IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010 1119 Low-Profile Multifrequency HF Antenna Design for Coastal Radar Applications James Baker, Hyoung-Sun Youn, Member, IEEE, Nuri Celik, Member, IEEE, and Magdy F. Iskander, Fellow, IEEE Abstract—A novel design for an electrically small high-fre- quency (HF) antenna suitable for coastal radar applications is presented. The principle design objectives were to develop an HF antenna resonant at multiple frequencies that is also compact and easily transportable for deployment to coastal sites and on floating platforms. The compact antenna achieves practical performance values for radiation resistance, bandwidth, and gain while maintaining small values of . The design presented in this letter consists of a meandering line antenna composed of helical elements and switchable folded arms. The value of ranges down to 0.16 at 5.7 MHz. The antenna is self-resonant at multiple frequencies including 5.7, 16.1, 20.6, and 28.1 MHz for open-circuit mode and 15.1, 18.5, and 26.1 for the short-circuit mode. In all cases, input impedances were easily matched to 50- coaxial feed lines, and the achieved bandwidths ranged from 1% to 12% within the HF band (3–30 MHz). The antenna is 90 cm high with a small ground disk of 60 cm diameter. Simulation results and prototype experimental measurements are presented. Index Terms—Coastal radar, electrically small antenna, ESA, helical, high-frequency (HF) antenna, meandering line, mean- dering line antenna (MLA). I. INTRODUCTION L IMITATIONS of current coastal HF radar systems include the large physical dimensions required for op- erations at the longer wavelengths of the high-frequency (HF) band (3–30 MHz). Coastal radar applications require vertical polarization for long-range surface-wave propagation over the ocean, so antenna elements tend to be vertical with heights ranging from 5 to 25 m. Many current systems use quarter-wave monopole antenna that are omnidirectional, requiring extensive arrays to accomplish the beamforming required to minimize clutter and backscatter from the surrounding terrain. Coastal HF radar system performance is also affected by ionospheric conditions that are constantly changing and impact the useable frequencies available. A typical antenna network involves ar- rays of quarter-wave monopole structures that, in the frequency range, correspond to antenna lengths of up to 25 m, with even larger ground radial networks. As a result, current HF surface wave radar (HFSWR) and over the horizon radar (OTHR) antenna systems tend to be located at fixed sites with extensive Manuscript received September 18, 2010; revised November 23, 2010; ac- cepted November 28, 2010. Date of publication December 03, 2010; date of current version December 13, 2010. This work was supported in part by the U.S. Department of Homeland Security under Grant 2008-ST-061-ML0002 to the School of Ocean and Earth Science and Technology, University of Hawaii. The authors are with the Hawaii Center for Advanced Communications, Col- lege of Engineering, University of Hawaii, Honolulu, HI 96822 USA (e-mail: jmbaker@hawaii.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2010.2096552 infrastructure and site preparation requirements. These radar arrays can extend for several kilometers with significant envi- ronmental impact. In modern coastal radar systems, though, it is desirable for the system to be readily transportable with short setup times for rapid deployment to remote and unprepared sites. For homeland security applications, it is also desirable that the system be deployable on floating platforms such as buoys, barges, ships, and other types of seaworthy platforms. Current coastal radar antennas are not suitable for these types of floating platforms due to their large size and infrastructure requirements. In addition to these requirements, it is also de- sirable to make the antenna structures less recognizable so that the position of the radar site can be disguised. For these mobile radar systems, particularly those suitable for surveillance and homeland security applications, the antenna size is considered one of the highest design priorities. Over the years there have been many contributors to the theory and design of antenna that are electrically small. Wheeler and Chu pioneered the field by developing funda- mental limitations in how small an antenna can be designed while maintaining practical performance [1]–[3]. The generally accepted criteria for what makes an antenna “electrically small” is based on the overall volume occupied by the antenna. Using the free-space wave number and the radius of the sphere enclosing the antenna, an antenna is considered to be electrically small if the value of is less than or equal to 0.5 [4]. For the coastal radar application, the primary char- acteristics of interest (and limiting factors) in antenna design are resonance, polarization, impedance, bandwidth, and phase stability. For the homeland security application, it is also desir- able that the antenna not be immediately obvious to the casual observer, yet it is usually not practical to disguise a vertical antenna that is 5–10 m tall. It remains important though that the low-profile HF antenna retain acceptable performance. It is also desirable for the low-profile antenna to be either broadband or at least have multiple resonances within the HF band to allow for system operation at different frequencies as atmospheric and other propagation conditions change. In this letter, a new HF antenna design is presented, one that is suitable for coastal radar applications. The new design is low- profile (less than 1 m high), relatively compact, and provides for effective performance within the HF band. This new design combines a helical antenna structure with a meandering line an- tenna (MLA) and has been shown to provide improved antenna radiation resistance and strong vertical polarization while main- taining an omnidirectional radiation pattern with low takeoff angle. The helical MLA also provides for multiple self-reso- nance frequencies ranging from 5.7 up to 28 MHz, allowing for selectable channels, even without the use of external tuning 1536-1225/$26.00 © 2010 IEEE