A Curved Monopole Antenna for HF Radar with Enhanced Gain and Bandwidth

This paper presents the design and simulation of a new curved monopole antenna optimized for skywave HF radar applications, with a systematic investigation of the effects of curvature and fixed-section length on antenna performance. The proposed design achieves improved impedance matching, broader bandwidth, and enhanced realized gain compared to a conventional quarter-wavelength monopole at 15 MHz. Parametric analysis shows that fully bending the monopole degrades performance, whereas introducing a straight section and carefully optimizing the curvature enables a 18.5% gain increase and a 400 kHz bandwidth expansion. The single-element design is further extended to a 12-element linear array with 0.45λ spacing (where λ is the wavelength), demonstrating stable embedded-element behavior and improved low-to- moderate elevation gain for skywave over-the-horizon radar operation. At θ = 30°, the proposed array achieves 14.04 dBi compared to 13.11 dBi for the reference array, corresponding to 24% gain enhancement, which is significant in high-power HF radar systems. These results confirm that the proposed curved monopole antenna provides a compact, broadband, and scalable solution for next-generation HF radar arrays.

💡 Research Summary

The paper addresses a longstanding challenge in high‑frequency (HF) over‑the‑horizon (OTH) radar: the large physical size of traditional quarter‑wave monopoles required for efficient sky‑wave propagation. By introducing a novel curved monopole geometry, the authors demonstrate that it is possible to reduce antenna height while simultaneously improving impedance matching, bandwidth, and realized gain.

The design methodology separates the antenna into a lower straight section of length Ls and an upper curved section described by a curvature κ = 1/R, where R is the radius of the tubular arc. The total electrical length is kept constant and equal to that of a conventional λ/4 monopole (≈467 cm at 15 MHz). This constraint ensures that any performance variation originates solely from geometric curvature rather than changes in electrical length.

A parametric sweep first varies κ while fixing Ls = 200 cm. The results show that increasing curvature up to κ ≈ 0.5 (R ≈ 200 cm) improves return loss and widens the usable bandwidth, but further curvature degrades performance due to excessive reactive effects. With κ fixed at its optimum, Ls is varied; the best trade‑off is found at Ls ≈ 200 cm, where the antenna exhibits the lowest return loss and the widest bandwidth. The optimal configuration therefore consists of a 200 cm straight segment and a curved arc of length 267 cm subtending 1.33 rad, yielding a curvature κ = 0.5 m⁻¹.

Compared with a reference straight monopole, the optimized curved monopole achieves a realized gain of 3.95 dBi versus 3.21 dBi (an 18.5 % increase) and extends the −10 dB return‑loss bandwidth by roughly 400 kHz around the 15 MHz center frequency. The improvement is attributed to a more favorable current distribution along the curved path, which produces a slightly more directive radiation pattern without increasing the overall height. Simulations were performed over a finite perfectly conducting ground plane (15 m × 15 m), reflecting realistic deployment conditions; consequently, the reported gains correspond to a half‑space with finite ground rather than an ideal infinite ground plane.

To assess scalability, the single‑element design is replicated in a 12‑element linear array with uniform spacing of 0.45 λ (≈9 m). This spacing balances mutual coupling suppression and grating‑lobe avoidance across the HF band. The array analysis focuses on low‑to‑moderate elevation angles (θ = 0°–45°), which are critical for sky‑wave OTH operation. At θ = 30°, the curved‑monopole array delivers a realized gain of 14.04 dBi, compared with 13.11 dBi for an equivalent array of straight monopoles—a 0.93 dB (≈24 %) increase in radiated power density. The array also exhibits reduced back‑lobe levels, indicating improved front‑to‑back radiation characteristics. Importantly, the embedded element impedance remains stable across the array, confirming that the single‑element benefits translate effectively to the array level.

The authors conclude that the curved monopole offers a compact, broadband, and higher‑gain alternative to traditional HF monopoles, making it well suited for modern OTH radar systems that demand long‑range coverage, high transmitted power, and flexible deployment (including mobile or space‑constrained platforms). The simplicity of the geometry—essentially a tubular arc attached to a straight rod—facilitates manufacturing and array scaling. Future work is suggested to include prototype fabrication, field measurements on real ground planes, and exploration of multi‑band extensions (e.g., VHF/UHF) to broaden the applicability of the concept.