5: MW Propagation

How MW signals reach you

The first thing you’ll probably notice when you listen to a MW radio is how many stations are audible and whether they are strong or weak. You’ll quickly notice a big different between daytime listening and night-time reception.

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. The transmitter radiates its signal from its antenna much like ripples in a pond spread out when you drop in a stone. But pond ripples only travel on the surface of the water. Radio waves can do this too but they also radiate upwards into the sky.

The signal that radiates up into the sky would normally be lost and you’d not hear it unless you were in an aeroplane. However waves at MW frequencies interact with upper layers of the atmosphere and this can have the curious effect of reflecting the signal back to the ground. Depending on the height and angle of this “virtual mirror in the sky” MW radio signals can return to the earth’s surface many thousands of km from the transmitter.

The mirror in the sky is in fact the ionosphere and this is made up several layers of ionised gas at various heights above the earth. It is not a solid mirror, in fact it is very variable. Some layers are good reflectors but others absorb rather than reflect. The height of the reflection, its angle and degree of reflection all depend on the degree of ionisation in the upper atmosphere, which is largely driven by energy arriving from the sun.

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, akin to ripples on the surface of a pond. 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, everything 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 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), ground-wave reception of stations up to 1500km 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 from day to day. During daylight hours this is the main method of receiving a MW radio signal up to 300km or so.

Skywaves

The ionosphere consists of several layers of high ionisation (Fig. 1). These 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 they don’t carry on upwards out into space. The more solar radiation that reaches the D layer the more complete the absorption is of MW signals.

With the approach of sunset, however, the ‘D’ layer absorption decreases rapidly and within a few hours MW signals can continue upward in the atmosphere.  But they then encounter higher regions of the ionosphere which instead of absorbing the signal tend to refract or bend the path of the signals travel. With enough refraction the signal is redirected back to the earth’s surface.

Depending on circumstances reflection occurs in the E region (about 100-120km up) or in the ‘F’ layer (225-300km). At distances over 300km from the transmitter listeners will receive strong night time signals from stations they’d not hear during the daytime. This is how Radio Luxembourg used to target a UK-wide audience and for that reason only broadcast its English service at night.

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 80-160km from a transmitter (Figure 2) where the groundwave and the skywave signals are received with roughly equal (but varying) strength, leading to severe distortion of the desired signal. Additionally the skywave from an unwanted distant station may arrive and cause co-channel interference to a local station, to the annoyance of the average listener.

During the transition hours of dusk and dawn, skywave signal strengths are found to be very frequency dependent. For example, signals at 1530kHz will be on average 15db stronger than a station on 700KHz – assuming that both stations radiate the same power of course. However, about two hours after darkness falls the difference in strength is only about 3 – 5dB and by about midnight any frequency dependence has more or less vanished.

MW DXers depend heavily on the state of the D layer. Long winter nights and periods of low solar radiation both conspire to reduce the D layer almost completely due to the reduced energy arriving from the sun.

For MW DXers interested in distances beyond 1000km more complex propagation paths exist.

Anomalous propagation

The conditions described above affect MW signals in a reasonably predictable way, so much so that it allows radio planners to choose frequencies and design coverage areas for new radio stations with a high degree of certainty. Planners are only interested in what happens 95-99% of the time – we DXers are interested in what happens the rest of the time.

But in real life nothing is quite black and white. Whilst many of the parameters are reasonably predictable (like the seasonal changes in the absorption in the D layer) there is always a statistical degree of uncertainty which can work to the advantage of the MW DXer.

Though it is well known that sun undergoes an 11 solar cycle which impacts upon the ionisation of the atmosphere, long term prediction of the strength and timing of the cycle is far from a precise science. Over a much shorter time frame predicting the behaviour of the sun over a matter of hours or days is notoriously difficult. But in recent years much better satellite based measurement is giving us advanced warning of changes that could either aid or hinder our DX listening.

Until the 1980s it was widely thought that transpolar reception was impossible in location like the UK and central Europe. But with better equipment and more practical experience DXers discovered that propagation paths existed that allowed reception from Alaska as far south as Italy.

Over recent years the rise of software defined radios that are able to monitor the whole MW band continuously has exposed MW propagation that many thought impossible including antipodal reception over distances of 20000km. Under rare conditions MW reception via “the long path” has been observed – where the path is greater than 20000km (example noted on Easter Island in 2007).

If signal propagation was entirely stable you’d hear the same signals every day, but fortunately for us this isn’t the case. Once you understand more about the mysteries of the ionosphere you will be able to use this knowledge to help you tune into exotic and interesting stations.