 2, with S11<−10 dB over the 0.9–3 GHz band for both substrates. Further, the denim LPDA gain is 5 dBi across the whole band, as shown in Fig. 3. Notably, the denim LPDA shows about a 1 dB loss versus the FR4 design in simulation [[17]]. Both designs are characterised by a high front-to-back gain ratio, as depicted in Fig. 4.
Table 1. Design parameters of LPDA in Fig. 1 (mm)
Dipole (i) Length (i) Width (i) Distance from feed (Di)
1 150.78 6.25 167.51
2 120.64 5.00 130.84
3 96.51 4.00 101.72
4 77.21 3.20 78.42
5 61.76 2.56 59.05
6 49.43 1.43 46.70
7 39.56 1.15 34.74
8 31.66 0.92 25.68
9 25.34 0.73 18.46
10 20.24 0.59 12.56
11 16.19 0.47 7.91
12 12.95 0.38 4.19
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Pictorial representation of LPDA embroidered on

(a) FR4 and, (b) Denim textile substrates

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Measured S11 of the FR4 and denim LPDA

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Measured end-fire gain of the FR4 and denim LPDA

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Measured front-to-back ratio of the FR4 and denim LPDA

3 Prototype construction and initial measurements

To demonstrate the viability of the textile LPDA, the denim prototype in Fig. 5 (right) was fabricated and measured with comparison to the FR4 design in Fig. 5 (left). The textile LPDA utilises a completely commercially available multi-layer embroidery textile stack-up design of ∼1 mm total thickness. Standard denim (ɛr = 1.6) was chosen as the textile substrate for a low cost and commonly available substrate [[16]]. The electrically conductive thread (E-thread) used was Elektrisola-7, with each 0.04 mm diameter strand consisting of seven Cu/Ag50 amalgam filaments [[18]]. Power handling is not considered in this work as the antenna is intended solely for mobile and conformal receivers.

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Fabricated LPDA on the FR4 (left) and denim textile (right) substrates

The denim stack-up depicted in Fig. 6 was found to be the ideal textile stack-up for repeatable and accurate embroidery of even 0.1 traces and 0.3 mm spaces. The dielectric value of this stack-up was chosen to be ɛr = 1.6 based on the average values given by a coaxial line reflection method measurement conducted as in [[19]]. The embroidery tolerances of this design structure ensure accurate fabrication, without compromising the antenna's textile advantages of flexibility and durability. Using off-the-shelf denim, the double-sided LPDA was fabricated and tested.

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Current multi-layer embroidered textile stack-up

The measured return loss is found in Fig. 2 with S11<−10 from 900 MHz to 3 GHz for both designs. Further, the measured end-fire (θ = 90°) gains of both LPDA are found in Fig. 3 with comparison to simulations. The measured levels were typically within 1 dB of simulated values, with the worst-case textile LPDA gain dropping by about 3 dB at 2.25 GHz. The front-to-back ratio is given in Fig. 4, with a measurement that exceeds the simulated values. This is reasonable due to the losses in the substrate that further suppress the back lobe. The measured results show good accuracy to simulations, with losses consistent with past textile papers [[17]]. The losses seen at frequencies above 1.5 GHz occur due to the relatively long length of the E-thread with respect to frequency. With a typical transmission loss of 0.25 dB/cm at 1.5 GHz, and the total length of the LPDA being 17 cm, up to 4 dB of loss can be expected from the simulation of the ideal perfect electric conductor [[17]]. The elevation pattern (xz plane) measurements in Fig. 7 show a consistency in performance between the FR4 and denim antennas.

