Polarization
Post date: Sep 08, 2013 10:49:53 PM
Revisiting a topic discussed a couple of years ago...
Electromagnetic waves have an electric component and a magnetic component. Both waves are transverse (vs. longitudinal), meaning the line of their motion is somewhere in the plane perpendicular to the direction of propagation away from the source; also the two components are always perpendicular to each other. When we talk about the direction of polarization, we normally are talking about the electric field; we know the magnetic field is always there and perpendicular, so we just ignore it to keep the discussion simple.
Linear Polarization
Practical polarization for radio amateurs is easy to remember. A vertical antenna produces (and receives) vertically-polarized signals; a horizontal antenna will produce horizontally-polarized signals (Now you know how to answer at least one license exam question! That was easy, huh?). Picture a simple vertical antenna. When radio-frequency alternating current is applied to it, the electrons are moving back and forth along the wire/pole, i.e. up and down. When that electric wave jumps off the side of the wire and flies through the air in all directions, it's still going to be going up and down, i.e. vertically. Can you picture it? Now picture a horizontal dipole. You're looking at it broadside, and you see the electrons going back and forth, left and right. As the wave comes off the wire towards you, the electric field is still going back and forth, left and right, parallel to the ground.
Why does polarization matter? By far the biggest concern to radio operators is that the polarization of both transmitter and receiver -- whether horizontal or vertical -- is the same. Picture that wave front coming from the horizontal dipole towards us. Now picture our own receiving antenna, also a horizontal dipole, broadside towards the oncoming wave. When the electric field (moving back and forth, right and left) hits the wire forming our antenna, it starts the electrons moving back and forth in the wire in the same direction, left and right, along the length of the wire. Presto! We now have an electric current that can be amplified and used.
Now picture that same wave hitting a vertical antenna? What direction is it going to try to move the electrons? Left and right along the -- what -- 2 millimeter thickness of our wire? Not going to get very far, is it! All the electric wave motion is horizontal; there's nothing to induce a proper current along the length of our vertical wire. We are said to be cross-polarized, and our received signal is going to be next to nothing.
Can you have linear polarizations other than horizontal or vertical? Certainly! You can tilt that antenna at any angle you want. Put it up at a 45-degree angle, and instead of having to worry about whether most people are using vertical or horizontal polarization -- and taking a good 20dB hit if you're using the wrong one -- you can take a 3dB hit (half the power, which isn't much in the grand scheme of things) on both polarizations without ever having to change orientation. (If course, if someone else is also doing 45 degrees...well, you get the idea, and then there's cases where the end of a semi-horizontal antenna is pointed at your way, in which case you kind of the same "pick it up with any orientation, just not as well" thing.)
Circular polarization
Want to get weird and wacky? How about you put 2 dipoles, a vertical and a horizontal, together in an X shape, perpendicular to each other, and feed them simultaneously ("in phase"). Vertical, horizontal, or what? Well, you have an equal amount coming out of the vertical and horizontal elements, so you can just add them together like vectors and you end up with -- again -- a 45 degree diagonal orientation. Okay, so what -- you can do that with a single dipole.
Now, insert an extra 1/4-wavelength of transmission line in the section that feeds one of the antennas. Its wave (a sine wave) is now trailing 90 degrees out of phase with the other element's (cosine?). At one point in time the resulting wave is vertical because the horizontal's amplitude is zero. Then it comes up as the vertical is going down, and at some point they're the same so it's diagonal; then the vertical goes to zero as the horizontal's goes up to max, and it's horizontal. If you were to display the angle change on a compass dial, you'd watch it go smoothly right around 360 degrees -- that pointer would just be going in circles. Congratulations, you've just discovered circular polarization. With circular polarization, instead of vertical and horizontal, you have right & left-handed polarization, depending on which direction the field is rotating.
What happens if you try to receive a circularly polarized signal with a normal linear antenna? Well, the rotating wavefront spends half of its time oriented about the right way and about half the time oriented closer to the wrong way for you, so you again end up with a 3dB drop no matter whether you're using a vertical, horizontal, or something in-between.
Satellites
What's the use of different polarization types? With horizontal & vertical, you can argue minor advantages with overcoming natural noise, ease of use w/ mobile vehicles (ever see a horizontal antenna on a car?), etc., but where it really comes into play is satellites.
Some communications satellites use linear polarization. Geostationary TV satellites usually have two antennas and transmit alternating polarizations on adjacent frequencies so they can pack the channels closer together without interfering with each other. You not only have to aim at the correct satellite and tune in the right frequency, but you have to get the polarization right, too. Now picture a satellite that's *not* geostationary, whipping across the sky, sometimes in different direction. As it moves, the orientation of its antenna is going to change with respect to you, and it can be hard to say which direction is "up and down" at any given time to any particular viewer. If the satellite is using circular polarization and you are either using the same polarization or a linear polarization (taking a mere 3dB hit), you don't have to worry, and can get consistent reception. (Similarly, if the satellite is linearly polarized but you use a circularly polarized antenna, you can receive both horizontal and vertical consistently.)
More on satellites next time. For now, here's a couple of articles that discuss polarization with a little more mathematical detail (and pictures!), if you're interested: