chapter 2 organic paper-based antennas · keywords: antennas, dual-band antennas, direct-write,...

CHAPTER 2

Organic Paper-Based Antennas

Dimitris E. AnagnostouElectrical and Computer Engineering, South Dakota School of Mines and Technology, USA.

Abstract

Advances in direct-write printing technology have enabled printable electronics practically on any substrates including paper. Paper is a material of very low cost, it is extremely flexible, and can be post-treated to obtain hydrophilic or hydropho-bic characteristics. Organic paper substrates have recently found extensive use in the development of antenna components. Many designs with different characteris-tics and for various applications have been made. In this chapter, a comprehensive description of printing technologies and recent dielectric measurements for paper substrates are described. Then, the time and frequency evolution of antennas on hydrophobic and hydrophilic paper substrates are presented. Antennas are approached from a principle of operation perspective and emphasis is given on their characteristics and the applications that span from omnidirectional radio fre-quency identification, WLAN and ultra-wideband antennas to recently developed QR code antennas for anti-counterfeiting and security applications.

Keywords: Antennas, dual-band antennas, direct-write, eco-friendly electronics, flexible antennas, nanoinks, organic antennas, paper substrates, printed electronics, ultra-wideband antennas.

1 Introduction

Antennas can be made of various materials, and these include conductive metals as well as non-conductive dielectrics. As seen so far, most planar antenna structures are fabricated on a dielectric substrate using mostly photolithography or etch-ing. Both technologies are subtractive, meaning that large material quantities are deposited and then removed, thus increasing waste. Moreover, the chemicals used in the fabrication process (photoresists, acetone, etc.) and the high temperatures the materials had to withstand (when placed in a milling machine) limit the selec-tion of available substrates to those that are robust and relatively inert.

www.witpress.com, ISSN 1755-8336 (on-line) WIT Transactions on State of the Art in Science and Engineering, Vol 82, © 2014 WIT Press

doi:10.2495/978-1-84564-986-9/002

26 InnovatIon In Wearable and FlexIble antennas

In the past decade, significant advantages took place in additive printing, and especially in direct-write inkjet printers (such as the Materials printer by Dimatix, the Maskless Mesoscale Material Deposition (M3D) system by Optomec™, and the nScrypt system by nScrypt) enabled a technology called additive manufacturing. Additive manufacturing is an eco-friendly technology that allowed for minimal waste (as it does not produce metallic or chemical photoresist waste as milling and chemical etching techniques do) and for ambient temperature deposition of printed metallic traces. This gave electrical functionality to the structures and led to the fabrication of printed electronic devices (including not only a large variety of antenna structures but also rectifying devices and even games on paper [1]) at a low cost, and without the need for expensive clean room or milling equipment. More-over, it allowed printing these devices on virtually any substrate, allowing for many uncommon (until today) substrates to be used, including paper. Before proceeding to the numerous antenna designs that have been realized on paper, it is worth to briefly overview the three major printing technologies. It is noteworthy that all three exist at the Direct-Write Laboratory of the South Dakota School of Mines and Technology. The antennas that will be described are listed in chronological order of development and are not grouped by printing technology because the results of all three technologies are effectively the same (a working antenna device) (Fig. 1).

2 Printing Technologies

2.1 Materials printer

The Materials Printer series (such as the DMP-2831 by FUJIFILM Dimatix) is a piezoelectric jet technology with MEMS on its print head that is a cost-effective, easy to use material deposition system designed for R&D and feasibility testing.

Figure 1: A QR code antenna on paper developed at SDSMT [2]. The antenna was thermally sintered on uncoated paper and resonates at 2.4 GHz with a return loss of 22 dB and smooth H-plane radiation pattern.


organIc PaPer-based antennas 27

These printers exist in various sizes and can deposit fluidic materials (inks) to create and define patterns on substrates by utilizing a disposable piezo inkjet car-tridge. Deposition takes place at <70°C. To manipulate the electronic pulses to the piezo jetting device and optimize the drop characteristics as it is ejected from the nozzle, a waveform editor and a drop-watch (fiducial) camera can be used. The printer has a MEMS-based cartridge-style printhead that allows users to fill their own fluids and print immediately in their own laboratory. Each single-use car-tridge has 16 nozzles linearly spaced at 254 µm with typical drop sizes of 1 and 10 pL (picolitres). The printer can jet (print) a variety of fluids, including aqueous and solvent-based fluids, solutions and particle suspensions. Ideally, the fluid viscosity should be between 2 and 30 cPs (centipoises) and ideally about 10 cP or (0.1 dyne sec/cm2) and have surface tension 30 dyne/cm2. Particles should not aggregate or settle. The drop size repeatability is around 0.5%–3.5%, and the smallest size spot is about 40 µm, which is sufficient to define fine features of approximately 50 µm (strips or slots). This feature size is sufficient (and subwavelength) for most micro-wave frequency applications. The smallest increment between one drop and the next can be 5 µm, which is equivalent to 5,080 dpi. At this spacing, we obtain drop overlap on the substrate because most spots will be at least 40 µm in diameter.

2.2 Aerosol jet technology

The M3D system by Optomec [3], as well as the industrial inkjet system PixDro and the wide area system Sono-Tek, are based on aerosol jet technology. In these systems, the conductive ink is first ultrasonically atomized (i.e. becomes a mist). The material stream is then aerodynamically focused using a flow guid-ance deposition head, which creates an annular flow of sheath gas to collimate the aerosol. The co-axial flow exits the flow guidance head through a nozzle directed at the substrate, which serves to focus the material stream to as small as a tenth of the size of the nozzle orifice (typically 100 µm). The head remains fixed, while the platen moves as required by the design. Patterning is accomplished by attach-ing the substrate to a computer-controlled platen, or by translating the flow guid-ance head while the substrate position remains fixed. The relatively large (>5 mm) standoff distance from the deposition head to the substrate allows accurate mate-rial deposition on non-planar substrates, over existing structures and into chan-nels. The aerosol is printed on a platen preheated at 40°C where the substrate is placed. The substrate is heated before printing so that the ink can bond onto it, preventing the ink from spreading out upon contact. The pressure and atomizing rates depend on the conductive ink’s properties, while the flow rates of the atom-ized ink and the fabrication speed can be adjusted using the manufacturer’s soft-ware to achieve the desired speed and high accuracy levels. As with the previous technology, Aerosol Jet systems can print fine feature electronic, structural and even biological patterns onto almost any substrate. The process utilizes aerody-namic focusing to precisely deposit electronic and other materials in dimensions as small as 10 µm (micrometres) and with thickness down to 0.1 µm thin. Com-patible material viscosities range from 1 to 1,000 cP. A wide range of materials



