Understand what is polarization and why do we need it?

[dervast]

Hi i am trying to understand what is polarization and why do we need it? We use the term of polarization in university in a wireless communications course.. Plz i need some articles to study..
Thx a lot

 

[woodsy2k]

I not sure where you can get articles on it, but ill offer my explainatio of it. An electromagnetic wave consists of two components. A magnetic field and an electric field. These oscilate about an axis that is parallel to the direction of propagation, and they oscilate at 90 degrees to each other. Take visible light for example. If you look at an unpolarised light source, the planes in which the electric field is osciliating will be in all possible directions (bare in mind that in polarisation it is the electric field component that is important). When looking through a linear polariser, Only one plane of polarisation is allowed through. So if you have a polariser that has its plane of polarisation parallel to a horizontal line on this page for example, only EM waves with an electric field component parallel to this line will pass through.
Thats the simple way of explaining it.
If you place now place a linear polariser in front of an unpolarised light source, you will as usual not really notice a chance in brightness. However if you now place another linear polariser in front of the current one at 0 degrees to it, again you will notice there is no change in brightness. However if you rotate the second filter, you will notice that the brightness falls slowly. it does not snap from bright to dark, like one might think - Specially if you think of it in terms of only one plane can pass (i know that's what i said, but it depends how deeply you want to understand the concept.). You now must think of light passing not being a statistical concept in terms of angle and brightness, but now think of it as a probability amplitude of whether or not the light passes. If the probability is high: the difference in angle of the two polarisers is small, then the brightness will be high. Hence is the angle is large, the brightness is low as this corresponds to a low probability amplitude.

What will really bake your noodle is the following. If you have the same setup as just mentioned, with two filters, that are at 90 degrees to each other, IE, no light should pass. if you now place a third filter at 90 degrees to the first, or 0 degrees to the second, you will get as expected, no light passing. however if you rotate the thrid, light will start to pass. This is again to do with the probability amplitude as a relation of the three filters.

Places where i find out about these sorts of things are quantum physics textbooks or any book on photonics.

Woody

EDIT: In terms of communications, you will know that you can block interference by polarising the EM waves that are being transmitted.

 

[dervast]

horizontal and vertical

Hmm in a wireless course in university teacher used the terms of horizontal and vertical polarization of a signal.. When do we use horizontal and when do we use vertical polarization .. What the main difference between them?

 

[armandowww]

try on wikipedia... then look at this applet: http://www.humuku.de/html/education/software.html
;-)

 

[woodsy2k]

Nothing except the angle of polarisation. A vertical polarisation will have the Electric field component oscillation from top to bottom, and the horizontal polarisation will oscillate from left to right and so on. I'd imagin some are used for different terrains perhaps, as in a particular polarisation state might travel over land better for example. I am not sure on that to be honest. My physics degree doesn't help with the uses, it simply informs me how it works.

 

[BobG]

Hmm in a wireless course in university teacher used the terms of horizontal and vertical polarization of a signal.. When do we use horizontal and when do we use vertical polarization .. What the main difference between them?

There is no difference between the two polarizations except the polarization is 90 degrees apart. If you line up your antenna to receive one signal at its maximum power, you'll receive none of the other polarization.

The advantage to this is that it allows you to squeeze analog frequencies closer together by alternating polarization. If you squeezed two horizontally polarized signals too close together, they would begin to interfere with each other, while vertical polarization would reduce the amount of separation in the frequency that would be needed.

The disadvantage is that your antenna can the only receive half of the signals available, if you need them at full strength. You can work around that by rotating the feedhorn around to receive both polarizations, but at less power since the antenna's actually misaligned for all of the signals. Of course, that also means you have to deal with the signals being very close together, increasing the chance of interference between them.

Another possible use of horizontal/vertical polarization is for duplex antennas that both transmit and receive. You don't want the signal you're transmitting to interfere with the signal you're receiving. Uplink channels are usually far removed from downlink channels within the spectrum. Having different polarization of the uplink/downlink channels would also help performance (I've never worked with antennas that did this with horizontal/vertical polarization, but it is very common to reverse the polarization for circularly polarized signals - CCW for one/CW for the other).

 

[RandallB]

These oscilate about an axis that is parallel to the direction of propagation, and they oscilate at 90 degrees to each other.

I think you mean to say both E and M oscilate pepindicular to direction of propagation here.
Like the way waves propagate in water, and not the way sound waves propagate though air.

 

[dervast]

Thx a lot.. You have really helped me a lot..
so polarization is used to remove interference.. :)
I want to ask if we can use other type of polarization except from horizontal and vertical.. I pressume no because there will be interference to vertical and horizontal if a signal is polarized to 45degrees for example...
I also want to know what is feedhorn
Do u know how can i understand from a receiver what kind of signals detect?
For example in Greece radio receivers are vertical i think... Do u know about television receivers... can u understand from the shape if it works for horizontal or vertical polarized signals http://www.sttsgroup.gr/antenna_3.jpg ?
Finally i can understand how polarization protects from interference in specific applications.. For example the tv in greece works on Mhz if all the signals are horizontally polarized then the interference still exists... right?
Finall what is feedhorn?
Thx a lot

 

[dervast]

Also i can't really understand how the recommended applet works
http://humuku.dnsalias.net/humuku/html/education/downloads/java/polarisation/index.html
What are the red and green lines? Are these the Pounting vectors? Or the electronic and mangetic lines?

 

[armandowww]

You could refer to the two linearly polarized light beams as s-wave and p-wave. It is only a choice saying that the former is the red one and the latter the green. But what lines represent is electrical field. It is allways a convention because magnetic field should be right too. Poynting vector should take a wrong location in this subject.