Propagation

with Steve Whitt

Most Listeners to the MW band will have noticed that long distance reception is normally only possible during the hours of darkness, but if you are a MW listener you' II know that skywave propagation is responsible for this effect. Skywave propagation is often quite variable in its behaviour, resuIting in great changes in received signal strength. This unpredictability is a problem for the broadcast engineer who is trying to plan a station's service area, but it is a boon for the listener who is trying to hear rare or unusual signals. Although it is difficult to say with certainty how strong a given signal will be at any specific time, there are some well proven guidelines.

 

FREQUENCY:

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.

 

LATITUDE:

This parameter Is a very, if not the most, influential factor in determining the received strength of a sky-wave signal. MW skywave field strength decreases with increasing geomagnetic latitude but unfortunately there is not much the average listener can do about latitude short of emigrating!

 

SOLAR and MAGNETIC ACTIVITY:

Evidence shows that increased solar activity reduces MW night-time signal strengths and this Is clearly evident in the better DX heard during the period of the sunspot minimum. AdditionalIy, magnetic storm activity causes very significant signal absorption particularly during the first five to ten days immediately following the onset of a storm. The absorption effect increases with increasing frequency and with higher latitude locations. Around 1500KHz signal absorption of 30db or more is not unusual as a result of storm activity.

 

SEASON:

In low latitude regions (such as the Caribbean) very little seasonal variation in night-time signal strength has been observed, but in Europe skywave signals are usually weakest in summer and strongest in spring and autumn, exhibit ing a seasonal variation of about 6-10db. In winter daytime skywave strengths show a very marked increase; particuIarly at mid to high latitudes the difference between daytime and night time skywave signals is often less than 25db (on average, however, skywave signals at noon are about 45db weaker than at midnight).

 

PROBABILITY:

Despite the above rules it is still possible for signals to be occasionally much stronger or much weaker than expected for the conditions prevailing. The DX-er is of course interested in when and how often the former case arises. Unfortunately it is very difficult to say when, but some figures do exist to say how often unusually strong signals may be heard. In a year of low solar activity

 

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