has been successfully deposited, including diluted thick film pastes, thermosetting polymers such as UV-curable epoxies, and solvent-based polymers such as poly-urethane and polyimide. Conducting inks including Ag, Pt, Pd, and Cu have been developed with cure temperatures down to 120°C, and higher temperature ceram-ics, ruthenates, and ferrites can be cured using laser treatment. Semiconductor, resistor, dielectric adhesives, and etch resist formulations also have been deposited on substrates such as polyester, polyimide, glass, C-Si, ceramic, FR4, and metal materials. Bio-degradable polymers such as PLGA have also been deposited with Aerosol Jet, as well as biomaterials like proteins and DNA, without denaturing or loss of bioactivity [4].

2.3 Microsyringe technology

The third technology described is contact microsyringe dispensing that deposits picolitres of the desired material using a high-pressure system, without atomizing it or electrically processing it like the previous two systems do. Examples of this technology are the Nordson EFD‘ and the nScrypt‘ that can provide with con-sistent dispensing for simple prototypes, or can print more complicated 3D struc-tures using a surface profiling option and 3D mapping to dispense accurately and consistently on conformal surfaces. The nScrypt‘ systems can dispense materials with a vast range of viscosity (from 1 to 1,000,000 cPs), and dispense dots or lines as small as about 50–100 µm in diameter. Additional features involve tracking of moving objects and printing during tracking, due to the capability of the printing head to move in different directions. The pump’s dynamic flow control allows for adjustment as material properties may change over time. Also the system has a micro mixer pump that allows real-time three-part mixing.

2.4 Screen printing

Screen printing is one of the most common methods for printing fast, larger quan-tities of radio frequency identification (RFID) antennas on conventional substrates or materials including paper. Screen printing is the process of using a stencil to apply ink onto a substrate, such as paper. In this technique, a woven mesh is used to support an ink-blocking stencil to receive a desired image. The attached stencil forms open areas of mesh that transfer ink or other printable materials, which can be pressed through the mesh as a sharp-edged image onto a substrate. A fill blade (or squeegee) is moved across the screen stencil, forcing or pumping ink into the mesh openings for transfer by capillary action during the squeegee stroke. A screen printing setup for antennas is shown in Fig. 2d.

2.5 Roll-to-roll printing

All the above technologies are sheet-based and suitable for low-volume, high-pre-cision R&D prototypes. A different technology is the roll-to-roll or rotary printing. It offers a finer resolution printing (flexography) and has gained strong support



from the RFID community, thanks to its suitability for significantly higher volume of prototypes and mass production than screen printing, and can reach 10,000 square meters per hour (m²/h). It is also suitable for quality-sensitive layers like organic semiconductors and semiconductor/dielectric-interfaces in transistors, due to high layer quality. Roll-to-roll printing equipment, such as the ROKO pilot line of VTT (Technical Research Centre) of Finland that is shown in Fig. 2e, was used to print large antennas on paper substrate. For quality, printing speed was limited to 2 m/min.

3 Paper as a Substrate

Currently, there are billions of electronic devices on Earth, and most of them were fabricated using subtractive manufacturing. Moreover, billions of electronic devices have been already outdated and are thrown or given for recycling. Also, these days technology progresses so quickly that many portable electronic devices last less than 3 years before being updated by a newer model. In addition to these, modern devices have components such as screen displays that may require high rates of repeatable structural flexibility. All these factors, when taken into consideration, lead to the necessity for a low-cost substrate material for electronic devices.

Paper is etymologically derived from Latin papyrus, which comes from the Greek ‘π ′απυρος’ (papyros), the word for the Cyperus papyrus plant. Paper is arguably the least expensive material made by mankind. Paper is biodegradable and friendly to the environment (aka ‘green’), a material that is also organic and flexible. As a substrate, it is compatible with additive direct-write, ink-jet, and cop-per lamination technologies [5,6]. It is also suitable for wearable devices and sensors due to its low profile (small thickness and light weight) that makes it an

Figure 2: Photo of the three different direct-write printing technologies. (a) Dimatix inkjet materials printer, (b) aerosol-jet printer M3D by Optomec, Inc., and (c) contact microsyringe printing system by nScrypt‘. (d) Screen printing semi-automatic equipment and Korsnäs paper. (e) Roll-to-roll setup for high-volume antenna printing using a ROKO Pilot printing line.



attractive substrate for modern RF applications including mass-produced RFID tags, antennas, microwave filters, and modules [7,8] for RF scavenging, smart devices, smart skins, wireless sensor networks, and anti-counterfeiting.

As printing technology evolved, paper became more attractive for low-cost fab-rication of recyclable electronics including antennas and RFIDs. Paper is an organic, thin material produced by pressing together moist fibres, typically cellu-lose pulp derived from wood, rags, or grasses, and drying them into flexible sheets. Its consistency defines its relative dielectric permittivity er at microwave frequen-cies, which has been found to be between 2.5 and 4. Coated paper (often photo-graphic paper) may have a thin layer of material such as calcium carbonate or china clay (Kaolin) applied to one or both sides in order to create a surface more suitable for high-resolution halftone screens, as well as to repel water. Coated or uncoated papers may have their surfaces polished by calendering. Coated papers are divided into matte, semi-matte or silk, and gloss and can be easily found in office and printer supplies stores. Glossy paper gives the highest optical density in a printed image. Although all paper types can be used for printing electronic devices, untreated or uncoated paper may result in less fine features due to its inability to limit ink spreading upon contact.

