April, 2012 Getting your bearings... - HF RF Surveillance VI HF RF surveillance monitoring is a complex and critical field and one that has grown significantly since its inception in 1902 when John Stone Stone patented the first direction finding system. It was not long afterwards that developments made it possible for HF Direction Finding (HF DF) to play a critical role in WW1, notably in the Battle of Jutland, and in WW2, where it was instrumental in locating German U-boats in the North Atlantic...
 At the monitoring station at Quoin Ridge in Hobart, Tasmania, a large array of V antennas, known as a Balcock Antenna, is in use for direction finding from 100KHz to 30 MHz. By switching in individual V sections, for example 2, 3 or 4 legs, highly accurate triangulation can be achieved in conjunction with other like equipped monitoring stations to plot signal direction. For professional HF DF base stations today, the choice is usually an Adcock type system or untuned Crossed Loops with Watson-Watt or a Correlation Interferometer. In non professional systems where sensitivity is not as critical and accuracy of bearing is not required, rotating antennas like directional beam antennas or loops may be acceptable. Directive ferrite rods, which use the magnetic component of the radio wave, may also be good enough for ground wave monitoring but these have considerably less sensitivity due to their very low effective height. Of course many of the antennas used for fixed site HF DF are too large to use in mobile applications. However, crossed loops are sometimes possible, especially on ships. Loops are also a possibility for close-in ground wave communications, as are ferrite rod antennas. Moonraker has developed a series of surveillance antennas that provide wideband performance for mobile, portable and semi-portable applications. BRX series antennas combine a number of different antenna types in the one antenna (monopole, dipole and discone) to provide the correct radiation characteristics throughout the bandwidth. These antennas generally cover from low frequency to SHF (150kHz to 1.5GHz) with active elements at HF for improved performance. With the active monopole and dipole elements of the extreme wide band ship and land based HF RXDPA system, it is possible to achieve simultaneous long range to NVIS communications from 0.01 to 30 MHz. By using multiple antennas mounted at different angles with a suitable switching network into a receiver, it is possible to determine signal bearing. The Moonraker BC Loop provides electrostatic screening to ensure minimum interference from nearby objects and a symmetrical pattern. With the advent of digital signal processing and super resolution DF, which provides a beam forming gain, greater accuracy in improving low and fading signal to noise ratios (SNRs) is possible today. The beam former manipulates the phase and relative amplitude at each transmitter to create a pattern of constructive and destructive interference in the wavefront, creating a phased array, so that the signals reach the receiver at the same time. A 12dB improvement in sensitivity can be achieved with an 8 element array. With the addition of coherent (constant relative gain) and noncoherent processing gain , it is possible to locate signals that are several dB below the noise level. The development of software-defined radio has facilitated these developments, reducing the need for extended hardware. Naturally, the role of the humble antenna is just as critical as it ever was… Directional Wideband Surveillance
- for transmission and reception
The antenna/mast can be erected by two men and may be either ground or roof mounted. The log periodic array has foldable elements (max length 1.5m) with a boom length of 2m. Maximum antenna length is 10.5m (34.5ft) in the fully vertical configuration. There are 3 sets of 4 RF transparent guys. Guy radius is 5m (20ft). - type BRX1500D
MF/HF Lightning Protection
- for MF/HF open wire antenna feeders
The unit is completely closed in an improved IP56 rated (weatherproof) cast aluminium housing, which ensures it is intrinsically safe from fire and explosion making it particularly useful on petroleum and gas vessels and the like. - type LPU
High Performance for Coastal Ships
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