Properties of Radio Waves — Page 10, Lesson 10

Let’s start with the physical picture, because everything else in this chapter hangs off it. When a radio wave is generated, the energy that leaves the aerial is electromagnetic energy — EM energy — and it’s made up of two components. One is the electrical field, which we call the E field, and it runs parallel to the wire of the aerial. The other is the magnetic field, the H field, and it runs perpendicular to the wire. So right at the source, you have an electric field and a magnetic field locked at right angles to each other, and that relationship is fixed by the orientation of the aerial itself. Now, that leads us straight into the first big property: polarization. Polarization of a radio wave is defined as the plane of the electric field. Not the magnetic field — the electric field. And that plane is dependent on the plane of the aerial. So if you have a vertical aerial, the electric field is emitted in the vertical plane, and you get a vertically polarized wave. If you have a horizontal aerial, you get a horizontally polarized wave. That’s the whole definition in one sentence: polarization is the plane of the E field, and it follows the aerial’s orientation. Why does that matter? Because to receive maximum signal strength from an incoming wave, your 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 put a horizontal aerial up against a vertically polarized wave, you’ll get a weak signal. So matching the plane is essential for good reception. 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 — a helix, a coil-shaped aerial. In circular polarization, the electrical field — and therefore the magnetic field too — rotates at the frequency of the radio wave. So instead of staying in one fixed 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 — same handedness — or you can use a simple dipole aerial. And here’s where the real payoff comes, because there are two significant advantages to circular polarization. First, in radar systems. If circular polarization is used, when the energy is reflected from water droplets, the circularity is reversed. That reversal means the precipitation returns with the opposite handedness, and the radar can discriminate against it — so the clutter caused by precipitation can be eliminated. That’s a huge operational benefit in weather. Second, if you use a dipole aerial for reception, the orientation of the aerial is no longer critical, the way it is with linear polarization. With linear polarization you have to match the plane exactly. With circular polarization and a dipole, you don’t. And clearly that’s a major advantage in mobile systems — cellular phones, satellite communication, and satellite navigation systems — where the aerial can’t be held in a fixed orientation. So let me tie it together. Polarization is the plane of the electric field. Linear polarization — vertical or horizontal — demands that your receiving aerial match that plane for maximum signal. Circular polarization has the field rotating at the wave’s frequency, with right- or left-handed rotation, and it gives you two big wins: it lets radar reject precipitation clutter because the reflection reverses the circularity, and it frees mobile receivers from orientation constraints when a dipole is used. That’s the core of polarization as a property of radio waves.