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, medium and long distances – without the need for expensive re-transmission devices, such as the VHF or UHF repeaters and 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 communications possible.

HF 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.

Radio Wave Propagation Illustrated

The following illustrations show the characteristics of Ground Wave, NVIS and Sky wave propagation during day and night time operation.

Daylight Hours:

During sunlight hours the ionosphere separates into three layers. The lowest layer (D-layer) does not refract radio waves but unfortunately attenuates the wave as it travels through in both directions. The lower the frequency the greater the attenuation. This encourages HF operators to use the highest frequency possible to make reliable contact with other stations.

The ‘E-layer’ refracts radio waves and is used for daylight communications up to approximately 500 kilometres. Frequency selection is important as if the frequency is too high it will pass through the E-Layer and be refracted by the next higher layer and will not return to earth within the 500 kilometre desired working distance.

The ‘F-layer’ separates during daylight hours into the indistinct F1 and F2 layers. These layers also refract radio waves and are used for ‘long-haul’ daytime communications over 500 kilometres.

Nighttime Hours:

Once the sun has set the D-layer decays quite rapidly. The D-layer attenuates radio waves during daylight and once it begins to decay, HF signals increase dramatically in strength and concentration.

The E layer decays more slowly and is often usable well after dusk for effective communications.

The daytime F1 and F2 layers combine very early after sunset and separate soon after sunrise and are called the F-layer. This layer settles at approximately the height of the daytime F2-layer. For most late nighttime operation only the F-layer is usable.

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.