The Bangladesh Ministry of Defence (MoD) has recently awarded Barrett Communications a contract valued at AU$15 million for tactical radio communications equipment (for both High Frequency – HF - and Very High Frequency – VHF -, in base station, vehicle and MANPACK configurations) for the Bangladesh Army.
Over the past 15 years, the Bangladesh Army has maintained an annual procurement programme with Barrett Communications, including both HF and VHF radio communication equipment in various configurations, in-country and Perth-based train-the-trainer training for radio operators, system enhancements and ongoing spare parts. This contract award is part of the ongoing relationship between Barrett Communications, through their local partner Radiant, and the Bangladesh MoD.
“The long standing and successful relationship we have with the Bangladesh MoD must be credited to the strong relationship we have with SA Bahadur, Managing Director, and his team at Radiant,” Greg O’Neill, Managing Director of Barrett Communications, commented. “The Bangladesh MoD represents a significant part of our sales in the Asian region and we look forward to continuing this relationship well into the future.”
The Very High Frequency (VHF) frequency band was an early choice for MANPACK radios used by ground troops to communicate within a local 8km (5mi) area; defined as the frequency range 30-300MHz, meaning that the size of antennas and tuning components used in VHF radio are much smaller and lighter than those of High Frequency (HF) radios. This is a big advantage for MANPACK radios. The higher frequency and shorter wavelengths of VHF radios have a profound effect on radio range.
Unlike HF, VHF transmissions lack the ability to utilise the Ionosphere and are limited to line-of-sight (LOS) communication.
This reduces radio emission clutter throughout an extended battlefield and limits the vulnerability to unfriendly interception. The wider channel bandwidth capabilities of VHF radios increase the efficiency of coding and encryption schemes and allow greater data throughput than that of HF radios. Wider bandwidth and limited range make these radios ideal for squad-to-squad communications.
Radio Wave Propagation Ground waves consist of three components: Surface waves, ground-reflected waves, and direct waves.
Surface waves travel along the surface of the earth, reaching beyond the horizon. Eventually, the earth absorbs surface wave energy. The frequency and conductivity of the surface over which the waves travel largely determine the effective range of surface waves. Absorption increases with frequency.
Ground-reflected waves are the portion of the propagated wave that is reflected from the surface of the earth between the transmitter and receiver.
Direct waves travel in a straight line, becoming weaker as distance increases. They may be bent, or refracted, by the atmosphere, which extends their useful range slightly beyond the horizon.
Transmitting and receiving antennas must be able to “see” each other for communications to take place, so antenna height is critical in determining range. Because of this, direct waves are sometimes known as line-of-sight (LOS) waves. VHF waves can be reflected off of dense surfaces like rocks or conductive earth, just like a beam of light can be reflected off a wall or a ceiling. Sometimes several paths exist between a transmitting and receiving antenna.
The suggested limits on LOS range are sometimes exceeded in practice. One of the principal reasons for this is an effect called “ducting.”
Under fair, dry, and moderate weather conditions; the slight variations in air density have negligible effects on the path of radio waves passing through it. Wave Reflection Frequently there are abrupt changes in air density due to weather fronts passing over an area or the heavy moisture burden of rain clouds. In such cases VHF radio waves can bend or duct between air layers of different densities. Sometimes this ducting bends the radio waves downward so that the radio waves tend to follow the curvature of the earth. In such cases the LOS range is considerably greater than the optical LOS range.
This type of wave propagation is impossible to predict; it is not practical to plan on it for range improvement. However, when ducting conditions exit, they generally do so for hours at a time.