To use paper as a substrate for electromagnetic devices, we need to first study its electromagnetic properties. In general, paper is a dielectric insulator high in cel-lulose. At frequencies below 100 MHz, paper gives satisfactory electrical proper-ties (especially high breakdown voltage ≈ 16 MV/m [9]). The dielectric loss curves are similar in shape and magnitude to those of wood flour and cotton. At frequen-cies below 100 MHz, paper, as well as other organic fillers (cotton, wood flour), have loss tangents, which change very little with absorbed moisture because there is a large number of OH groups already present in the cellulose molecule although as the moisture content increases above 1% the loss tangent increases more rap-idly. The effect of absorbed moisture is smaller at lower temperatures and higher frequencies because the absorbed moisture mainly contributes to the ionic conduc-tance losses and these losses occur at low frequencies [10]. At higher than 100 MHz frequencies, recent studies showed that paper has a relative dielectric permit-tivity between er = 3 and er = 4, and relatively high losses, e.g. tan d ≈ 0.05 at 2.45 GHz [11]. Further studies related losses in printed RF electronics to two main factors: (1) conductive nanoparticle ink loss and (2) dielectric loss, of paper. In another recent study [12], the average dissipation factor of paper substrate was found to be tan d = 0.072 from 0.5 to 2.5 GHz. Permittivity results from these stud-ies for different frequency ranges are shown in Fig. 3. Consequently, and due to this relatively high value (which is more than 10× larger than that of commercially available high-frequency substrates), the majority of paper research has been lim-ited to applications for frequencies lower than 1 GHz, and especially focused on RFID tags. The relatively high loss tangent, however, should not deteriorate sig-nificantly the performance of antennas that are designed to radiate fields that travel perpendicularly through the paper substrate, which is very thin (typically 25–200 µm), and thus cannot absorb much energy. This fact had been disregarded for years, until 2009 when the first high-frequency antenna above 1 GHz (specifically,



a flexible 2.45 GHz Wi-Fi antenna for flexible displays) was presented [13,14] and published [11]. Since then, numerous examples followed.

An additional characterization of paper at microwave frequencies was made by Anagnostou et al. [11] to ensure using the microstrip line method for Kaolin coated HP Advanced Glossy Photo Paper‘. In that work, a microstrip line was printed to validate the permittivity and losses (tan d) of the substrate. Signal atten-uation versus frequency was measured and it showed that attenuation increases with frequency. From that result, the loss tangent was determined using its estima-tion formula [16]:

tan( )

. ( ),

,

,

dm f f

f f=

−

−

α

πd r r eff

r r eff

0 1

8 686 1 (1)

where l0 is the free-space wavelength, ad is the dielectric attenuation constant, and er,eff is paper’s effective permittivity, which is given here by an empirical formula [17]:

Figure 3: (a) Dielectric permittivity of paper based on measurements at various frequencies, extracted using the microstrip ring resonator method [12] and the split-ring resonator method [15]. The error bar shows a 95% two-sided confidence interval in a linear regression model using the method of least squares for the er. (b) The measured dissipation fac-tor (loss tangent tan d) of paper measured at specific resonant frequen-cies and its variance with 10% uncertainty in measured quality factor Q for tan d. (c) Measured tan d versus frequency using different methods though a printed microstrip transmission line [11].



ee e

r effr r

,

/

t

w= +

−+

+−1

2

1

21 12

1 2

(2)

where t is the substrate thickness (approximately 250 µm) and w the line width. Making the reasonable assumption that paper is a non-magnetic material, the above method gave er ≈ 3.4 at 2.45 GHz and a dissipation factor tan d ≈ 0.07, which matched well with other published results.

To create efficient direct-write printed antennas, another very important factor is the conductivity of the nanoparticle ink used for the direct-write printing of the device. Conductivity typically increases with curing temperature and varies with curing time. Various nanoinks are available commercially, while more can be made at a lab. With most nanoinks, conductivity varies from 5% to 70% of bulk silver. Simulations, however, have showed that efficient antennas on paper are those that form their fields to radiate perpendicularly to the substrate and do not utilize the substrate as part of their resonance enabler. For example, microstrip or cavity-based antennas are affected by the high losses of paper and have reduced quality factor Q, while dipoles (that form a standing wave of current), monopoles, PIFA-based, as well as TEM-planar horn antennas (such as the Vivaldi), and other travelling or leaky wave antennas have less of dependence on the substrate param-eters and can use a silver ink with down to 5% bulk silver conductivity without compromising its overall efficiency.

Curing of the printed nanoink pattern can take place by heating it for a specific amount of time (usually between 1 and 3 h) to a temperature that is typically between 120°C [18] or 150°C and 400°C. Substrates need to withstand the high temperature of curing. As most of the early commercial inks required curing tem-peratures above 200°C, the use of paper was very limited at the beginning, and many inkjet antennas were printed on other materials such as liquid crystal poly-mer. However, nanoink developments on both commercial and R&D inks [18,19] resulted in inks that are cured at temperatures as low as 120°C. This enabled the curing of paper for longer time without physically burning it, as the curing tem-perature is below the deformation or melting point of paper (180–233°), as well as other substrates including but not limited to: Teflon (200–327°C), RT/Duroid (260°C), Liquid Crystal Polymer (280–315°C), silicon and ceramics (>1,000°C). These developments allowed practically any solid material to be used as a sub-strate. As an example of the effect of curing temperature, Fig. 4 shows the mea-sured resistance of printed conducting lines on paper versus the curing temperature for 1-h exposure. The measurements were carried out using a 4-point probe. The printed lines have the same dimensions (25 × 0.5 mm2) and were printed under the same conditions. As the curing temperature increases, the lines’ resistance decreases. This is explained because when the curing temperature is low, there are large spaces between the nanoparticles of the ink resulting in a high resistive path for the RF current. In contrast, high curing temperatures reduce these gaps provid-ing a high conductive path for the current.



