Abstract
In this paper we document the design, development, results, performance and field applications of a compact directive transmit antenna for the long-range High Frequency ocean RADAR (HFR) systems operating in the International Telecommunication Union (ITU) designated 4MHz and 5MHz radiodetermination bands. The antenna design is based on the combination of the concepts of an electrically small loop with that of travelling wave antenna. This has the effect of inducing a radiated wave predominantly in a direction opposed to that of energy flow on the antenna structures. We demonstrate here that travelling wave design allows for a more compact antenna than other directive options, it has straightforward feed-point matching arrangements, and a flat frequency and phase response over an entire radiodetermination band. In situ measurements of the antenna radiation pattern, obtained with the aid of a drone, correlate well with those obtained from simulations, and show between 8dB and 30dB front-to-back suppression, with a 3dB beam width in the forward lobe of 100∘ or more. The broad-beam radiation pattern ensures proper illumination over the ocean and the significant front-to-back suppression guarantees reduced interference to terrestrial services. The proposed antenna design is compact and straight forward and can be easily deployed by minimal modifications of an existing transmission antenna. The design may be readily adapted to different environments due to the relative insensitivity of its radiation pattern and frequency response to geometric detail. The only downside to these antennas is their relatively low radiation efficiency which, however, may easily be compensated for by the available power output of a typical HFR transmitter. Antennas based on this design are currently deployed at the SeaSonde HFR sites in New South Wales, Australia, with operational ranges up to 200 km offshore despite their low radiating efficiency and the extremely low output power in use at these installations. Due to their directional pattern, it is also planned to test these antennas in phased-array Wellen RADAR (WERA) systems in both the standard receive arrays: where in-band radio frequency noise of terrestrial origin is impacting on data quality, and in the transmit array: to possibly simplify splitting, phasing and tuning requirements.
Highlights
High Frequency RADAR (HFR) measurements of surface currents in the coastal ocean have become a standard and cost-effective component for ocean observing systems globally [1,2,3]
It is worth noting that an over-scaled design such as the Prototype ×4 caused a loss of directivity in the simulated pattern
Simulations with the NEC-2 [58,59,60,61] software demonstrate that the travelling wave loop antenna (TWLA) design combines the desired characteristics of broad beam directivity, with predominantly vertical polarization and the required level of compactness
Summary
High Frequency RADAR (HFR) measurements of surface currents in the coastal ocean have become a standard and cost-effective component for ocean observing systems globally [1,2,3]. Developed more than four decades ago [4], oceanographic HFR provides highly accurate synoptic observation of large scale coastal circulation features at high temporal and spatial resolutions not readily obtained using conventional instrumentation [5,6]. HFR systems rely on the backscatter of vertically polarized electromagnetic energy in the 3–50 MHz frequency range to observe the sea surface state. An individual HFR system operated in a conventional monostatic configuration maps the depth-integrated component of an ocean current advancing towards or receding from its associated receiver [4]. Two or more HFRs overlooking the same patch of ocean from different locations are required to resolve the two-dimensional current field in the area of common overlap
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