Introduction
With the continuous growth of the IoT in various sectors and the emergence of advanced wireless communication technologies like 5G and beyond, the significance of antennas has become increasingly essential [Reference Patnaik and Kartikeyan1, Reference Peng, Du and Wang2]. A wide array of devices, spanning from wearable fitness trackers to industrial machinery, enables the possibility of real-time data monitoring, analysis, and the ability to make well-informed decisions based on the acquired information [Reference Abir, Anwar, Choi and Kayes3–Reference Singh, Gunjan, Chaudhary, Kaluri, Victor and Lakshmanna5].
Radio frequency (RF) technology holds applications across various sectors [Reference Aziz, Farhan, Sharif, Ijaz and Safdar6], and ultra-high frequency radio frequency identification (UHF RFID) provides improved features such as good tag sensitivity and better read range for the tag antenna [Reference Lu and Liou7, Reference Kamalvand, Pandey and Meshram8]. RFID tags offer a distinct identifying code for physical entities, enabling IoT devices and systems to recognize, determine locations, and monitor objects inside the physical realm. Passive RFID tags are of paramount importance in facilitating the automated acquisition of data and the development of the IoT, hence enhancing effectiveness and precision across various sectors and applications [Reference Shen, Yang, Wang, Kang and Mao9–Reference Parada and Melia-Segui11].
The recent research focuses on UHF RFID tags for wearable applications [Reference Hughes, Horne, Brabon and Batchelor12]. A novel dual-band tag antenna using artificial magnetic conductor structures for UHF-RFID frequency bands has been proposed by [Reference Bansal, Sharma and Khanna13] to achieve optimal impedance matching with the Alien Higgs 4 chip. The authors [Reference Morales Pena, de Oliveira, da Silva and Benjo da Silva14] proposed a flexible passive UHF RFID small and slim tag antenna and employed a flexible commercial substrate for frequency bands of 902–928 MHz. A UHF-RFID circularly polarized bio-inspired tag antenna has been designed using a characteristic mode analysis (CMA) in CST to analyze the various characteristic angles and characteristic modes for low-permittivity and metallic substances [Reference Sharif, Kumar, Althobaiti, Alotaibi, Safi, Ramzan, Imran and Abbasi15]. This design features two shorting stubs and slots etched on an F4B substrate with a leaf-shaped radiator.
Body area RFID tags are widely used in IoT applications for sensing and communication with other IoT devices. For IoT applications, a novel UHF-RFID passive glasses frame tag antenna has been proposed using CMA in the CST microwave studio [Reference Hanif, Farhan, Sharif, Paracha and Ijaz16]. In Bajaj et al. [Reference Bajaj, Upadhyay, Kumar and Kanaujia17], the authors present a compact, circularly polarized cross-dipole antenna that integrates the RFID band and GPS band for an amalgamated identification mechanism. This antenna was designed to improve the efficiency and accuracy of the reader-tracking device within an IoT network. This work [Reference Hanif, Farhan and Sharif18] presents a UHF-RFID tag stitched onto a face mask for IoT applications to operate in the 902–928 MHz band using CMA.
Logo tag antennas provide a distinctive combination of visual appeal and usefulness, rendering them a significant product for branding, monitoring, and consumer interaction across many industries. It functions as an aesthetically pleasing symbol of a brand while integrating RFID technology. The authors in ref. [Reference Sreelakshmy, Kumar and Shanmuganantham19] developed a wearable patch logo antenna using jeans material as the substrate and fabricated it through embroidery. The researchers proposed a simple antenna, not a tag, so it could not be used for tagging purposes.