Apart from thermal curing (heat), another curing method involves photonic sin-tering using pulses of high-power ultra-violet light. Photonic sintering is particu-larly effective for uncoated paper, which may discolour at temperatures as low as 120°C, such as the common letter-sized (or A4 sized) blank sheets of paper used in printers and copy machines. Photosintering is effective in obtaining sufficient conductivity of the silver ink. Typical parameters that have been found to be effec-tive in sintering large silver nanoparticles, as well as paper printed antennas (e.g. using a Novacentrix Pulseforge PCS-1100), involve using a white lamp with set-tings of 800–1,200 V for 500–900 µs pulses, or 1,200 V and 900 µs pulses for maximum sintering [20]. It has been noticed that as the ink particle size decreases, melting temperature also decreases [11,21], implying that less energy is needed for sintering to occur between silver nanoparticles than for bulk silver. This makes photosintering attractive for creating conductive macrostructures. Conductivity typically decreases with thicker ink depositions and less sintering seems apparent, possibly due to delamination of the sintered surface material. Nevertheless, after being exposed to toluene and being photocured, paper did not seem to discolour, despite some discolouration may appear on traces of silver.

Another important factor for paper antennas is the ink adhesion to the sub-strate. Studies [22] showed that the adhesion of the ink to paper may be weak after photonic sintering; light brushing against any surface can remove material. This may be due to incomplete sintering from the low penetration of the light of the lamp. Specifically, photonic light penetrates up to approximately 5 µm, sug-gesting that sintering deeper than this distance could be only due to thermal con-duction of the ink [20]. It has been suggested to use a thin polymer coating over the deposition or thermally assisted photosintering to sinter more thoroughly the underlying material.

The effects of photosintering on printed silver appear in Fig. 5. Before sintering, the ink deposition appears blue (Fig. 5a). After sintering, the silver turns a metallic light gray, or almost white (Fig. 5b). In addition to changing colour, stress cracks tend to form on the surface of the silver (Fig. 5c).

Photonic sintering tests made on 1 mm × 10 mm strips printed on printer paper were also observed under a scanning electron microscope (SEM) before and after

Figure 4: Resistance of lines fabricated on a paper substrate as a function of the curing temperature after 1-h thermal curing [11].



sintering (Fig. 6). The silver ink printed on paper to thermally sinter was very flat, where its height was on the order of the roughness of the paper (Fig. 6a and b). Ink printed on paper and cured by photosintering was also flat (Fig. 7a). After sinter-ing, the nanoparticles seemed to expand upward, making the sintered part of the

ink less dense (Fig. 7b) [22].

Figure 5: Images of photosintered 1 mm × 10 mm silver (Ag) nanoparticles strip samples before and after curing, as well as using different photosinter-ing parameters [22]. The samples showed some stress fractures on their surface. (a) Before sintering Ag NP strips. (b) After sintering Ag NP strips. (c) Photocuring using 1,000 V, 800 µs, and 2 pulses. (d) Photo-curing using 1,200 V, 900 µs, and 1 pulse.

Figure 6: (a) SEM image of ink on paper before thermal sintering. (b) Ink on paper after thermally sintering. (c) Close up image of ink on paper before thermally sintering. (d) Close up image of ink on paper after thermally sintering [22].



4 Organic Paper-Based Antennas

There are many antenna designs that have been implemented on paper substrate. In fact, with the advances in direct-write technology, it is reasonable to theorize that practically any planar antenna can be fabricated on a paper substrate. The scope of this section is to present some of the most characteristic antennas that have been implemented on paper, and to describe their functionality, physical principles of operation, and specifications. In general, most designs exhibit a high efficiency (above 80%) as the substrate is thin. The most common (and often significant) fea-ture of organic (carbon fibre based) paper-based antennas is their low cost. Some of the earliest antennas on paper are related to the well-known RFID technology.

Work and discussion on RFIDs began in 1945 by Léon Theremin, who had invented ‘The Thing’, a covert listening device energized and activated remotely using radiowaves, without batteries. The first patent associated with RFIDs was filed in 1973 and granted in 1983 to C. Walton [23]. In the 1990s, RFIDs found uses in timing and tracking applications, while in the next decade the technology was adopted in multiple commercial applications by retailers, casinos, airports, and others.

RFIDs on paper substrates have used both the 13.56 MHz (e.g. for smart cards and ticketing services that require short scanning range), as well as the 433 and 930 MHz bands with read range of up to 100 and 12 m, respectively. Most 13.56 MHz RFID designs are single-sided or double-sided rectangular spirals and resemble the structure of a radiating planar inductor rather than an antenna. Designs targeted at the 860–930 MHz range (which covers the U.S. 915 MHz band and the recently approved in Asia 920 MHz band) utilize a variety of shapes, some also appliqué, inspired by the specific application where the RFID was being used. This is achieved by using a dipole antenna configuration and making the ground asymmetric, and relatively large compared with the other antenna arm. This results in a monopole antenna driven over a planar ground plane, in which case the ground size may vary without altering significantly the radiating fre-quency of the monopole. Slight tuning then leads to an efficient design.

Paper as a substrate for antenna applications was characterized using the split-C resonator technique [24] by Rida et al. [7] and Yang et al.. [25] who measured values of er = 3.2 and tan d = 0.06. Designs they proposed include an inkjet-printed U-shaped RFID passive antenna with integrated matching stubs that functions as a narrowband dipole at 870 MHz. This design was later extended into a Paper-Based

Figure 7: (a) Ink on paper before photonic sintering. (b) Ink on paper after pho-tonic sintering (1,200 V and 900 µs) [22].



Wireless Sensor Module by Vyas et al. [8] and is shown in Fig. 8. By inverting one of the arms, the ‘U’-shaped antenna can be converted to an ‘S’ shape that has higher polarization purity and uses the same matching stub [26] (Fig. 9). Another dipole design shaped as a planar bowtie and fed through a planar Tee-matching balun was also presented by Yang et al. [12] in a paper described more extensively the dielectric permittivity measurements of paper as well as the antenna design and measurements (Fig. 10).

