Thursday, February 14, 2013

Polarity

Some people just don't understand polarity, even some engineers who really ought to. The short version is that all electrical signals have polarity, RF, audio, and electrical. Even gravitational waves are known to have polarity. Polarity is the sine wave’s position or voltage above or below the median line. If you shift a wave 180 degrees you reverse its polarity. You can do this the ham handed way by chopping off half a cycle. In audio and RF we also use hardware which inverts the polarity of a signal by swapping positive voltage to negative voltage. It's easier with home wiring, you just switch the positive and negative terminals. More here.

But polarity has consequences. The fact that energy travels in a wave is interesting on it's own. But that behavior has other less obvious properties.  It you electrically sum a signal with an inverted-polarity time-synchronous copy of itself the result is complete and total cancellation. In other words the sum is zero. Yes, it ceases to exist. This is part of Kirchhoff's law of current: The sum of all currents entering a node is equal to the sum of all currents leaving the node.  See also Kirchhoff's law of voltage: The directed sum of the electrical potential differences around a loop must be zero. Sometimes combined as Kirchhoff's Circuit laws, they are named for Gustav Kirchhoff, who described them in 1845. The same is true of audio and RF. In fact that is how noise-cancellation works. More here.

Polarity is not a 2 dimensional construct. Waves can exhibit oscillations with more than one orientation. There are three types of polarization: linear, circular, and elliptical. Linear is the traditional model, like waves upon the water, like the image at the top of this post. It's most easily visualized. Linear has two sub types vertical and horizontal . Horizontal is parallel to the plane of the earth's surface and vertical is perpendicular to it. Circular polarization has two sub types. If the rotation is clockwise looking in the direction of propagation, it's called right-hand-circular (RHC). If the rotation is counterclockwise, the sense is called left-hand-circular (LHC). The polarity literally corkscrews. Elliptical polarization is similar to circular but the two perpendicular waves (one positive one negative) are of unequal amplitude. With circular polarization they have equal amplitude. the result being that elliptical polarization wobbles along it's axis. More here.
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