The logo antenna of the famous brand Louis Vuitton, capable of operating on two bands, 2.45 GHz and 4.5 GHz, was proposed by Tak and Choi [Reference Tak and Choi20]. A leather substrate and conductive textile have been used for the fabrication of the proposed antenna. A Chanel-logo shaped antenna has been proposed in Li et al. [Reference Li, Chung and Li21], and modal radiation patterns and modal significance have been studied for different modes through CMA. The study [Reference Sim and Chi22] designed an Olympic-like logo square-shaped patch antenna for 900 MHz UHF RFID reader applications. A UHF RFID tag antenna for the company logo of CTCI Corporation was designed using CST and attached to the helmet so that the headcount can be quickly carried out when workers are passing through a wide control portal [Reference Huang, Lin and Kuo23]. An 865 MHz logo dipole antenna has been developed for RFID applications, and CST software has been used to model and simulate the antenna [Reference Kumar24]. The authors [Reference Tribe, Oyeka, Batchelor, Kaur, Segura-Velandia, West, Kay, Vega and Whittow25] fabricated a UHF RFID wearable antenna for the logo of the University of Kent and Loughborough University by using silver conducting paint on adhesive tape.
Integrating a logo shape into the antenna design serves a dual purpose in wearable IoT applications. Firstly, it addresses the growing demand for aesthetic and brand-oriented customization in wearable electronics, where visual appearance and brand identity are often as important as functionality. The logo-shaped antenna can be seamlessly embedded into clothing, event badges, or institutional wearables, enhancing user acceptance and promoting unobtrusive integration of IoT components into daily attire. It also facilitates covert identification or tracking while preserving reliable RFID readability.
Secondly, the logo’s geometry was carefully optimized using CMA to align its structural features, such as curves and gaps, with the antenna’s resonant characteristics, ensuring efficient current distribution and minimal performance degradation. By strategically embedding the radiating elements within the logo’s contours and tuning key parameters, including feed position and substrate flexibility, the design achieved UHF-RFID operational requirements, i.e., read range and impedance matching, without compromising mechanical flexibility or radiation efficiency.
The key novelty of this work lies in the CMA-optimized integration of the institutional logo of Government College University Faisalabad (GCUF), as the functional radiating element of a flexible UHF-RFID tag, eliminating traditional matching networks while maintaining both aesthetic integrity and RF performance for IoT applications. By employing CMA to exploit the logo’s intrinsic modal characteristics, we achieved impedance matching with commercial RFID chips without the need for conventional matching techniques, such as vias, stubs, or additional lumped components, a significant advancement over conventional logo antennas that typically sacrifice performance for aesthetics. This proposed method preserves the visual identity of the logo while delivering robust electromagnetic performance, demonstrating a significant advancement in the design of visually discrete, wearable RFID systems. The approach introduces a new direction in logo-based antenna design by ensuring practical usability without compromising performance, mechanical flexibility, or integration ease.
Proposed design using CMA
The logo of GCUF has been selected for the proposed research design. The 2D geometric model of the GCUF logo was initially created in AutoCAD. This model was then imported into CST to perform the simulation for CM analysis, aiming to identify its resonating modes at the required frequency band of 0.9 GHz. The actual GCUF logo and simulated logo are portrayed in Fig. 1(a) and (b). The simulation for the eigenvalues in Fig. 1(c) concluded that modes 1 and 2 are resonating modes, mode 3 is capacitive, and mode 4 is the inductive mode. A modal significance graph in Fig. 1(d) illustrates that mode 1 and mode 2 are resonating and significant modes. Far-field patterns for the initial four characteristic modes of the GCUF logo are represented in Fig. 2. The radiation pattern of mode 1 and mode 4 is bi-directional in a horizontal plane, showing a cardioid shape, whereas the radiation pattern for mode 2 and mode 3 is also bi-directional but in the vertical plane.
(a) Original Logo of GCUF, (b) initial GCUF logo-simulated design, (c) eigenvalue graph, and (d) modal significance graph.
Far-field patterns for the initial four modes.
Characteristics current pattern for all modes of logo antenna are illustrated in Fig. 3. Mode 1 and mode 2 need to be excited in CST by placing a chip at the appropriate position. The maxima and minima of mode 1 and mode 2 are also illustrated in Fig. 3. The RFID chip possesses a capacitive impedance, allowing it to be conveniently placed at the current pattern’s minima without the need for any additional external source or component. Modes 1 and 2 have the same mimima points; therefore, mode 1 is finalized for excitation and selected for further analysis. For the said purpose, a UHF RFID chip (Alien Higgs H3) is attached to the minima of the current pattern for mode 1.