The first efficient antenna on paper radiating above 1 GHz is the printed inverted-F antenna (PIFA) by Anagnostou et al. [11,13,14] that is shown in Fig. 11. The design enables Wi-Fi reception on flexible displays. The antenna was aerosol-jet printed using the M3D technology using an in-house made silver nanoparticulate ink that was cured at 120°C and is shown in Fig. 11 during and after fabrication. The developed antenna on paper radiated at 2.4 GHz with more than 82% effi-ciency. The antenna design differed from RFID designs as it is not based on a dipole structure. Instead, it consists of a planarized inverted F, which explains the PIFA name, and is derived from a quarter-wave half-patch antenna with a shorted (metallic strip) half-patch that decreases the resonance frequency using. It reso-nates at a quarter-wavelength (thus reducing the required space needed on the mobile device), and also typically has good SAR properties due to the use of a ground plane that partially reflects radiation thus creating a slightly asymmetric

Figure 8: Paper-based wireless-sensor module (a) with and (b) without electronics [8]. © 2008 IEEE.

Figure 9: ‘S’-shaped RFID tag architecture showing the location of the integrated circuit (IC) terminals (feeding point) [26]. © 2007 IEEE.



Figure 10: T-match folded bow-tie RFID tag module configuration, return loss that covers the universal UHF RFID band, and measurements demon-strating good agreement between both paper metallization approaches (inkjet and copper lamination). The normalized patterns (bottom) and measured RFID tag reading distance illustrate an omnidirectional radi-ation pattern with directivity about 2.1 dBi [12]. © 2007 IEEE.

Figure 11: Direct-write printing of the 2.4 GHz antenna on paper using the M3D aerosol printer. The fixed head of the M3D and its movable platen can be seen (left). Photo of a fabricated antenna prototype on organic paper-based substrate (right). The SMA feed has been connected and is shown for size reference [11].



radiation pattern, stronger in one direction (and usually toward the free space, away from the user). This type of antenna also has low profile and is a common cellular phone built-in antenna. This antenna was also tested under flexing condi-tions. The high level of flexibility, in conjunction with the relatively minimal dis-tortion of its radiation pattern, made it very suitable for flexible display applications. One key difference in the fabrication of this antenna with respect to previously inkjet-printed antennas was the method of fabrication.

During the design process of this antenna, paper was evaluated (successfully) as a potential substrate for antenna applications at the GHz range and the results were presented earlier. Despite the relatively high dielectric loss of paper (tan d ~ 0.07 at 2.45 GHz), the maximum measured realized gain of the fabricated antenna is +1.2 dBi and its total efficiency is approximately 82%, which is very satisfactory for such a low-cost substrate. The measured resonant frequency is 2.5 GHz (only 2% shifted from the expected). With its 10 dB bandwidth of ΔfBW = 24%, this antenna covers sufficiently the entire WLAN frequency range. As with traditional PIFA antennas, the effect of increasing the length s of the compensating stub is to increase the antenna inductance and tune out the antenna capacitance to make the input impedance purely real (Fig. 12a). The dimensions h, d, and s were then varied to minimize the return loss at 2.45 GHz and obtain the dimensions of the final prototype. The antenna design and equivalent circuit appear in Fig. 12b. The measured return loss and radiation pattern are shown in Fig. 13. The F/B ratio is 6 dB. An efficiency comparison with a reference low-loss PIFA antenna designed on a 32-mil thick high-frequency RO4003C substrate is also shown in Fig. 13. The flexibility of the design, including the ink adhesion, cracking during flexing, and longevity, was also tested as shown in Fig. 14. The ink maintained its high conduc-tivity (and the antenna its high efficiency) for more than 2 years after curing, while being exposed to a laboratory ambient air environment. These results were excel-lent considering the extremely low cost of the paper used.

For this antenna, the M3D fabrication process was repeated ten times in order to achieve a 3-µm thick conducting layer and reduce the skin depth effect. The total fabrication and curing time for this antenna was 3 h. Direct-write printing can be much faster than photolithography for small R&D prototypes as it does not involve masks. For mass-production applications, fabrication time can be dramatically reduced by using commercial inkjet printers with multiple heads [29]. Roll-to-roll [30] and screen-printing [31] techniques can also be used. Fabrication speed may be further reduced using commercial equipment that sinters at room temperature large areas of ink within a few seconds exposing it to ultraviolet (UV) light or sequences of laser pulses [32–34].

The feasibility of this proof of concept of low-cost antennas on paper substrates at frequencies above 1 GHz spurred several more prototype designs. In Fig. 15a, a 3–11 GHz ultra-wideband (UWB) antenna with two slots for double-frequency rejection at the WLAN (Wi-Fi, 5.2 and 5.7 GHz) bands was printed using a com-mercial Dupont‘ ink cured at 170° and the nScrypt‘ microsyringe technique. The principle of operation of most such single-layer (uni-planar) and conformal UWB antennas is that a coplanar waveguide feed line (often l/4 long) transmits a



TEM electromagnetic wave from feed point to the antenna terminals. The electro-magnetic wave is divided into two parallel and planar TEM horns, one on each side of the antenna. The horns can be linear or tapered and match the transmission line to the high impedance of free space (Z0 = 120π or 377 Ω). The antenna is made larger to minimize reflections of the current from the ends of the structure that would affect the input impedance and limit the bandwidth. The current traces the external edges of the antenna, which bends appropriately to result in a nearly omnidirectional radiation pattern. The rejection can be achieved using two slots (Fig. 15a) or two parallel open-circuit l/4 long stubs (detail from another antenna shown in Fig. 15b). The open circuit becomes a short circuit at the point where the stubs connect to the antenna. When the stubs are placed at the area where the cur-rent is maximized at 5.2 and 5.8 GHz, they create an electronic short circuit for that current and draw it inside the stubs. The opposition of current flow directions between the antenna edges and the stubs increases the input impedance of the

Table 1: Final dimensions of the paper-based IFA antenna.

Dimension x y w d h l sSize (mm) 46 25 0.57 10 5 26.15 6

Figure 12: (a) Effect of the length of the compensating inductive stub s on the return loss of the antenna. An s value of 6 mm tunes out completely the antenna capacitance, giving purely real input impedance and thus a good 50 Ω match. (b) PIFA antenna design and its equivalent circuit. (c) Dimensions of PIFA antenna on paper substrate [11].



Figure 13: (a) Measured and simulated return loss of the IFA antenna on paper, (b) radiation pattern, and (c) total efficiency compared with the effi-ciency of the same antenna fabricated on a high-frequency, low-loss RO4003C substrate [11].

