Radio Propagation Theory — Page 37, Lesson 38

Let's pick this up right where the skip-distance idea lands, because this next bit is the classic exam trap. We just established that skip distance grows as frequency rises. Now I want you to see the operational consequence: the range you actually need to communicate over will dictate your frequency choice. Short ranges call for lower frequencies; longer ranges call for higher frequencies. That's the direct relationship to hold onto. Here's the classic problem that tests whether you truly understand it. 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: what frequency can the pilot expect to use with the USA? Let's reason it out before I give you the answer. The aircraft is roughly halfway between the two continents. The wave to the UK gets refracted halfway between the aircraft and the UK — that's the skip point. The wave to the USA gets refracted halfway between the aircraft and the USA. Now, sunrise: the sun is coming up in the east. So the midpoint toward the UK is already in daylight, while the midpoint toward the USA is still in darkness. Daytime ionisation is strong, so the ionosphere reflects higher frequencies — you need a relatively high frequency to get a sky wave back down during the day. Night-time ionisation is weak, so you must drop to a lower frequency for the wave to be refracted at all. The pilot is already using 12 MHz toward the UK, which is the daylight side. Toward the USA, it's night, so the frequency must be lower. The answer is 6 MHz — exactly half. The wave is refracted halfway to the UK in day, requiring the high frequency; halfway to the USA in night, requiring the low frequency. Now let's lock in the whole picture with the propagation summary. This table is your memory anchor for every band. LF, low frequency, propagates primarily by surface wave, with sky wave present but not normally used. MF, medium frequency, same story — surface wave primary, sky wave in brackets. HF, high frequency, flips: sky wave is the primary path, surface wave is the bracketed one. Then VHF, UHF, SHF, and EHF all propagate by space wave, with no bracketed alternative at all. So the pattern to remember: the lower bands rely on surface wave, HF is the sky-wave band, and everything from VHF upward is space wave. That's the complete frequency-band picture you need for propagation theory.