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Polarization

The last fundamental theory we must understand is polarization. Do not be scared off, this theory was NOT saved till last because it was more difficult to understand! It is just as easy as any of the other concepts! A radio wave is actually made of up two fields, one electric and one magnetic. These two field are perpendicular to each other, shown in figure 7. The sum of the two fields is called the electro-magnetic field. Energy is transferring back and forth from one field to the other - This is what is known as "oscillation".

We are interested in one field primarily here - the electric field. Its position and direction with reference to the earths surface (the ground) determines wave polarization. Below, are two figures you can click on for an animated demonstration.

Do not think these animations are an actually representation of a signal, they are to merely demonstrate how the electric field is parallel to the radiating elements.

Horizontal polarization - The electric field is parallel to the ground.

Vertical polarization - The electric field is perpendicular to the ground.

In general, the electric field is the same plane as the antenna's element (antenna element is that actually metal part of the antenna that is doing the radiating). So if the antenna is vertical, then the polarization is vertical. The horizontal dipole in figure 3 is horizontally polarized. In figure 8, this is a vertical arrangement for a dipole, and its polarization is consequently vertical. It is important to note that the 1/2 wave dipole in a vertical position has a different radiation pattern (and consequently different gain over an isotropic) than the 1/2 wave dipole in the horizontal position. When most people talk about a dipole, they usually mean a 1/2 wave dipole in the horizontal position.

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Figure 7 - Makeup of an Electro-Magnetic Field

Waves do not have to be either horizontal or vertical. This is merely an arbitrary setup. Antennas are arranged like this so that others can orient their antennas in a similar matter. These terms horizontal and vertical are in reference to the earths surface, take away the ground, say go into outer space and these terms would have no meaning! This is why we arrange our antennas for either a horizontal or vertical polarization though.

For instance, if the receiving antenna is receiving a signal that does not have the same polarization of itself, then the signal is reduced about 20db (almost 7 times) compared to if the signal had the same polarization as the receiving antenna. Some operators turn their beams at a 45 degree angle trying to achieve an in between polarization. This is just an old trick, that sometimes works out good.

The thing is, as our signals travel they usually reflect off of objects, and the field can change polarization! So your signal may end up loud and clear to another station even if you are not using the same polarization as each other because the signal may be bouncing off some object (a water tower for example) that might be flipping the polarization before it gets to the other station!

More people choose horizontal polarization for DXing because receiving using horizontal polarization is generally more quiet. This is because most man made noises (interference) are vertically polarized. There is no proof that horizontal waves are better for "skip" signals.

There is one special polarization known as Circular polarization. It should be of special interest to antenna experimenters! As the was travels it actually spins, not maintaining a set polarization but covering every possible angle in-between. This is good because it helps reduces signal fade (QSB) and flutter during DX contacts. It can either be right handed or left handed circular polarization depending on which way its spinning (think of it spinning clockwise or counter-clockwise as it leaves the antenna) I will detail how to make your beam radiate circular polarization under "Performance Tips" - only for beams capable of both vertical and horizontal polarization.

 

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Figure 8 - The dipole in a vertical configuration.

This four part article has covered a lot of antenna theory . It was arranged to flow as logically as possible and the author has also simplified several issues, so that you could get the theory was easier to understand . It might however be necessary to go back and re-read some sections to thoroughly understand it and I hope that you found this article enjoyable.

Our Thanks to for his permission to reproduce this article on our website.

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