Figure 14: Photos of the fabricated antenna on paper bent around cylindrical rolls with radii R1 = 1.25 cm and R2 = 2.7 cm. The antenna showed no sign of permanent deformation, conductivity deterioration (i.e. cracks) or ink detachment after extensive and repeated bending. To the right is shown a conceptual illustration of the application and embedding of this antenna into a flexible display for potentially rollable screens and computers. Antenna locations are shown with dashed and dotted yellow lines. Original screen captured from [28] and modified for the purposes of this work [11].



antenna at the cutoff frequencies. To achieve this, accurate definition of the stub dimensions is needed. nScrypt, as well as the other direct-write technologies can achieve features with less than 200 µm size, as shown in Fig. 15b. Other antennas that were developed and use the principle that the fields should radiate perpen-dicularly to the surface of the paper (in other words, antennas that did not form a cavity that would have low Q because of the relatively high loss tangent of the paper substrate), include a 1.2 and 1.4 GHz dual-band dipole covering the remote sensing L-band of salinity and surface roughness radiometers (Fig. 15c). The

Figure 15: (a) nScrypt microsyringe direct-write printed ultra-wideband antenna (3–11 GHz) with double WLAN rejection, cured at 170°C. (b) Fine feature detail from an nScrypt printed dual-rejection UWB antenna. The stubs are 200-µm wide and have a 100-µm slot and are from a UWB antenna with double WLAN rejection, also described in [35]. Note that nScrypt is the least fine-feature capable technology from the three discussed in this chapter. (c) A dual-frequency dipole at 1.2 and 1.4 GHz for remote sensing, and (d) a 0.9 and 1.8 GHz dual-frequency monopole for GSM applications. Antennas (a) through (d) were devel-oped at SDSMT. (e) 3–10 GHz UWB antenna made with a Dimatix inkjet materials printer at Georgia Tech [15]. © 2011 IEEE.



antenna functions by feeding in series two dipoles that each resonates at a different frequency. The dipoles are placed close to each other to prevent them from acting as a director or reflector of one another. Also, a simple dual-band monopole is shown in Fig. 15d. Each arm of the monopole is resonant at a different frequency, the long arm at 0.9 GHz and the short at 1.8 GHz, to cover both GSM bands. The current divides into two different paths, and the antennas are well-matched to 50 Ω. On both of these antennas, a standing wave of current is created on the surface of the antenna to enable radiation.

Later, in 2011, an UWB antenna without the band-rejection slots (of that in Fig. 15a) was printed on a paper using the CCI-300 ink from Cabot‘, and cured at 120°C for 2 h. Its return loss is less than 10 dB throughout the UWB band and its group delay variation (which should be small to prevent distortion of the trans-mitted or received signals), was measured to be about 0.6 ns. The antenna is shown in Fig. 15e.

The same year, a facile pen-on-paper approach was demonstrated for flexible printed antennas [37] such as the one shown in Fig. 16. The 1.9-GHz antenna is composed of eight tapered meanderline arms (w = 650 µm, centre-to-centre spac-ing = 1 mm) and was printed by a desktop printer on adhesive-back paper. Then, conductive silver traces were hand-drawn on the guide lines using a silver ink-filled rollerball pen. The pattern is then cut to form a pinwheel structure that is conformally adhered to the surface of a hollow glass hemisphere (radius = 12.7 mm). The 3D antenna is completed by attaching the patterned hemisphere to a low loss laminate substrate (Duroid 5880, Rogers Corp.) with copper feedlines. Mea-surements showed an efficiency of 20–30%, which is significantly lower than that achieved by conformal printing conductive features directly onto glass hemi-spheres. However, those 3D antennas required heat treatment to 550°C to obtain silver meanderlines with electrical resistivity similar to that of bulk silver ( ~1.6 × 10−6 Ω·cm) [38].

In a notable system integration work, a bio-patch using a fully integrated low-power system-on-a-chip (SoC) sensor and paper-based inkjet printing technology was implemented [39]. Specifically, both the electrodes and the interconnections

Figure 16: Photo of a 3D pen-on-paper drawn antenna, and its reflected power, illustrating an efficiency of 20–30% [38].



of the bio-patch were implemented by printing conductive nanoparticle inks on a flexible 270-µm thick photographic paper (HP Advanced Photo Paper) substrate using inkjet printing technology. Other components were then connected. The NPS-JL ink by Harima Chemicals was used and was printed using a materials printer by Dimatix. Images of the printed electrodes and the SoC are shown in Fig. 17. Conductivity of about 10% of bulk silver (63 × 106 S/m) was measured. The bio-patch was used for ECG measurements.

A U-slot monopole antenna with three bands [40], optimized using slots to direct the surface current so that it radiates at 1.57, 3.2, and 5 GHz, was printed on a paper substrate using a conductive silver ink from UT Dots, Inc. with 10-nm sized nanoparticles dispensed in a hydrocarbon solvent. This antenna is shown in Fig. 18. The conductivity of the ink was 1.2 × 107 S/m. In that work, it is noted that ink conductivity improves by adding more layers, but only up to a certain number of layers. In that work, five layers were used, after which, conductivity did not increase significantly. Curing was made at 160°C for 1 h. Two layers were glued back-to-back (one with the radiating structure and another with the ground plane), for a total substrate thickness of 0.44 mm for the microstrip line. The simulated efficiency of the antenna, based on the achieved gain (which varied from 6 dBi to +2 dBi with frequency), was 55% at 1.57 GHz, 79% at 3.2 GHz and 71% at 5 GHz.

