What is radio?
A radio signal is an electromagnetic wave, that it a phenomenon with both and electric and a magnetic field element to it. To produce a radio signal you need to produce the two parts which interact to produce the wave, which does start to interact some distance from the antenna. These travel at the speed of light in free space, but do slow down when travelling through denser materials, hence you get refraction as well as reflection.
Then there is the business, very misleading, of "radio frequency" or RF. As radio exists anywhere between DC and light it's a misnomer. Very low frequencies can be used for communication, such as SAQ's 17.2kHz or up into the microwave region for satellite broadcasting at around 12GHz.
One important equation at this point is:
Frequency (Hz) x Wavelength (m) = Speed of light (m/s)
For example a signal of 100MHz has a wavelength of 3m.
Longer wavelengths tend to be able to go round obstructions and follow the surface of the earth. Very low frequencies (such as 10kHz or so) are used for submarine communications as they are able to penetrate sea water. Likewise these can be used underground. However, antenna sizes at this frequencies can be immense (several km long) or just be a long wire trailing in the wake of a submarine.
In Europe there is still "Long Wave" broadcasting, notably such as BBC Radio 4 on 198kHz, which covers the UK with two transmitter and all the coastal waters. Hence their use for the Shipping weather forecast. You would notice if you go into a metal framed building that R4 reception on LW reduces - the metal frame screens the signal.
Going up a little, frequencies between 3 and 30MHz have been used for international broadcasting. These are reflected/refracted (it's not a precise science!) by the upper atmosphere so can be heard thousands of miles away. It used to be the way Albania's old government told the world how many tractors it had produced that week if you go back to the 1970's "Short Wave" scene. This is where a lot of "Hams" inhabit in the chunks of spectrum available, where you stand a (usually) good chance of getting reliable long distance contacts.
Between 100 and about 800MHz there is a nice "sweet spot". The characteristics are such that the ground following aspect is fairly good but it doesn't have the extreme range of lower frequencies. The same frequencies can be used within the same country for broadcasting and communications, and these are the ones in most demand. Shorter wavelengths don't have the same sort of problems inside buildings, the signal can come through doors and windows which are larger than a wavelength.
Up from this signals get a lot more directional, get absorbed by walls etc and are more "line of sight". With only short ranges reuse of frequencies is easy although some are getting rather crowded, just don't try to use a wifi router (2.4GHz) on the top floor of a block of flats in an urban area. Directionality and small antenna size mean that your home satellite dish just picks up the one it's pointed at.
That's the simple waves dealt with. They aren't a lot of use without actually conveying any sort of information. Look out for the next installment.
