86 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 17, NO. 1, JANUARY 2018 Ultralow-Profile and Flush-Mounted Monopolar Antennas Integrated Into a Metallic Cavity Nghia Nguyen-Trong, Member, IEEE, Sree Pramod Pinapati, Student Member, IEEE, David Hall, Andrew Piotrowski, Senior Member, IEEE, and Christophe Fumeaux, Senior Member, IEEE Abstract—This letter presents an evolution of a low-profile monopolar antenna specifically designed for the integration onto large vehicles or house roofs. The use of a shallow cavity is investi- gated for a reduction of the antenna height seen from the ground plane. The tradeoff between the cavity size and effective height of the antenna is studied, where it is demonstrated that the sunken geometry is well suited for applications requiring omnidirectional patterns. Two antenna prototypes were developed: a low-profile one and a flush-mounted device both enclosed in a thin plastic radome. The experimental validation shows satisfactory antenna performance over a larger than 3:1 bandwidth with a monopolar pattern. Index Terms—Cavity antennas, low-profile antennas, mono- cones, monopoles, omnidirectional antennas, vehicular antennas. I. INTRODUCTION L OW-PROFILE monopolar antennas are a well-known class of radiators and have an interesting history. These antennas can be designed simply as a monocone or a metal patch with shorting wires as demonstrated early by Seeley [1] in the 1950s and Burberry [2] in the 1990s. Recently, with the aid of numerical simulation tools and newly developed tech- niques, significant efforts have been spent to miniaturize these antennas while maintaining broad impedance bandwidths and monopolar patterns [3]–[12]. An antenna with a size close to the fundamental miniaturization limit has been shown in [13] with capacitively coupled feeds with parasitic elements. A dual-band reconfigurable antenna has been proposed in [14] for height reduction while providing significant coverage for two indepen- dently tunable frequencies. This letter examines the wideband low-profile monopolar an- tennas from a perspective of its integration into a platform with large metallic surfaces such as vehicles or house roofs capable of encasing them in a cavity. In fact, flush-mounted antennas with directive broadside patterns are commonly implemented, Manuscript received September 28, 2017; revised November 5, 2017; ac- cepted November 18, 2017. Date of publication November 20, 2017; date of current version January 10, 2018. This work was supported by the Defence Science and Technology Group Cyber and Electronic Warfare Division. (Cor- responding author: Nghia Nguyen-Trong.) N. Nguyen-Trong is with the School of Information Technology and Electrical Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia (e-mail: n.nguyentrong@uq.edu.au). S. P. Pinapati, D. Hall, and C. Fumeaux are with the School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, SA 5005, Aus- tralia (e-mail: sreepramodpinapati@gmail.com; david.hall@hecdesign.com; christophe.fumeaux@adelaide.edu.au). A. Piotrowski is with the Defence Science and Technology Group, Edinburgh, SA 5111, Australia (e-mail: Andrew.Piotrowski@dsto.defence.gov.au). Digital Object Identifier 10.1109/LAWP.2017.2776290 Fig. 1. (a) Design of low-profile monopolar antenna with top view (top) and side view (bottom). (b) Integration of the antenna into a shallow cavity (top) with PETG cover (bottom). The cavity is circular. but monopolar antennas sunken into a shallow cavity to fur- ther lower their profile are often desirable for omnidirectional applications. This type of structure is particularly attractive for vehicular applications. A few antennas have been recently re- ported, such as with carbon fiber cavities in [15] and [16] or as a resonant hole antenna in [17] and [18]. This letter shows a systematic tradeoff between the cavity size and the monopolar antenna height above the ground plane. It is also noted that an emphasis of the design is placed on reducing its complexity, and thus its production cost, while achieving ultralow profile with a performance that is still comparable to those of existing designs found in the literature. The proposed geometry and technique provide designers with a convenient selection guideline of the antenna structure geometry depending on the specification. II. ANTENNA DESIGN A. Low-Profile Monopolar Antenna The antenna structure follows the classical approach, i.e., a metallic cone loaded with shorting pins or wires. This approach keeps the antenna simple, easy to manufacture, and relatively in- sensitive to parameter variations. To reduce the antenna weight, the cone can be made hollow as shown in Fig. 1(a). This does not change the antenna performance since the currents excited by the feed for radiation are mostly confined at the lower (outside) surface of the cone. For miniaturization, the design is modified by adding a metal patch patterned on a low-cost FR-4 substrate attached at the top of the cone. A capacitive gap of width g is used to optimize the impedance matching [6]. Three shorting wires are placed evenly around the antenna perimeter. These wires provide a shunt inductive compensation to enhance the 1536-1225 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.