Characteristics current pattern for first four modes and minima and maxima of the first two modes.
The proposed logo design is simulated in CST Microwave Studio as shown in Fig. 4(a). The simulation result of the impedance graph is shown in Fig. 4(b). The value of impedance at 0.9 GHz is 6.5 + 150 j Ω, which is significantly low as compared to the impedance of the UHF RFID chip Alien Higgs H3 (27–200 j Ω). Hence, further modifications are required to achieve a perfect conjugate match.
(a) Simulated logo tag antenna with the RFID chip at the minima and (b) Impedance graph.
To enhance impedance, the current path was incrementally increased by modifying the lines beneath the chip area, as shown in Fig. 5 (ver.1–3). Initial adjustments (ver.1 to ver.3) resulted in minimal improvement, with an impedance value of 9.08 + 145 j and stabilizing at 10.4 + 145 j, which could be observed in Fig. 6(a–c), still unsuitable for matching the RFID chip. A significant improvement was achieved in ver.4 by creating a loop above the chip, raising the impedance to 27 + 213 j (Fig. 6d), closely matching the Alien Higgs H3 chip (27–200 j). Final adjustments in ver.5 introduced symmetry, resulting in a minor impedance shift to 26.89 + 215 j (Fig. 6e), which remains acceptable for matching. Since the ver.5 logo achieves a perfect conjugate match with the RFID chip, this logo shape has been selected as the proposed tag antenna. The detailed dimensions of the proposed tag antenna design are graphically shown in Fig. 7.
Modifications from ver.1 to ver.5.
Impedance graphs (a) ver.1, (b) ver.2, (c) ver.3, (d) ver.4, and (e) ver.5.
Dimensions for the final version of the logo tag.
Simulated and experimental results
The finalized logo tag antenna with a substrate attached (blue background) has been simulated in the CST Microwave Studio as illustrated in Fig. 8(a). Rexin, an artificial leather with a dielectric constant of 1.8, is utilized as the substrate [Reference Ahmed, Ahmed and Shaalan26]. This simulated tag antenna with substrate provides an impedance value of 27.38 + 211.5 j on the rexin sheet and an impedance value of 26.89 + 215 j in free space. In both cases, the proposed tag antenna is perfectly conjugate-matched with the RFID chip. A return loss graph of −45 dB at 0.9 GHz with a bandwidth of 110 MHz is shown in Fig. 8(b).
(a) Simulated proposed logo tag antenna with a substrate and (b) Reflection coefficient.
A prototype of the proposed logo antenna was fabricated by transferring the design onto Rexin, coating it with silver–copper conductive paint, and attaching the RFID chip using conductive adhesive, as shown in Fig. 9(a). Impedance was measured following the method in Qing et al. [Reference Qing, Goh and Chen27], with the chip removed and SMA connectors attached as shown in Fig. 9(b), and the setup illustrated in Fig. 10(a). The reflection coefficient was derived from the measured impedance, and the resulting reflection coefficient (S11) graph has been compared to the simulation in Fig. 10(b). The discrepancy in measured and simulated results is due to fabrication imperfection. The prototype was tested on various objects, i.e. bag, a notebook, and a T-shirt, with corresponding S11 plots shown in Fig. 10(c). Measured gains were 2.484 dB (notebook), 2.418 dB (bag), and 1.1 dB (T-shirt). The decrease in gain and reflection coefficient observed for the T-shirt is attributed to energy absorption by the human body. On each object, the tag provides a decent (good) read range. This makes the tag antenna an excellent option for IoT systems.
(a) Fabricated prototype of the proposed logo tag antenna and (b) SMA connectors attachment.
(a) Impedance measurement setup, (b) simulated and measured reflection coefficient plot, and (c) reflection coefficients for bag, notebook, and T-shirt.
