To make the most of MW listening you'll need to have a basic understanding of how a radio signal arrives at the receiver from a distant transmitter. A great deal of scientific work has been under-taken investigating the propagation of radio waves, but fortunately for the MW DXer things can be greatly simplified by considering just two dominant propagation modes. MW propagation takes place by means of two different and distinct mechanisms, namely groundwaves and skywaves.
Groundwaves
The groundwave, as its name implies, travels along a path close to the earth's surface. How far such a signal goes is dependent on a number of factors, principally transmitter power, operating frequency and earth conductivity. Groundwave propagation is heavily dependent on the frequency, with low frequency signals travelling greater distances. In fact, every thing else being equal, groundwave signals from a station on 550kHz will travel twice as far over land as those radiated by a station on 1500kHz. The earth conductivity is also a very significant factor and it is found that the better the conductivity the further the signal travels. Sandy or rocky soil is the worst terrain whilst sea water is best and in regions such as the Caribbean, where the sea is particularly saline (and therefore more conductive), groundwave reception of stations up to 1000 miles distant is possible. In contrast, a similar signal travelling over rocky terrain would carry only about one quarter of this distance. Groundwave propagation is very stable resulting in consistent reception conditions. It is, however, usually only associated with daytime (although equally present at night) since at night long distant reception is predominantly via the sky wave. Because of its stable daytime behaviour, radio stations usually optimise their aerials to radiate as much of their signal as possible via the groundwave in order to improve coverage.
Skywaves
There exists a rarefied region of the earth's upper atmosphere that absorbs the intense solar ultra-violet radiation thereby protecting life on the earth's surface. This radiation results in a region of ionised gases known as the ionosphere, which, depending on diurnal and seasonal variations, consists of several fairly distinct layers of high ionisation (Fig. 1). These layers have a profound effect upon radio waves approaching them from transmitters on the ground below. Under certain conditions refraction of waves occurs, resulting in the 'reflection' of signals back down to the earth, whilst at other times signals can be totally absorbed by the ionised gases. During daylight hours solar radiation penetrates the atmosphere far enough to form the lowest layer of ionisation, the 'D' layer roughly 60km above ground. The 'D' layer so completely absorbs signals on MW frequencies that any radio signals radiated by a station other than those parallel to the earth's surface are completely lost.
With the approach of sunset, however, the 'D' layer absorption decreases rapidly and within a few hours MW signals are being reflected back to the ground from higher regions of the ionosphere; depending on circumstances reflection occurs in the E region (about 100-120km up) or in the 'F' layer (225-300km).
Figure 3 & 4 illustrate this process and shows the skip distance which for MW frequencies turns out to be about 100 to 500 miles. Longer distance reception is possible when multiple reflections occur between the ionosphere and the earth's surface. This occurs with least signal loss over ocean paths hence the possibility of good reception of Brazilian stations here in Europe.
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Figure 3: The Ionosphere and MW Propagation
Whilst the skywave enables good MW DX at night, it also leads to a deterioration in reception quality for the normal broadcast listener. Firstly there is a region about 50-100 miles from a transmitter (Figure 4) where the groundwave and the skywave signals are received with roughly equal (but varying) strength, leading to severe distortion. Additionally all skywave signals are affected by fading as a result of the continually changing characteristics of the ionosphere.
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Figure 4: Skywave / Groundwave Interference
Anomalous Propagation
A previous section examined some of the basic factors governing MW (and LW) reception, in particular the effect of the ionosphere and the influence of solar radiation and ground effects. We deliberately restricted the subject to effects of a regular or predictable nature; the sort of parameters that a planner takes into account when planning the reception area for a new station.
There are however many other occurances that have a bearing on radio propagation at these frequencies; each with a greater or lesser degree of unpredictibility. Although it is nice to be able to predict when good DX will be heard on the MW band, it is the possibility of the unusual occurring that adds a touch of excitement to the DXing hobby. One of the overriding features of MW propagation is the effect of solar radiation on the upper regions of the earth's atmosphere. Predictable effects of solar radiation can be seen as diurnal and seasonal variations in MW propagation as well as in the influence of the 11 year sunspot cycle.
Less predictable events include ionosperic storms, shortwave fadeouts and polar disturbances. These somewhat esoteric events result from disturbances occurring in the sun, which is, under such circumstances, referred to as 'active'. The mechanisms behind such events are both complex and in some instances not yet fully understood but fortunately the average DXer is likely to be more interested in knowing the effect rather than the cause. In addition it could be very helpful to know when such an event was in progress and to be able to gauge its possible effect on DXing. A number of institutes around the world keep a watch on the sun and the ionosphere but the DXer is faced with the problem of obtaining (and interpreting) this extensive scientific information.
Fortunately the American National Bureau of Standards provides this information via the standard time and frequency broadcasts of station WWV. This station, which is most likely to be heard on 5.0, 10.0 or 15.0 MHz, transmits regularly up-dated radio propagation data during the 18th minute past every hour. It is also possible to obtain the same message by phoning a pre-recorded announcement: the US phone number is +1-303-497-3235.
One piece of information transmitted via WWV that is particularly interesting, is the Fredericksburg 'A' Index (more properly called the Fredericksburg Index of Geomagnetic Activity in the Earth's Magnetic Field) which can be used as a simple guide to propagation on the MW band. It is a simple matter to construct a daily graph of the A indices from which basic propagation predictions can be made. High values (above 20) indicate that MW signals in high latitude paths are likely to be absorbed, leaving signals propagating via paths closer to the equator to dominate. Low values over a period of time indicate a likelyhood of improved reception via higher latitude paths. Long periods of very low (6 or less) values are needed to raise the possibility of good high latitude reception throughout the entire MW band.
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