By: Suad Čobo
Even though we take any kind of media over radio waves for granted, it is still being used today: millions of amateur radio operators use the HF and VHF spectrum all around the world, military personnel use for their internal communication, road operators relay traffic information over commercial radio stations in tunnels, etc. Besides, in case of emergencies, where cell signal, internet connections… stop working, radio is the only thing left. However, the important thing for broadcasting on air is radio propagation, which is essentially how radio waves behave in the environment. Considering this behavior, which can be different depending on what time of day it is or how complex the terrain around a transmitting radio station is, we can determine how far can your voice be heard.
Radio propagation with ionosphere schematics (Photo: Wikimedia Commons) 
During the daytime, ionizing radiation from the Sun ionizes a part of the Earth’s atmosphere, from 60 to 1000 km of altitude, which is called the ionosphere.  There are four layers of ionosphere during daytime: D, E, F1 and F2; at night the D layer vanishes since there is no ionizing radiation from the Sun and there are only F and E layers. Different frequencies of radio waves behave differently depending on the time of day: Low Frequency waves (LF: 30-300 kHz) gets refracted by the lower part of D layer during daylight, while at night they get refracted by the E layer, making them go higher and changing the angle at which they are refracted; Medium Frequency waves (MF: 300-3000 kHz) can only be oscillated as ground waves (which follow the curvature of the Earth rather than being transmitted through the atmosphere) during the day (because they get absorbed by the D layer) and get refracted by the E layer during the night; High Frequency waves (HF: 3-30 MHz) can sometimes be refracted by E or F layer; and Very High Frequency waves (VHF: 30-300 MHz) can go through the ionosphere in outer space. That’s why VHF waves are used by the International Space Station and the satellites. If they aren’t intended for broadcasting in outer space, a Line-of-sight propagation is used, where waves travel in a straight line and can reach only the parts of the world which aren’t blocked due to the curvature of the Earth. 
Why is this important?
AM broadcasting at the time of dramatic changes of ionosphere between daylight and night can cause stations to be heard much further if transmitting using the same power. For commercial AM stations, this would cause chaos in the spectrum. That’s why they need a special permission from government authorities to transmit at a higher power during the night, or else they need to significantly reduce power.
Amateur radio operators transmitting AM waves, on the other hand, don’t need the permission, and if they have their equipment in good conditions, they could hear operators from other continents or very distant islands, for example from Europe to USA or from Europe to the Far East.
FM broadcasting isn’t affected by ionospheric but usually by meteorological conditions, i.e. The current cloud layer or temperature inversions, dew, frost, fog, ice storms, etc. Since mobile phone signals use Ultra High Frequency waves (UHF: 300-3000 MHz) ranging from 700 to 2600 MHz, they are even more prone to weather-induced propagation changes, because of the shorter wavelength (For comparison, HF waves have wavelengths ranging from 100 to 10 meters, while VHF and UHF range from 10-1 and 1-0.1 meters respectively). That’s why in urban areas, smaller cell antennas are installed, using lower effective radiated power, thereby increasing user capacity. 
 (Wikimedia), M. (2010, 7 21). File:Ionospheric reflectionDay and Night.PNG. Retrieved from Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Ionospheric_reflectionDay_and_Night.PNG
 Ionosphere. (n.d.). Retrieved from Wikipedia (English): https://en.wikipedia.org/wiki/Ionosphere
 Line-of-sight Propagation. (n.d.). Retrieved from Wikipedia (English): https://en.wikipedia.org/wiki/Line-of-sight_propagation
 Radio Propagation. (n.d.). Retrieved from Wikipedia (English): https://en.wikipedia.org/wiki/Radio_propagation