Radio Propagation Theory — Page 37, Lesson 38

Right, let's pick this up with the frequency selection problem. We've established that skip distance increases as frequency increases. That single relationship drives everything about choosing a frequency for a given communication range. Think about it this way: if you need to talk over a short distance, you want a wave that bends back down to Earth quickly, close to the transmitter. That requires a lower frequency. Conversely, if you need to reach a station a long way off, you want the wave to travel further before it returns, so you need a higher frequency. So, short ranges require lower frequencies, and longer ranges require higher frequencies. Now, let's apply that to a classic exam problem. An aircraft is flying from London, UK, to New York, USA. It's mid-Atlantic at sunrise. The pilot is talking to the UK on 12 MHz. The question is: what frequency can the pilot expect to use to talk to the USA? The key to this is the geometry of the path. The wave to the UK is refracted halfway between the aircraft and the UK. The wave to the USA is refracted halfway between the aircraft and the USA. So we have two reflection points, one on each side of the aircraft. Now, here's the sunrise part. Midway between the aircraft and the UK, it is day. During the day, the ionosphere's D-layer absorbs lower frequencies, so we need a relatively high frequency to get through. That's why we're using 12 MHz to the UK. Midway between the aircraft and the USA, it is still night. At night, the D-layer disappears, and lower frequencies propagate much better. So, for the night side, we can use a relatively low frequency. The answer is 6 MHz. Exactly half of the 12 MHz we're using for the UK. The wave to the USA is refracted on the night side, so the frequency is halved. The wave to the UK is refracted on the day side, so it's the higher value. This is a classic example of how time of day and path geometry dictate your frequency selection. Now, let's consolidate all of this into a propagation summary. This is a table you need to know cold. It lists each frequency band and its primary propagation path. Where a path is in brackets, it means that path is present but not normally utilised. Starting with LF, Low Frequency. Its primary path is the Surface Wave. The Sky Wave is present but in brackets, so it's not normally used. MF, Medium Frequency. Same story: Surface Wave is primary, Sky Wave is in brackets. HF, High Frequency. This flips. The Sky Wave is primary, and the Surface Wave is in brackets. Then we get to VHF, UHF, SHF, and EHF — Very High, Ultra High, Super High, and Extremely High Frequency. All four of these use the Space Wave as their propagation path, with no bracketed alternative. So the pattern is: the lower bands, LF and MF, rely on the surface wave. HF is the sky wave band. And everything from VHF upwards is space wave. That's the complete picture of how each band propagates.