As this is a wearable tag antenna, the proposed design has also been simulated with a human body model, maintaining a 3-m gap between the human model and the tag. The presence of the human body model causes an increase in the real part of the impedance of the tag antenna, as the human body absorbs electromagnetic fields. As a result, the reflection coefficient decreases to −17 dB, which is acceptable. The simulation is illustrated in Fig. 11(a), and the characteristics of the human body model are presented in Table 1. Impedance graphs and reflection coefficient are portrayed in Fig. 11(b, c), respectively.
(a) Simulation of proposed logo tag antenna with human body model, (b) Impedance graph, and (c) Reflection coefficient plot.
Human body model characteristics
Read range measurements
To validate the simulated results, a read range test of the fabricated prototype has been performed. The read range of the tag antenna can be calculated using the Friss equation formula presented in Eq. (1).
\begin{equation}R=\frac{\lambda}{4\pi}\sqrt{\frac{P_{TX}G_{TX}G_{RX}}{P_{RX}}}\end{equation} Where 
${P_{TX}}$ and 
${G_{TX}}$ are the power and gain of the reader antenna, and R is the maximum range, respectively. λ is the wavelength, 
${G_{RX}}$ and 
${P_{RX}}$ are the gain and power of a tag antenna, respectively. Read range measurements were conducted using a bag, notebook, and T-shirt, as shown in Fig. 12(a–c). The tag was gradually moved away from the reader in line of sight to observe the RSSI-based detection limit. Maximum read ranges of 3 m were achieved for the notebook and bag, with RSSI values of −61 dB and −59 dB, respectively. For the T-shirt (on-body), the range was 2 m with an RSSI of −62 dB. The proposed UHF RFID logo tag is compared with existing logo antennas and tags in Table 2. Unlike previously reported antennas [Reference Sreelakshmy, Kumar and Shanmuganantham19, Reference Tak and Choi20, Reference Monti, Corchia, De Benedetto and Tarricone28] operating at 2.45, 4.5, and 1.8 GHz, the proposed tag supports diverse identification scenarios at UHF frequencies. It achieves a 3 m off-body read range, outperforming the 1.5 m range of the logo antenna [Reference Choi, Kim, Lee and Chung29].
Measurement setup for the proposed tag on (a) notebook, (b) bag, and (c) T-shirt.
Comparison of the proposed logo tag antenna with existing literature
Finally, the proposed CMA-based design approach is inherently scalable and adaptable to a wide range of shapes and materials. This geometry-driven technique evaluates the natural resonant behavior of any arbitrary conductive structure, making it highly suitable for non-standard and visually distinctive shapes such as institutional logos, icons, or symbols. This enables designers to explore a variety of shapes while maintaining control over modal excitation and radiation performance. Regarding material adaptability, the methodology can be extended to various flexible and rigid substrates by incorporating the dielectric properties into the full-wave simulation environment used alongside CMA. Additionally, since CMA provides a clear understanding of how structural features influence current distribution and radiation, it allows for systematic adaptation when scaling the design for different frequency bands or adjusting to material constraints. This scalability and adaptability make the proposed approach highly versatile for diverse IoT and wearable antenna applications where both functional and aesthetic considerations are critical.
Conclusion
The logo of GCUF has been transformed into a novel UHF RFID tag antenna, and the proposed tag can work on the whole UHF RFID band (860–960 MHz). Simulations are conducted using CST Microwave Studio, and the CM analysis has been performed step-by-step throughout the design process. The Alien Higgs H3 is used as an RFID chip for impedance matching. A prototype has also been fabricated and tested to validate the simulated results. The read range measurements of the fabricated prototype are also performed using a UHF RFID portable reader. The read range measurements are carried out after placing the proposed tag design over different objects like notebooks, bags, and T-shirts to prove the robustness of the tag design. The results depicted a read range of 3 m, 3 m, and 2 m for bag, notebook, and T-shirt (wearing scenario), respectively. These results proved the versatility and usage of the proposed tag antenna for numerous IoT and next-generation applications. A logo RFID tag antenna is proposed and tested on various objects, and has demonstrated a good read range. This innovative tag can offer versatile applications, including library inventory management, efficient organization and categorization of documents, projects, or notes, and integration with students’ T-shirts for automated attendance recording.