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Elevation patterns (x–z plane) of LPDA on the FR4 (left) and denim (right) substrates at 1, 2, and 3 GHz

4 Pattern reconfiguration through folding

The denim textile substrate has a great draw of conformity that enables many more applications than the rigid end-fire PCB prototype. Therefore, the performance of the textile LPDA was studied with two simple bending configurations. Specifically, the folding of the antenna at the mid-point of the LPDA, between the 4th and 5th dipoles (see Fig. 1) was simulated and measured at an angle of 45° and 90°. For measurement, Styrofoam mounting structures were used to ensure an exact bend radius and consistency with the simulation. RF tape was used to adhere to the LPDA to the Styrofoam for measurement in the antenna chamber. Further, to show the reliability and repeatability of these measurements, the fabricated antenna was taken off its Styrofoam mounting structures after each measurement, manipulated, and re-taped to the mounts for the next measurement, a total of three times. These repeated elevation pattern (xz plane) measurements in Fig. 8 show a consistency with previous measurements. That is, there is an estimated 1 dB drop in measured gain versus the simulated value. However, the direction and shape of the simulated and measured patterns agree well. As expected, the frequency-dependent radiating components of the LPDA at low frequency correspond to the longer dipoles at the long edge, and the higher frequencies correspond to the shortest dipoles at the narrower end. The lower frequency components, still flush to θ = 90° radiate out to θ = 90° and the high-frequency component at 3 GHz radiate towards its end-fire pointing direction of θ = 45° for the 45° bent case and θ = 0° for the 90° bent case. Interestingly, the mid-band 2 GHz pattern shows that since the resonant dipole for this band lies in the region of the folding vertex, the pattern displays a broader beam covering the quadrant from the end-fire pointing direction (θ = 45°/0°) to θ = 90°.

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Conformal study of LPDA with 45° bend (left) and 90° bend (right) with measured elevation patterns (x–z plane) versus simulations at 1, 2, and 3 GHz

5 On-tissue simulation and measurements

The LPDA was placed on a 2 cm thick organic medium to emulate human tissue. The 2 cm thickness was chosen as an average of typical human tissue thickness and related to 0.1λ in the tissue at the central operational frequency of 1.5 GHz. The medium consists of 80% lean and 20% fat ground beef in accordance with [[20], [21]], and follows closely to the human value reported in [[22]]. The simulation and measurement setups are given in Fig. 9. The simulated setup involves an LPDA mounted on the top of a 2 cm thick human tissue model with a 1.5 mm layer of skin between the antenna and tissue. Likewise, for measurement, a 2 cm slate of ground beef, sealed in plastic bags, rests beneath the LPDA. The simulated and measured azimuth (xy plane) patterns in Fig. 10 show that on-body loading removes the directionality of the LPDA. Instead, the LPDA radiates with a more-omnidirectional pattern due to the conductivity of the organic matter. At the mid and high-band frequencies of 2 and 3 GHz, the simulations closely match the measurements in terms of pattern shape. Notably, measured patterns show a higher gain than simulations. This is due to the lower losses of the plastic cover used in measurements, as compared to human skin. Given this fact, the LPDA is a good candidate for UWB omnidirectional receivers when mounted on-body. In contrast to body-worn patch antennas [[8]], this allows greater multi-function to the antenna, with a near-omnidirectional pattern on-body and a directional behaviour stand-alone. Due to the scope of this work as a receive oriented antenna, the specific absorption rate (SAR) of the antennas was not studied. However, SAR and power handling can be investigated in future studies, using methods outlined in [[23]].

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Setup of denim LPDA while on the beef tissue phantom in simulation (left) and measurement (right)

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Measured azimuth (x–y plane) patterns of the denim LPDA while on the beef tissue phantom at 1, 2, and 3 GHz

6 Conclusion

A novel planar LPDA antenna, a textile substrate was presented. The antenna was embroidered using off-the-shelf conductive threads and denim fabrics. The measurements agree well with simulations, with a matched bandwidth of 900 MHz to 3 GHz and 5 dBi average gain. The LPDA proves to be a conformal antenna with the ability to beam-steer with frequency dependency by bending the structure at specific physical lengths. The conformal antenna was also considered for body-worn receive applications, where it was found to be a good candidate for UWB omnidirectional receivers. The advantages of this antenna are its UWB performance and high gain with flexible and lightweight substrates.

7 Acknowledgments

The authors thank Dieff Vital for their advice in embroidery techniques. This work was supported in part by AFOSR grant no. FA9550-18-1-0191.