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Ground Wave - Sky Wave - ITU Recommendations

LF and MF PROPAGATION - ground wave, sky wave, fading zone, sea gain, day time, night time, Millington's method, polarisation coupling loss and anti-fading radiators

Unlike propagation at UHF and higher frequencies which is broadly based on Line of Sight and Curved Earth, propagation at LF, MF and even HF is often regarded as a black art.  Indeed, empirical data based on many measurements over different topography for a wide variety of conditions is the preferred methodology.   The International Telecommunications Union (ITU) produce many documents, recommendations and guidance relating to propagation prediction at these frequencies.

 
Ground Wave

The day time service area of an LF or MF station is determined by the characteristics of the ground wave mode of propagation.    This is affected by the carrier wave frequency and the effective conductivity of the ground.   FanField carries out computer predictions to ITU-R Recommendation P.368-7.   An example of a coverage area prediction is shown to the right.

Ground conductivity in obtained from direct measurements or an assessment of the topography, climate and geology of the area.  Millington’s mixed path method is used to assess multi-section paths in cases where the ground conductivity varies significantly over the coverage area.

 

Sky Wave

 

As well as the ground wave, which propagates at all times, at night a sky wave is also propagated via the ionosphere.  This is not useable during the day because, at frequencies below HF, there is excessive absorption by the D-layer of the ionosphere which is present during daylight hours.

The sky wave mode is capable of covering a much larger area than the ground wave and can be used for international coverage.   FanField can perform sky wave coverage computer predictions to ITU-R Recommendation P.1147-1, as shown in the example left.

The path loss for ionospheric sky wave propagation is generally independent of bearing and thus the coverage plots would normally reflect the shape of the antenna's horizontal radiation pattern.   However, in some cases, two factors influencing the sky wave propagation introduce a dependence on azimuth bearing into the path loss equation.    These are known as the excess polarisation coupling loss and the sea gain.   

Sea Gain

The sky wave originating from sites located within a short distance of the coast can be enhanced by an effect known as Sea Gain.  

The standard sky wave path loss equation takes into account the reduction in efficiency due to the ground losses in the immediate vicinity of the transmitting antenna.   However, when the antenna is located very close to the sea, an improvement in efficiency is obtained for those azimuth bearings where the distance to the sea is less than a few km, although the effect is only noticeable at ranges in excess of 1000km from the antenna.   

Sea Gain is dependent on the overall path length and the distance between the site and the coast (measured along the path)  and can be up to 10dB.   This effect is fully taken into account in FanField’s predictions.   

It should be noted that, whilst a coastal location can be useful for launching a ground wave service to an overseas country because of the reduced propagation loss over the high conductivity salt water, Sea Gain affects only sky wave propagation.

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Polarisation Coupling Loss

The polarisation coupling loss is a term included in the sky wave path attenuation equation to take account of the efficiency with which the vertical polarisation radiated by the antenna couples with the ionosphere.   This coupling mechanism is normally fairly efficient and thus the small direction independent term included within the main equation is usually sufficient to take account of this effect.    However, in a band of latitudes close to the magnetic equator, the polarisation coupling loss for azimuth bearings close to the east west direction increases significantly.   To take account of this, an additional term, known as the excess polarisation coupling loss, is included in the path loss equation.   

The excess polarisation coupling loss is ignored by many available computer prediction tools, but again FanField has incorporated the full effect in its software.

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Fading Zone

Close to the station, the field strength due to the sky wave is generally very much lower than that due to the ground wave, but whereas the latter decreases with distance, the sky wave initially increases in strength.  At some point, the two signals can be of the same order and fading can be apparent.   This is because, while the relative phase of the ground wave signal at a given point is largely constant, that of the sky wave varies due to movement in the ionosphere.   

Having predicted the ground wave and sky wave field strength contours, FanField can use the results to determine the fading zone, as shown in the example right.   Night time fading is generally only relevant to high power stations, where the ground wave still provides a usable signal at a distance where the sky wave reception zone starts.

 

 


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Anti-fading Radiators

The sky wave coverage is dependent on the vertical radiation pattern of the antenna.  At shorter ranges the higher elevation angles are relevant and vice versa.  Thus the start of the sky wave coverage zone can be moved further away from the transmitting site by suppressing high angle radiation from the antenna.  This is commonly achieved by increasing the antenna height, from the normal quarter wavelength to around half a wavelength.   The effect on the ground wave coverage is only minor, as the gain at zero elevation increases by only 1.7 dB, but the effect at high elevation angles is much greater and the distance to the start of  fading zone can be increased significantly.

Consequently, FanField can help maximise your antenna system's performance, ensure that your proposed new installation operates as planned or simply provide experienced personnel for that unusual or unexpected task.

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FanField Ltd., Oxley House Cottage, Oxley Hill, Tolleshunt d'Arcy, Maldon, Essex, CM9 8EN
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