As a general rule, daytime propagation conditions will be best on frequencies from about 12-14 MHz and higher. Propagation at night will generally be best on the 2-15 MHz bands. These general rules often have exceptions, of course; during periods of high solar activity, the 21 MHz band may, for example, he excellent for long distance propagation well into the night. However, it would be highly unusual for the bands below about 8 MHz to support transoceanic propagation throughout the daytime period.
When reading broadcast station schedules, one must consider not only the time of day at one's own location, but also the time of day at the transmitter location. Let us examine the example of two broadcast stations, one in Tokyo and one in Moscow, both operating on 6 MHz at 6:00 PM local time (for our example, let us say that you live in New York City, USA). Because there are nighttime conditions across most, if not all, of the North Atlantic path that a signal would follow on its way from Moscow to New York, one would normally have a good chance of receiving the broadcast from Moscow. However, the path from Tokyo to New York is largely a daylight ; path, and our rule of thumb discussed earlier would fell us that it would be difficult, if not impossible, to hear Tokyo at that time on that frequency.
However, if one reads further down the schedule of the Tokyo schedule, a broadcast on 15 MHz at the same time may be found. The 15 MHz region (±5 MHz) is a middle ground which often supports round the clock propagation. You would have a much better chance of hearing Tokyo on 15 MHz, because the path is largely over daylight.
Broadcast station managers are well aware of this phenomenon, and this is the reason why their schedules indicate "North American Service" or "Programming Beamed at Southeast Asia." They take propagation conditions into consideration, and aim their antennas carefully, so as to have the best chance of reaching their target area at a time when people will be at home to listen.
The time of year is important for several reasons. For example, at 4:00 PM in New York in June, the sun is still high in the sky. But at 4:00 PM December, twilight is fast approaching, and night time conditions are taking over on the North Atlantic path. Broadcast station managers adjust their schedules so as to use the lower frequencies (below 10 MHz) more heavily in the wrote months, because of the increased distance covered by darkness during the winter.
Signals do not always follow the shortest distant: from point A to paint B (called the "Great Circle path). They sometimes follow a bent path, or on, exactly the opposite of the great circle. This is why it is sometimes possible to hear Tokyo from New York on 7 MHz late in the afternoon in the winter, even though the Great Circle path is in daylight; the signals are traveling along a darkness path around the world. The fact that many stations are louder, and that the transmitting antenna may not be beamed on the optimum path at that time, makes reception extremely difficult. But this is the excitement of shortwave listening hearing the unexpected. Under tough conditions such as this, the AM-N (narrow bandwidth) position of the mode switch will prove itself to be a highly useful feature.
To conclude our discussion on propagation conditions, we would stress the following general rules. First, use the higher frequencies (15 MHz and up) as your main daylight bands. Secondly, use the low frequencies (below 1 MHz) as your prime nighttime bands. Thirdly, look for peaks in propagation when there is sunrise or sunset at one end or another of a propagation path. For example, look for a peak in 26 MHz propagation towards the East for the hour or so after your sunrise, and toward the West around your sunset.
Careful planning of your operating times, proper choice of listening frequencies, and diligent study of schedules from overseas broadcast stations will pay rich dividends in entries in your log book We hope that this section will have helped yon understand the fascinating world of shortwave radio propagation better.