HF Communication Systems

Overview of HF operation
HF (High Frequency) is the radio spectrum with frequencies between 1.6 and 30MHz. Within this radio spectrum an efficient form of transmitter modulation, SSB (Single Side Band), is used. This, combined with the use of the ionosphere - a layer of ionisation gases that resides between 100 and 700km above the earth's surface, provides efficient, cost effective communications over short, medium and long distances - without the need for expensive re-transmission devices, such as the VHF or UHF repeaters or satellites, all of which have on going operational costs and a reliance on a physical infrastructure.

In many remote areas, HF/SSB is the only form of communication possible.

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

- The ground-wave, which travels directly from the transmitting antenna to the receiving antenna following the contours of the earth.

- The sky-wave, which travels upward and at an angle from the antenna, until it reaches the ionosphere (an ionised layer high above the earth's surface) and is refracted back down to earth to the receiving antenna.

Generally speaking, ground-wave is used to communicate over shorter distances usually less than 50km. Because ground-wave follows the contours of the earth, it is affected 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 up to 3,000km. Whilst the nature of sky-wave propagation means it is not affected by the type of terrain as in ground-waves it is affected by factors involving the ionosphere as described below.

Radio wave propagation illustrated
The following illustrations show the characteristics of ground-wave and sky-wave propagation during day and night time. In each illustration the height of the ionosphere above the ground is shown.

In both illustrations Station A communicates with Stations B, C and D. Propagation from Station A to B is by ground-wave. The diagrams illustrate that the ground wave is not affected by the time of day and the height of the ionosphere above the ground.

Propagation from Station A to C and D, however, is by sky-wave and as the diagrams illustrate the sky wave is significantly affected by the time of day and the height of the ionosphere above the ground.

Under each diagram there are recommended working frequencies listed. Please note that these will vary according to time of year and other factors. They are intended only as a guide and are subject to change.

Day

The sun is higher, the best frequency to use is higher

A to B - Possible optimum working frequency is 3 MHz

A to C - Possible optimum working frequency is between 7 - 9 MHz

A to D - Possible optimum working frequency is between 13-16 MHz

Night

The sun is lower, best frequency to use is lower

A to B - Possible optimum working frequency is 3 MHz

A to C - Possible optimum working frequency is between 5 - 7 MHz

A to D - Possible optimum working frequency is between 9 -12 MHz

Factors which affect HF/SSB communications
There are a number of different factors which will affect the success of your communications via HF/SSB radio. These are outlined below:-

Frequency selection
Frequency selection is perhaps the most important factor that will determine the success of your HF/SSB communications.

Generally speaking the greater the distance over which you want to communicate, the higher the frequency you should use.

Beacon Call, a Selcall (selective call) function built into the Barrett 950 transceiver, makes finding the correct frequency to use easy. Beacon Call is based on the network transceivers all having a selection of frequencies that will accommodate most ionospheric conditions. When in standby the network transceivers scan these frequencies waiting for a call (Selcall or Beacon Call) from another transceiver. The transceiver wishing to check for the best frequency to operate on sends a Beacon Call to the station he wishes to contact. If his call to the other station is successful he will hear a revertive call from the station he is calling, indicating the channel he selected was suitable for the ionospheric conditions prevailing. If he does not hear this revertive call or it is very weak, he tries on another channel until a revertive call of a satisfactory signal strength is heard.

(Refer to Selcall (selective call) section of this manual for full details on Beacon Call operation).

Time of day
As a rule, the higher the sun, the higher the frequency that should be used. This means that you will generally use a low frequency to communicate early morning, late afternoon and evening, but you will use a higher frequency to cover the same distance during times when the sun is high in the sky (e.g. midday). You will need to observe the above rule carefully if your transceiver has a limited number of frequencies programmed into it, as you may only be able to communicate effectively at certain times of the day.

Weather conditions
Certain weather conditions will also affect HF/SSB communications. Stormy conditions will increase the background noise as a result of ‘static' caused by lightning. This background noise could rise to a level that will blank out the signals you are trying to receive.

Man-made electrical interference
Interference of an electrical nature can be caused by overhanging power lines, high power generators, air-conditioners, thermostats, refrigerators and vehicle engines, when in close proximity to your antenna. The result of such interference may cause a continuous or intermittent increase in the level of background noise.