# Conductive Ink Printed Fabric Antenna with Aperture Feeding Technique
## 1 Introduction  
Flexible wearable devices have been at the center of recent smart clothing research as they can be easily integrated into regular clothing with minimal discomfort to the end-user; all the while maintaining the fashion aesthetics and intended communication capabilities. A 2020 in-depth analysis of recent advances in the wearable research industry [1] indicated that a critical concern in smart textile development is the elimination of on-body coupling while incorporating additional functionality to the antenna system. The others are the use of bendable, waterproof, and highly conductive fabric-based radiators. In this regard, various research [2; 3; 4; 5; 6; 7; 8] has been conducted on material choice and manufacturing technologies to achieve highly conductive and functional fabrics. These technologies can be grouped into two general categories. First, is the use of conductive sheets or threads attached, woven, or embroidered into fabrics with differing meshing techniques. The second is the use of conductive inks and/or paint applied on fabrics using special screen-printing techniques. The former has been investigated by [5; 6] and many others with the use of conductive yarns woven into radiating shapes such as a spiral or monopole. [7; 8] developed wearable antennas by integrating fabrics with thin copper sheets shaped into radiator sizes for resonation. In addition to the obvious aesthetic discomfort to the wearer with the use of these solutions, these antenna technologies are difficult to integrate into existing clothing and pose a major challenge in input impedance matching. That is, the potential air gaps between the radiator and fabric substrates, irregular thread dimensions due to yarn coating, etc., result in unpredictable results. Conductive ink applications [3; 9; 10], on the other hand, have overcome some of these challenges with the use of highly conductive inks such as silver base inks printed directly(inkjet printing) on the target fabric or with the use of thin fabric bases such as polyester, cotton, or nylon which acts a conductive fabric bed. The latter technique, known as screen-printing, forms a thick quasi-conductive fabric that can be cut into various radiator shapes and integrated into the target fabric. Both of these techniques will be investigated in this letter to obtain performance characteristics, similar to what is presented in [3]. After choosing a suitable conductive ink technique, another major challenge with textile antenna integration is feeding. The widely used method of exciting the printed and/or woven antenna is with a SubMiniature version A (SMA) probe or edge feeding as implemented in [3; 4; 9] among others. Although very excellent radiation and return losses are obtained, the bulkiness of the SMA connector inhibits the general adaption of these antennas into mainstream clothing. To this end, as shown in Figure 1, simple patch antennas have been developed and presented in this paper to show progressive advancements in the field of flexible, on-body compatible antennas using ordinary fabric textiles. An in-house, silver-ink screen-printed polyester base conductive fabric has been developed with high conductivity, high stretching, and bending resistance with the screen-printing technique [4]. The antennas (shown as model #1, #2 and #3) were developed at 2.44 GHz and 868 MHz, which are widely used frequency bands for long-range communication. Bluetooth low energy (BLE) operating at 2.4 to 2.48 GHz and long-range wireless access networks (LoRaWAN) operating at 863 to 870 MHz are some of the popular communication standards used in long-range track and detection, Internet of Things (IoT) communication devices, etc. Additionally, a coaxial feeding technique has been developed and tested at these LoRa and BLE frequency bands. The feeding technique is a proximity-fed aperture coupling of the radiating patch with a coaxial cable attached to the ground plane.