RESEARCH ARTICLE Simple design of broadband circularly polarized two-arm Archimedean spiral antenna Huy Hung Tran 1,2 | Nghia Nguyen-Trong 3 1 Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam 2 Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam 3 School of ITEE, University of Queensland, St Lucia, Queensland, Australia Correspondence Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam. Email: tranhuyhung@tdtu.edu.vn Abstract This article presents a broadband circularly polarized two-arm Archimedean spiral antenna with a novel simple feeding method. Contrary to the conventional spiral antenna excited by a vertically or horizontally balun, the presented design is directly fed by a coxial cable with a planar feeding section optimized to provide a broadband input impedance matching. An antenna prototype has been fabricated and measured to validate the concept. Measured results of reflection coefficient and axial ratio show a good performance over a frequency range from 1.0 to 3.5 GHz, equivalent to ~111.1%. Within this band, the antenna prototype exhibits bidirectional radiation with broadside gain values of better than 3.6 dBic. KEYWORDS Archimedean spiral antenna, circularly polarized, UWB 1 | INTRODUCTION Archimedean spiral antennas, which are characterized by constant input impedance and circularly polarized (CP) radia- tion patterns over a wide range of frequencies, have emerged as leading candidates for various applications requiring broad- band operation. 1 Due to its balanced structure, feeding the antenna from a coaxial line would require a broadband balun, which is one of the main challenges in spiral antenna design. The conventional method is to use vertical balun situated in the center of the spiral. 2-5 This kind of feeding method leads to the high- profile antenna and it also causes difficulties in antenna assem- bly. Many efforts have been made to feed the spiral antennas in the planar plane, including Dyson-style balun, 6-10 coplanar waveguide feed, 11-13 and parallel-plane perpendicular current feed. 14 Although good performance can be achieved, these designs typically require more space for feeding circuit and the antenna's footprint is therefore bigger in comparsion with the center-fed spiral antenna. Furthermore, using coplanar wave- guide feed is only suitable for slot spiral structure 11,12 or 3-arm spiral 13 and multi-layer is another disadvantage of the approach in Reference 14. Recently, a technique of using tapered transmis- sion line transformers integrated in the center of the spiral antenna is proposed. 15 As a consequence, the antenna can be simply excited by directly connecting to the coaxial cable. How- ever, this method requires multiple feeding ports. In this article, a simple, yet effective technique to realize a broadband two-arm Archimedean spiral antenna is pres- ented. This design is directly fed by a coaxial cable through a transition section, which is integrated into the spiral arms at the center. This transition section is optimized to work as a broadband impedance matching network between a 50-Ω coaxial line and the spiral antenna. It is verified through simu- lation that this feeding also significantly reduces the leakage current. Thus a balun is not required which makes the antenna very simple and low-cost to implement. The measured results indicate that the fabricated antenna can operate with an opera- tion bandwidth (BW) of 111% (1.0-3.5 GHz) and broadside realized gain values of greater than 3.6 dBi. 2 | ANTENNA DESIGN AND PROPOSED TECHNIQUE Figure 1 shows the top and cross-section views of the pres- ented spiral antenna. The antenna is etched on two different Received: 11 February 2019 Revised: 1 April 2019 Accepted: 16 April 2019 DOI: 10.1002/mmce.21813 Int J RF Microw Comput Aided Eng. 2019;e21813. wileyonlinelibrary.com/journal/mmce © 2019 Wiley Periodicals, Inc. 1 of 6 https://doi.org/10.1002/mmce.21813