All these prototypes so far have been low gain, near-omnidirectional antennas, suitable for receivers. High-gain antennas on paper substrate can also be made possible using inkjet printing. In [41], the complete characterization of inkjet

Figure 17: (a) Printed electrode using silver nanoparticle ink on a photo paper substrate, (b) cross-sectional view of the printed electrode, and (c) prototype of the bio-patch on paper substrate with printed electrodes, interconnections, bioelectric SoC, and a connected soft battery [39]. © 2012 IEEE.



printing process was made for frequencies up to 12.5 GHz, and high gain and wideband antenna designs were demonstrated. Specifically, a Vivaldi type of UWB antenna that has gain up to 8 dBi, and a slow-wave log periodic dipole array with ridged arms employing a new miniaturization technique achieving 20% width reduction were printed and then laser and heat sintered. The antennas are shown in Figs 19 and 20, and were printed using a materials printer with Sigma Aldrich conductive silver nanoink. Due to the high sintering temperature needed (200°C), laser sintering was used. A Professional Systems 10 nm laser with beam width 100 µm was aligned with the design and the laser was raster scanned over the traces at several power levels from 0 to 75 W at a resolution of 1,000 dpi. Sintering for a 10 × 10 mm area was performed in seconds and resis-tance of about 0.5 Ω/square, similar to that achieved by heat curing at 200°C for 1 h was achieved when the laser power was set to 80%. The achieved conductivity was 1.2 × 107 S/m. Although the log periodic design with t = 0.93 and s = 0.11 parameters was anticipated to have a gain of 8 dBi at 525 MHz, only a 4 dBi gain was measured, indicating that the effect of the substrate loss was stronger in this slow-wave ridged structure.

The efforts made to expand the repertoire of antennas on paper include self-similar and spiral designs that have very large (ideally infinite) bandwidth. The large electrical size of these antennas at microwave frequencies makes them less suitable for direct-write printing, and more suitable for other printing technolo-gies such as screen printing and roll-to-roll printing. A comparative study of the

Figure 18: (a) Fabricated prototype of triple-band U-slot monopole antenna. (b) Measured and simulated |S11|. (c) Measured and simulated realized gain and simulated |S11| [40]. © 2012 IEEE.



different printing methods [42] (including direct-write) of a 915-MHz RFID antenna on paper showed that all methods can provide sufficient conductivity and ink thickness (via many printed layers) for antenna applications. The design shown in Fig. 21a and b are screen printed and roll-to-roll printed. The antenna is a narrowband bowtie with curved ends, matched using a Tee-matching balun. For screen-printing and roll-to-roll purposes, the ink was printed thicker than in pre-vious designs by using Asahi conducting paste that has also good mechanical flexing properties. The prototypes were post-annealed for 2 h at 140°C to improve conductivity.

Results from another study of screen-printed antennas [43], such as the proto-types shown in Fig. 22, indicate that although screen printing may affect the lin-earity of antenna edges, imperfections are usually a few microns in size and do not affect significantly the effective antenna area and the antenna performance at the GHz range.

Figure 19: (a and b) Photo of a high gain antipodal Vivaldi antenna on paper, front, and back sides, and (c) measured and simulated |S11|, gain (reaching about 7.5 dBi), and radiation patterns demonstrating pattern stability versus frequency up to 11 GHz [41]. © 2008 IEEE.



A series of wideband spiral antennas has also been developed on paper. These antennas targeted the 0.8 to 3 GHz frequency range and were printed using the materials printing inkjet technology. First, an Archimedean spiral antenna [44] was designed and printed (Fig. 23a) on Felix Schoeller photographic paper. A Xerox conductive ink with silver loading 40 wt% was used. Results showed that the measured input resistance (Fig. 23b) from 1.1 to 1.4 GHz fluctuates around 150 Ω, which is in accordance with spiral antenna theory. The input reactance (Fig. 23c) is also stable and fluctuates around +j8 Ω, as expected of frequency-independent antennas. The return loss is also shown and the antenna broadband behaviour, as well as its pattern stability with frequency, is verified. With 7.5 turns, this spiral achieved approximately 4.4 dB gain. Such wideband antennas can be used to simultaneously integrate a wide range of modules as well as RFID tags.

Figure 20: (a) Slow-wave log periodic design with fishbone arms for miniaturiza-tion, with an attached infinite balun. (b) Measured and simulated |S11|, and (c) measured and simulated gain of the slow wave log-periodic dipole array on paper, achieving 4 dBi, possibly affected by the high substrate losses [41]. © 2012 IEEE.



Figure 21: (a) Screen-printed antenna on Korsnäs. (b) Roll-to-roll printed antenna on Korsnäs paper. (c) Comparison of antenna input resistance variation and (d) of input reactance variation, for various printing methods. (e) Measured read range. (f) Measured and simulated return loss. From this study, it can be seen that the different printing technologies, when used correctly, have minor effect on antenna performance [42, repro-duced courtesy of The Electromagnetics Academy].

By adjusting the gap between the adjacent tracks of the spiral arms, we can achieve a larger number of turns within a given aperture size.

More wideband antennas have been developed to cover the 0.8–3 GHz band. One example is the planar wideband two-arm sinuous antenna [45,46] on a paper, shown in Fig. 24a and b. The sinuous curve is defined by angles and a growth rate (expansion ratio) s. Results in Fig. 24c and d show a wideband response with mea-sured input resistance around 150 Ω, and radiation pattern that is stable and does not vary significantly with frequency.

Another example for the same 0.8–3 GHz frequency band is the planar log-spiral antenna [45,47] on paper, developed using a similar approach as the previ-ous antenna, and shown in Fig. 25. The curve of the antenna, which has 1.25 turns, is calculated by: r = r0e

αϕ, where r0 is the inner radius, which has 1.36 cm to accommodate for the area of the multi-chip module package. The expansion coef-ficient α is 2.88 and is related to the pitch angle y by: α = ln(t)/2p. The antenna has also tapered ends to minimize reflections from the end of each arm at the lower frequencies. The structure is self-complementary and should have input imped-ance 60p Ω for free space (with no dielectric loading) at all frequencies. To avoid



soldering that requires heating to above 250°C, the antenna was attached to a test fixture with CW2400 silver conductive epoxy cured at 24°C for 4 h.

