HF: BACK TO THE FUTURE
By Adam Baddeley, Deputy Editor, BATTLESPACE
Historically, growth in High Frequency (HF) military usage can largely be put down to the development of ALE (Automatic Link Establishment) technology. ALE has eliminated the need for much of the lengthy experience previously necessary to ensure effective communication in the 1.6-30Mhz-frequency range, allowing trained signallers to communicate effectively. Once ease of use had been achieved, users are free to use this unique form of Beyond Line of Sight (BLOS) communication as a bearer system throughout the battlefield. Subsequent IP-based developments are enabling this once this largely stove-piped communications to be fully integrated into existing VHF-UHF Tactical Internets and applications.
HF has seen its greatest renaissance in the ground tactical environment, allowing it to become a stand-in or alternative to satcom and offering essentially free-to-use, long range communications in a manpack form factor. The advent of third-generation ALE, codified in STANAG 4538, takes HF into the IP world enabling it to support services such as Microsoft Outlook email.
HF’s inherent capabilities, which are also suited to the circumstances of urban warfare, are being increasingly called upon as militaries are drawn into operations in this environment. Application of HF’s NVIS (near-vertical-incidence skywave) transmission avoids the clutter of built up areas and the need for VHF/UHF repeater and rebroadcast stations which could be affected by enemy attacks and require personnel to be tied down in defence of these fixed positions. That this frequency has utility for the ‘urban jungle’ should come as no surprise. The British Army, for example, has always heavily used HF manpacks for jungle operations giving each platoon a company’s worth of HF equipment, coupled with wire antennae trailing behind patrols, to overcome problems of dense terrain.
The US has been the most important convert to HF on the battlefield. Since the 1960s and 1970s HF had started to fall out of favour as a mainstream solution. Its BLOS capabilities were increasingly poorly regarded when compared to those of satcom. However, it has been clear that far from being less significant than either satcom or LOS, HF can be utilised when these other technologies are unavailable or in insufficient quantities.
When it came to equipping the new Stryker Brigade Combat Teams (SBCTs) with HF, the Army found that its ageing AN/PRC-104/GRC-193 family of HF equipment, which dated from the 1970s, would be inadequate to the task. The Army then acquired an off-the shelf solution with the AN/PRC-150(C) from Harris RF Communications. Although this marked HF’s re-entry into the Army mainstream, the frequency had always continued to maintain a foothold in US procurement through niche roles. Most notable was the AN/PRC-138 and -150 already in service with US Special Operations Command (as the Improved SOF HF Manpack Radio System (ISHMRS)) and the Army Medical Department. Now designated the Transformation High-Frequency Radio System (THRS), when deployed to SBCTs, the AN/PRC-150(C) has been fielded in 20W and 150W vehicular and 400W base-station configurations. The antenna used is the OE-505, which at 3m in length is still short for an HF antenna, but overcomes some of these challenges with the use of highly a capable coupler, matching antenna length to impendence.
Although the AN/PRC-150 is best known for its operation in the 1.6M-30MHz frequency, the radio also operates up to 60Mhz, dipping a toe into the VHF frequency band. Although it is not strictly a multi-band radio, when compared to the AN/PRC-117, AN/PRC-150s still offer one, five or 10W frequency-modulated 16Kbps throughput for voice and data. The radio is also interoperable with the Single-Channel Ground and Airborne Radio System in the non-hopping, encrypted or plain-text digital-voice mode and is perfectly secure against an un