Properties of Radio Waves — Page 10, Lesson 10

Let’s start with the physical picture, because polarization is really just a property of the wave’s geometry. When a radio wave is radiated, the electromagnetic energy is made up of two components: an electrical field, which we call the E field, and a magnetic field, the H field. The E field is parallel to the wire of the aerial, and the H field is perpendicular to the wire. So if you imagine a vertical wire, the electric field runs up and down along it, and the magnetic field sticks out sideways, at right angles to it. Now, polarization is defined as the plane of the electric field. That’s the whole definition — it’s the plane in which the E field lies, and it depends on the plane of the aerial. So a vertical aerial emits radio waves with the electric field in the vertical plane, and that gives you a vertically polarized wave. A horizontal aerial does the opposite — it produces a horizontally polarized wave. So the aerial’s orientation dictates the polarization. Why does this matter? Because to receive maximum signal strength from an incoming wave, the receiving aerial must be in the same plane as the polarization of the wave. A vertically polarized wave needs a vertical receiving aerial. If you tried to receive it with a horizontal aerial, you’d get a very weak signal — the geometry just doesn’t match. Now, there’s a second kind of polarization: circular polarization. It can be produced in several ways, and one of them is using a helical antenna — that’s a coil-shaped aerial. In circular polarization, the electric field — and hence the magnetic field too — rotates at the frequency of the radio wave. So instead of staying fixed in one plane, the field spins around as the wave travels. The rotation can be right-handed or left-handed, and that depends on the orientation of the aerial array. For reception of a circularly polarized wave, you need an aerial of the same orientation — so the same handedness — or you can use a simple dipole aerial. And here’s where the two big advantages come in. First, in radar systems: if you use circular polarization, when the energy reflects off water droplets, the circularity is reversed. That means the reflected signal comes back with the opposite handedness. Because of that reversal, the clutter caused by precipitation can be eliminated. So rain returns get filtered out — that’s a huge operational benefit in radar. Second, if you use a dipole aerial, the orientation of the aerial is no longer critical, unlike with linear polarization. With linear polarization you have to match the plane exactly. With circular polarization and a dipole, you don’t have to worry about that alignment. And that’s a major advantage in mobile systems — cellular phones, satellite communication, and satellite navigation systems — because the user can hold the device in any orientation and still get a good signal. So to tie it together: polarization is the plane of the electric field, it’s set by the aerial’s orientation, linear polarization demands matching planes for best reception, and circular polarization trades that strict alignment for rotation — which gives you the radar clutter rejection and the orientation freedom that mobile and satellite systems rely on.