Wednesday, January 7, 2009

Radio Waves and Polarization

So far we have discussed some aspects of radio wave creation and propagation and in those discussions we have touched on polarization but tonight we will go a bit deeper into what it is and why it's important to understand.

Most hams know that a vertical antenna creates a radio wave with vertical polarization and a horizontal antenna creates a horizontally polarized wave. This may seem pretty obvious but we need to take a closer look at this in order to describe another useful polarization, namely circular polarization.

Before we get into polarization, let's look at what makes up a radio wave, also known as an electromagnetic wave. There's a reason we call them electromagnetic waves and not just electric waves. In an electromagnetic wave there is an electric field and a magnetic field. These fields vary over time depending on the frequency of the signal. The interaction of the two varying fields is what causes an electromagnetic wave that can travel through empty space. As it happens the electric field, called the E field is parallel to the antenna's radiating element. The magnetic field is perpendicular to the E field and in the same phase. You don't really need to bother with magnetic fields any more, I just wanted you to be aware of them.

Linear Polarization is the usual kind and what it means is that the polarization of the radio wave as it is sent out doesn't change over time or space, it always points in the same direction, whether vertical, horizontal or otherwise. In HF where we use the ionosphere to bend our signals back to earth, the polarization is changed by the ionosphere in complex ways which I'm not prepared to go into now. I might try to tackle that in a future net. But for local communications in VHF and UHF the polarization stays vertical or horizontal and this can have quite an effect on how well you can communicate with another station. In theory, a horizontal antenna would not pick up a vertically polarized radio wave at all. In practice it can severely reduce the signal to make it hard to communicate even if your antenna does pick it up. And even in local communications radio wave polarization can be affected by reflections or other interfering bodies.

There is one kind of ham communication where linear polarization is not suitable and that is space communication in general and satellite communication in particular. In ham radio, we communicate with satellites using VHF and UHF signals where the polarization is not seriously affected by the ionosphere. What's more, it is not practical to try to always know the orientation of the satellite and match your antenna to it. For these reasons we use circularly polarized radio waves that don't depend on knowing the orientation of either party. So what is circular polarization and how do we make it?

Circularly polarized radio waves sound pretty strange. A circularly polarized wave is one with constant amplitude where the orientation of the wave turns through a circle according to the frequency of the wave rather than the amplitude changing and the orientation staying constant. As it turns out, this is pretty easy to create in theory and in practice as well. The way you do it is to create two sine waves that are at right angles to each other and 90 degrees out of phase.

Let's now follow it through part of a wave transmission. Assume that the first wave is vertical and the second horizontal pointing to the right. When the first one is at it's peak amplitude the second is zero resulting in a vertically polarized field. As the amplitude of the vertical signal is reduced, the amplitude of the second one is increased exactly in such a way that the total amplitude stays constant and the direction of the field rotates to the right. Finally, when the vertical wave has reduced all the way to zero, the horizontal wave has reached it's maximum and the resulting electric field is horizontal and pointing to the right. This works only because both waves are sine waves. But since our transmitted waves are sine waves with only minor variations, it works in practice for ham communications.

So how do you make such a wave? Clearly we don't have transmitters that create two waves that are 90 degrees out of phase. The answer is to have a T-connector splitting the signal into two paths and have one path be a quarter wavelength longer than the first. Now by connecting one side of the T-connector to one driven element of a yagi and the other side to a driven element of a yagi at right angles to the first, you get your circular wave.

There is another common circular wave antenna called a Helical antenna. Unfortunately I didn't have time to prepare a discussion on this antenna. Like the ionosphere effect on polarization, I may tackle this topic in a future net.

Good web page on polarization used with satellites

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