4.1 Novel designs and future trends

A new type of antenna was recently designed and printed on an organic, untreated (uncoated) paper substrate. This antenna is shaped as a bar-code, and in particular a QR (Quick Response) code [2,48,49] (Patent Pending). QR codes are 2D barcodes generated for any text, URL, or alphanumeric content, and consist of an array of square modules arranged in a square pattern. Each area of a QR code has a specific purpose (e.g. alignment, error correction, encoding, etc.), but each QR code is dif-ferent (even for QR codes with the same message), and not all square modules are connected to each other, posing a challenge when the QR code is made to act also as an antenna (in addition to an optical message conveyor) because a continuous path of metal with significant length is typically required. The optical code is vis-ible, but the RF code is invisible and readable only with a specific reader. Enabling QR codes with radiating properties (as a receiving or transmitting antenna) adds a new dimension in their applicability as security devices that are hard to detect and replicate. In a realistic application, a QR code antenna may convey wirelessly the same or a completely different message than its visual code. Moreover, the radio transmitted code may be used as a ‘key’ signal to decode the visual message (or vice versa), both of which may convey individually other, innocuous messages. Without the transmitted ‘key’, the QR code could be a counterfeit. In this way, a

Figure 22: (a) Screen-printed antennas on flexible paper substrate. (b) Recon-struction of screen printing edge behaviour in a simulation model [43]. © R. Zichner and R. R. Baumann, 2013.



Figure 23: (a) Archimedean spiral inkjet-printed antenna on Felix Schoeller paper. (b) Input resistance and (c) input reactance variation demonstrating constant impedance with frequency (and thus a frequency-independent design). (d) Measured and simulated return loss demonstrating wide bandwidth. (e) Measured far-field radiation pattern demonstrating pattern stability versus frequency [44,45, reproduced courtesy of The Electromagnetics Academy].

QR code antenna can be used as an additional measure for security and anti-coun-terfeiting [48]. Moreover, QR code antennas can be made visible, near-invisible (i.e. unscannable wire-grid structures, or embedded inside a substrates), or even entirely invisible to the naked eye [50,51] using upconverting nanoparticles. A challenge is to feed these antennas in a manner that is similar for most designs,



Figure 24: (a) Photo of the two-arm sinuous antenna on paper. (b) Design param-eters. (c) Return loss, showing wide bandwidth. (d) Radiation pattern exhibiting similar shape at all frequencies [45].

Figure 25: (a) Log-spiral antenna printed on paper. (b) Design parameters. (c) Return loss, showing wide bandwidth. (d) Radiation pattern exhib-iting similar shape at all frequencies [45].



to minimize the engineering needed to make each one resonate at a specific fre-quency. Since each QR code is different, each antenna model will also differ from its predecessor.

All QR codes have one large ‘positioning pattern’ on their three corners, and one or more, smaller ‘alignment square’ patterns, fixed and connected to a large part of the visual message. This alignment pattern can be used as a feed for the antenna. This design and feeding methodology can be applied to any QR code. Simulations (Fig. 26) show the surface current density, from which one can extract the radiated field (that is near omnidirectional) and polarization (that can be made linear, elliptical, or circular). Moreover, these antennas can be used with repro-grammable RFID tag antennas to serve multiple functions.

As a proof-of-concept prototype on paper, a QR code antenna at 2.4 GHz was designed and was printed on untreated (photocopy machine) paper for the first time. This paper is hydrophilic (unlike most photographic paper which is partially

Figure 26: Surface current density distribution on simulated models of QR code antennas. An example of QR code antenna prototype that was aerosol-jet printed on thin and flexible organic Kapton‘ is also shown while being successfully read by a low-resolution camera. All antennas have dimensions 3.5 × 3.5 cm. The antennas convey optically the message ‘http://www.sdsmt.edu’.



hydrophobic), so additional care needs to be given in the ink preparation and depo-sition to prevent expanding the printed pattern too much through the paper fibres.

A silver nanoparticle ink was formulated from nanoparticles approximately 4 nm in diameter, capped with decanoic acid and dispersed in toluene. The deca-noic acid capping agent prevents particle agglomeration and improves particle dis-

Figure 27: (a) Thermally sintered and (b) photo-sintered QR code antenna on uncoated paper substrate [2]. The paper on the photo-sintered antenna shows significantly less signs of discolouring. (c) Photo of the ther-mally sintered antenna showing the feeding cable. (d) Thermally sin-tered antenna during pattern measurement inside the anechoic cham-ber, showing the bazooka balun at its feed point. (d) Measured return loss (of the thermally sintered antenna) indicates radiation at 2.4 GHz. (e) The antenna has a smooth H-plane pattern, indicating good recep-tion characteristics.



persion in the solvent. Particle loading was 60%wt [52] and provides conductivity 35-40% of bulk silver, which enables very high efficiency metallic devices such as this antenna. The skin depth at 2.4 GHz is 1.6 µm, and five layers were printed to minimize any resistive losses. The printed antenna was fabricated on the untreated paper substrate and is shown in Fig. 27. Its return loss is 12 dB at 2.4 GHz and its measured gain is 1.25 dBi, which is representative of a good receiver.

The latest technological development in antennas on paper is the reconfigurable antenna shown in Fig. 28 [53]. Reconfigurability is a much desired feature in antennas as it adds more functionality in a given (and usually limited) space or volume. The design of this antenna is based on a coplanar folded-slot dipole that is reconfigured by diodes that are connected to metallic islands inside the slot that alter its circumference [54,55]. Its major characteristic is the achievement of two resonant frequencies on demand (specifically at 4.5 and 4.9 GHz), with the same pattern and polarization, which practically doubles the bandwidth of the antenna, as shown in Fig. 28b. This prototype is also the first-ever reconfigurable antenna on paper.

5 Conclusions

Paper antenna research is on-going, and as technology evolves continuously, more prototypes and proof-of-concept designs will be demonstrated and more chal-lenges will be tackled. Current efforts are not only limited on the conductivity but also include ink viscosity, overspraying, uniformity in the thickness of print-ing, adhesion on different surfaces, resistance to scratching, and flexibility with-out cracking. Achievements have been really impressive, and more advances are anticipated. This technology direction has made it possible to print simple, passive electronic components from our homes, using a computer and an inkjet printer, and to experiment with them, or even share designs with others. The impact in education and in research and experimentation is tremendous.

Figure 28: (a) Photo of the first-ever reconfigurable antenna on paper substrate [53]. (b) Measured return loss of the reconfigurable antenna on paper, when the diodes are ‘ON’ and ‘OFF’, indicates radiation at 4.5 GHz and at 4.9 GHz, respectively.



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chapter 2 organic paper-based antennas · keywords: antennas, dual-band antennas, direct-write,...

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