signals the overall polarization of the signal can change slightly although it remains broadly the same. Different types of polarization are used in different applications to enable their advantages to be used. Linear polarization is by far the most widely used. Vertical polarization is often used for mobile or point to point applications. This is because many vertical antennas have an omnidirectional radiation pattern and this means that the antennas do not have to be reorientated as positions are changed if, for example, a vehicle in which a transmitter or receiver is moved. For other applications the polarization is often determined by antenna considerations. Some large multi-element antenna arrays can be mounted in a horizontal plane more easily than in the vertical plane and this determines the standard polarization in many cases. However, for some applications there are marginal differences between horizontal and vertical polarization. For example, medium wave broadcast stations generally use vertical polar- ization because propagation over the earth is marginally better using vertical polarization, whereas horizontal polarization shows a marginal improvement for long-distance communications using the ionosphere. Circular polarization is sometimes used for satellite communications as there are some advantages in terms of propagation and in overcoming the fading caused if the satellite is changing its orientation. How radio signals travel Radio signals are very similar to light waves and behave in a very similar way. Obviously there are some differences caused by the enormous variation in frequency between the two, but in essence they are the same. A signal may be radiated or transmitted at a certain point, and the radio waves travel outwards, much like the waves seen on a pond if a stone is dropped into it. As they move outwards they become weaker as they have to cover a much wider area. However, they can still travel over enormous distances. Light can be seen from stars many light years away. Radio waves c