Competing interests
The authors report no conflict of interest.
Maria Hanif received an MS degree in Electrical Engineering from the University of Engineering and Technology and a PhD in Electrical Engineering from Government College University, Faisalabad, Pakistan. She has more than 10 years of professional experience in teaching and research. She is currently working as a Research Officer in the Department of Electrical Engineering and Technology at Government College University, Faisalabad, Pakistan. Her research interests include communication systems and Antenna design.
Muhammad Farhan received a Ph.D. degree in Control Science and Engineering from the Beijing Institute of Technology, China, in January 2016. He has more than 10 years of professional experience in teaching and research. He is currently working as an Assistant Professor in the Department of Electrical Engineering and Technology at Government College University, Faisalabad, Pakistan. His research interests include communication systems, energy harvesting, control engineering, electric vehicle charging, and renewable energy.
Abubakar Sharif (Member, IEEE) received M.Sc. degree in electrical engineering from the University of Engineering and Technology, Lahore, Pakistan, and a Doctor of Engineering (Electronics engineering) from the University of Electronic Science and Technology of China (UESTC). He is working as an Assistant Professor at the Government College University, Faisalabad. He is also working as a Research Fellow with the University of Glasgow. He is the author of several peer-reviewed international journals and conference papers. His research interests include wearable and flexible sensors, antenna interaction with the human body, antenna, and system design for RFID, metasurfaces, passive wireless sensing, Machine learning, and the Internet of Things.
Dr. Muhammad Shahzad received his B.Sc. and M.Sc. degrees from the Islamia University of Bahawalpur, Pakistan, in 2011 and 2013, respectively. He received his Ph.D. degree from North China Electric Power University, Beijing, P.R. China. Currently, He is an Assistant Professor at Muhammad Nawaz Sharif University of Engineering and Technology, Multan, Pakistan. He has published more than 50 research articles, conference papers, and book chapters. His research interest includes Probabilistic and Statistical Analysis of Electric Power Systems, Renewable Energy, Energy Management, and Power System Operation and Control.
Nouman Q. Soomro received both the B. Eng. and M. Eng. degrees in Software Engineering and Information Technology from Mehran University, Pakistan, in 2008 and 2011, respectively, and the PhD degree in Computer Vision and Machine Learning from Beijing Institute of Technology (BIT), China, in 2015. Currently, He is Chair & Associate Professor at the Department of Software Engineering, Mehran University of Engineering & Technology, SZAB Campus, Khairpur, Pakistan. His major research interests include machine learning, Computer Vision, Medical Imaging, Remote sensing Image processing, pattern recognition, multimedia processing, and data science.
Kashif Nisar Paracha (Senior Member, IEEE) received a B.S. degree (Hons.) in electrical engineering (EE) from the University of Engineering and Technology (UET), Taxila, Pakistan, in 2004, the M.S. degree in electrical engineering from the King Fahd University of Petroleum and Minerals (KFUPM), Dahran, Saudi Arabia, in 2008, and the Ph.D. degree from the Wireless Communication Center (WCC), Electrical Engineering Department (FKE), Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia. He has been serving as an Associate Professor with the EET Department at Government College University Faisalabad (GCUF), since 2011. He has published more than 17 journal articles and technical proceedings on antenna design, stochastic algorithms, and renewable energy resources, in international journals and conferences. His research interests include signal processing and algorithms, communication systems, antenna design, metamaterials, and renewable energy resources.
Engr. Dr. Umer Ijaz is an Assistant Professor in the Department of Electrical Engineering at Government College University, Faisalabad, Pakistan. He holds a PhD in Electronics and Communications Engineering and an MS in Communication Engineering from Politecnico di Torino, Italy. He has extensive teaching and research experience and has published widely in impact factor journals, and has contributed significantly to areas such as antenna design, IoT, LiFi systems, and renewable energy.
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