Radio waves travel outwards in straight lines from the antenna. In much the same way as light, they can be blocked, reflected, or refracted (bent) by the objects which they encounter. As the distance from the antenna increases, the strength of the radio waves decreases rapidly. This decrease in strength will ultimately lead to the strength of the radio signal being less than that of the background noise, and reception will no longer be possible. This behaviour is again similar to beams of light. If a light is shone onto a surface, a circle of light will be seen. The further the surface is from the source of the light, the wider spread and dimmer it appears to be. Eventually, no discernable light will be seen. The way the radio waves behave after they have left the antenna is known as "propagation".
As the radio waves travel away from the antenna they will encounter buildings, hills and other objects. Each of these will have an effect on the waves, the effect is largely dependant on two factors, wavelength of the transmitted signal and the density/conductivity of the object encountered.
Radio waves of a lower frequency are less influenced by physical obstacles than those of a higher frequency. This is because their wavelengths are much larger. A small building is of insignificant size to a 80m long wave, but a large mountain is not. Similarly even the smallest buildings are a significant size when the wavelength is only a couple of centimetres.
Radio waves are attenuated more by dense obstacles than by less dense ones. For this reason, high frequency (VHF and UHF) radio waves are heavily attenuated by walls of buildings, but a window, whose size is significant relative to the wavelength, will present much less attenuation. You may have observed that a transistor radio often works better if placed near a window.. The attenuation by walls etc is also why outdoor antennas invariably work better than those mounted indoors.
Highly conductive structures like metal factory buildings are invariably earthed. This causes them to block most radio waves completely. Only those signals where the wavelength is much larger than the building will be able to penetrate, and even those will be attenuated to some degree. Note that most concrete structures contain large amounts of metal in the form of reinforcing bars which are normally bonded to earth at the foundations, these will have a similar attenuating effect on incoming radio waves.
All radio waves will be attenuated (made smaller) when passing through a wall or other obstruction. In some cases the attenuation will be sufficient to prevent a useful signal being received. Best results will always be achieved when there is a clear "line of sight" path between the antenna of the transmitter and that of the receiver. This is why TV antennas are usually mounted on masts above the roof line where they have a clear path between them and the TV transmitter antenna. This is also why the TV transmitters are usually mounted on very tall masts, so that even in a valley, a receiving antenna can still "see" the transmitter.
In
the top picture, the radio waves from the transmitter, 'A' on the left, cannot
be received by the receiver 'B' on the right. because they are blocked by the
hill. Raising the height of the transmitting antenna allows the signals to be
received since a "line of sight" path has been restored.
At lower frequencies less attenuation would be caused by the hill, however a useable signal may still not be received, Raising the height of the receiving antenna would have the same effect.
You will see that while increasing the transmitter power may also allow
reception of the signal at the receiver by overcoming attenuation caused by the
hill, return transmissions from 'B' to 'A' would still not be possible. It is
therefore always preferable to increase the height of the antenna as this will
aid in both transmission and reception of signals.
