High Frequency (HF) Radio Communication PDF Print E-mail

HF Radio has come a long way and has developed rapidly in the last decade and “automate” those difficulties that the HF operators of the past were faced with.

Technology such as Automatic Link Establishment (ALE) and Selective Call (Selcall) has drastically increased the reliability of HF Radio Communications.  

Not to mention the advanced technology in Transmitters, Receivers, Peripheral Equipment and actual Signal Processing technology.

We understand the art of HF Radio and offer the best in specialized HF solutions that money can buy.

About HF Radio

HF (High Frequency) is that portion of the radio spectrum between 3 and 30 MHz. Within this radio spectrum an efficient form of transmitter modulation, SSB (Single Side Band) is utilised. SSB is very energy efficient, only transmitting when the operator actually speaks. No signal is transmitted between syllables or when the operator stops speaking.
This combined with the use of the ionosphere – a layer of ionised gases that resides between 60 and 500 Kms above the earth’s surface, provides efficient, cost effective communications over short (<50Km) and medium (300Km) to long (3,000Km+) distances – without the need for expensive re-transmission devices, such as the VHF or UHF repeaters and satellites, all of which have ongoing operational costs and a reliance on a physical infrastructure.
In many remote areas, HF/SSB is the only form of radio communications possible. (Not referring to Satellite Communications)

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HF Propagation

HF Propogation

Simplified illustration of HF Radio wave propagation

When HF/SSB radio waves are generated by the radio transceiver there are usually two components:

(i) The Ground or Surface Wave, which travels directly from the transmitting aerial to the receiving aerial following the contours of the land or surface of the sea.

(ii) The Sky Wave, which travels upward and at an angle from the aerial, until it reaches the ionosphere and is refracted back down to earth at a distance from the transmitting aerial dependant on the angle it is refracted by the ionosphere.

Generally speaking, ground wave is used to communicate over short distances usually less than 50 Kms. Because ground-wave follows the contours of the earth, it is effected by the type of terrain it passes over. Ground wave is rapidly reduced in level when it passes over heavily forested areas or mountainous terrain.

Sky wave is used to communicate reliably over medium to long distances from approximately 150Kms up to 3000Kms. Whilst the nature of Sky wave propagation means it is not effected by the type of terrain, as in ground waves, it is effected by various factors involving the ionosphere.
Near Vertical Incident Sky-wave (NVIS) propagation techniques are used to communicate using the ionosphere for short distances, up to 150 Kms from the transmitting aerial. This distance was, in part, traditionally covered by the Ground Wave up to 50 Kms but a ‘dead-zone’ was often experienced where Ground Wave faded out and Sky Wave began. NVIS requires some specialisation in aerial erection and selection of frequency to ensure a high angle of radiation is achieved to return the radio wave to the earth with in the first 150 Kms.

The ability of the ionosphere to refract HF radio waves is caused by ionisation of particles in the upper atmosphere by ultra-violet, cosmic rays and X-ray radiation from the sun. This effect naturally varies greatly depending on time of day, seasonal conditions and the eleven-year cyclical variation of sunspot activity.

The challenge for the HF radio user is to work with these variances to select the optimum frequency for best results.

Frequency Selection
Frequency selection is perhaps the most important factor that will determine the successfulness of HF/SSB communications. Generally speaking - the greater the distance over which to communicate - the higher the frequency selection. This basic rule however, needs to take into account the time of day. Rule two basically states - the higher the sun - the higher the frequency to be used.
Based on this rule, a lower frequency should be use during early morning, late afternoon and early evening to communicate over the same distance as the frequency selected at mid day. Even lower frequencies should be selected for late night operation.
Weather Conditions
Certain weather conditions will also effect HF/SSB communications. Stormy conditions will increase the background noise level as a result of ‘static’ caused by lightening and ‘rain static’ in the upper atmosphere. This increase in noise level can rise to such a level that it will block-out weaker legitimate signals.
Man-Made Electrical Noise
Interference of an electrical nature can be caused by overhanging power lines, high power generators, air conditioners, thermostats, refrigerators, television sets and vehicle engines when in close proximity to the aerial.
By far the greatest bane for the HF user is the proliferation of personal computers throughout the country, PCs, generate large amounts of electromagnetic radiation across the HF spectrum and within close proximity can completely obliterate even the strongest of HF signals.

Why use High Frequency (HF) Radio Communications?

Government and private organisations are continually searching for the most flexible, reliable and cost effective solutions for their remote, emergency and security communications needs. In today’s world of instant voice, email, messaging, fax and GPS navigation, what has High Frequency (HF) radio to offer? Surprisingly perhaps, HF radio provides all of these options and has several important advantages over other communications media. HF radio will never replace fixed and mobile telephony as the first communications option for the general public, but for organisations involved in emergency, remote and military communications it is a vital and irreplaceable wireless communications tool.

Long Range Communications Capability
Whilst VHF and UHF radio is also commonly used for short-range line-of sight (LOS) communications, only HF is capable of communicating over distances of 3000 km or more.
Minimal Infrastructure Requirements
Unlike conventional, Voice Over IP (VoIP), cellular and satellite telephony, which all rely upon land-based infrastructure, an HF radio network requires minimal infrastructure. As such it is often the only reliable means of communication when disaster strikes.
Full Mobility
HF radio is simple and quick to deploy and provides communications capability for users no matter where they are. Fixed base stations can be used to communicate with other bases or to provide command and control for mobile (vehicle-mounted) and portable (manpack) users in the field.
Interoperability
HF radio can be used to communicate with existing VHF and UHF radio systems, cellular telephones and land-based telephones through developments in cross-patching technology that make this as easy as dialling a telephone number.
Low Cost of Ownership
Compared with satellite telephony, the most common alternative technology for communications of last resort, HF radio is the economical choice. Once the initial investment in equipment is made, there are no call costs or ongoing monthly line or equipment rentals. Also, HF radio equipment is built tough to withstand the extreme conditions, which proves to be very cost-effective.
Command and Control
The nature of emergency planning requires that simultaneous communications be made to and amongst a number of operators in a command and control style network. This facilitates situational awareness amongst the users and external organisations that can also be included in the network. HF radio provides this capability in all base station, mobile (vehicle-mounted) and portable manpack configurations of the radio network.
Security of Communications
Where communications are sensitive, HF radio offers technologies to ensure the security of voice and data transmissions. The military uses HF radio as its primary communications medium and from the military have come a number of enhancements in voice encryption and frequency hopping which guarantee secure communications. For non-military use, different levels of encryption are available to suit the individual organisation’s security requirements.