Circular Polarization vs. Linear Polarization


Circular Polarization vs. Linear Polarization

Circular Polarization vs. Linear Polarization

This is an analysis of circular and linear polarization, tryn to avoid confusion and misconceptions surrounding these forms of technology.

An antenna is a transducer that converts radio frequency (RF) electric current to electromagnetic waves that are then radiated into space. For C, Ku, Ka bands the antenna designed as a horn.

Polarization

the polarization in general. Electromagnetic wave is a combination of electric and magnetic fields. They always appear simultaneously. The electric field vector is perpendicular to magnetic field vector and they are both perpendicular to the direction of wave travel.

The direction of electric field vector specifies polarization of the antenna.

Polarization Types

Linear Polarization

From a technical perspective, linear polarization is defined as polarization of an electromagnetic wave in which the electric vector at a fixed point in space remains pointing in a fixed direction, although varying in magnitude. there is no phase shift between electric vector and magnetic vector,

This polarization is vertical or horizontal depending on the orientation of the electric vector with respect to the  the Earth’s surface ( for earth surface transmission) or the equator plane (for Space-earth trnasmissions).

There are two forms of linear polarization:

vertical,   by vertically linear polarized antennas., where the electric field vector of EM(electro-magnetic) wave is perpendicular to the Earth’s surface, example of a vertical antenna is a broadcast tower for AM radio or the whip antenna on an automobile

horizontal, by Horizontally linear polarized antennas. where the electric field  vector of EM(electro-magnetic) wave is parallel to the Earth’s surface. Example. UHF/VHF television transmissions in theUSA use horizontal polarization. Thus, TV antennas are horizontally-oriented.

For an effective communication link, both transmitting and receiving antennas should be in the same polarization, and the implementation  of polarization

 Both directions can be used simultaneously on the same frequency, allowing  the frequency re-use.

Figure 1: Linear Polarization

Circular Polarization

Technically speaking, circular polarization ( In a circularly-polarized antenna) involves the plane of polarization rotating in a corkscrew pattern, making one complete revolution during each wavelength. The circularly polarized wave will radiate energy in the horizontal plane and vertical plane, as well as every plane in between with a phase shift of  ±90° . The  90° shift (positive or negative) means that when electric field reaches .its maximum, the magnetic field is equal zero and vice versa.

There are two directions of propagation that come with circular polarization:Depending on the sign before 90°, we have either right hand circular polarization (RHCP) or left hand circular polarization (LHCP).

Right-Hand-Circular (RHC) which follows a clockwise pattern, and

Left- Hand-Circular (LHC) which follows a counterclockwise pattern.

As with linear polarization, both directions can be used simultaneously on the same frequency, allowing higher revenue generation through the doubling of capacity on the satellites.

Theoretically, if we have other values of phase shift (neither 0/180° nor ±90°), we have elliptical polarization, but this is not used in satellite transmission.

Figure 2: Circular Polarization .  The perpendicular planes Y,X,Z . The Electric field (E) and magnetic field (H) are Perpendicular. Two E waves (from Horizontal and Vertical Polarized antennas) are generated perpendicular but de-phased in  90°.The resultant  wave is a circular polarizated.

 

Why Linear or Circular Polarization Antenna?

Advantages of Linear or Circular Polarization

There are several key advantages for  circular polarization over linear polarization.

*Easier installation

  as signals pass through the atmosphere they become depolarized, causing undesirable reception of the opposing polarity.  One result of incorrect alignment is increased interference.

For circular feeds  no need for exact signal alignment. The only requirement is ensuring that the  antenna is aimed in the correct direction on the satellite; simply  point and transmit. This allows to  be set up quicker, and there is less of a risk of being misaligned.   For linear feeds,. Linear polarized feeds are aligned in such a way to compensate for the Faraday effect, usually with the help of a tracking device; corrections can be made either by rotating the feed system or using adjustable polarizer’s within the feed system. This can be very time consuming because the alignment must be exact.

Feed horns (The LNBF) :  In the receiver  Operationally, the feed horn conveys radio waves between the transceiver and the reflector. The feed horn also separates the two polarities (vertical from horizontal or left from right hand) of the signal being received. This removes unwanted signals, (for example, left hand circular), from the desired signal, (for example, right hand circular) of a given frequency.

for the linear polarization signals , It is generally easier to manufacture  with a  good performance, to properly separate the polarizations, it must be precisely rotated to the exact alignment with the satellite’s signals (adjust the skew of LNBF depending on your geographical location).

Switching from linear polarization to circular polarization simply requires changing the feed horn that is mounted at the antenna’s focal point

A circular feed horn However , one of the well known disadvantages is the necessity to The LNBF for the circular polarization is more difficult to manufacture and then become it more expensive, (That’s why the majority of Ku-band LNBF use linear polarization) but as advantage Is that with this is not needed  adjust the skew of the in the dish focal point.

 *Feed Horn cost : The price difference can be almost negligible. For example, a low cost VSAT antenna with two port feeds is a mere $200 more than a linear feed horn. However, in some cases the feed horn can be significantly more expensive for circular polarization, an investment which will be at least partially offset by

* The dish dimension: The diameter of the disk in the receiver decrease with the frequency use. For C-Band  is more large than Ku-band. This factor can be highly inportart  from the requirements of  the user equipments. The size may influence on the band selection and the band selection ans others factor in the polarization type.

*Atmospheric Conditions:  attenuation

The effect of a high frequency  signal passing through rain can cause  signal attenuation and accounts for the majority of the problems  with rain fade. Moisture laden clouds are also a factor;  by the time a signal passes through a cloud system it  can be attenuated by as much as 1dB. Water droplets  on the feed horn may also cause detrimental effects.  However, the most important  aspect to note is that  higher frequencies (like Ku-Band) degrade faster,  harder, and longer than their frequency counterparts  (C-Band).

Inclement Weather: Rain and snow cause a microcosm of conditions  (i.e. reflectivity, absorption, phasing, multi-path and line of sight) Circular polarization is more resistant to signal degradation due to inclement weather conditions for all the reason stated above.

Reflectivity: Radio signals are reflected or absorbed depending on the material they come in contact with.  linear polarized antennas: are able to “attack» the problem in only one plane, if the reflecting/absorbing surface does not reflect/absorb  the signal precisely in the same plane, that signal strength will be lost. circular polarized antennas send and receive in all planes, the signal strength is not lost, but is transferred to a different plane and are still utilized.

Absorption: As stated above, radio signal can be absorbed depending on the material they come in contact with. Different materials absorb the signal from different planes. As a result, circular polarized antennas give you a higher probability of a successful link because it is transmitting on all planes.

Phasing Issues:  linear polarization When it is used in High-frequency systems (i.e. 2.4 GHz and higher)  typically require a clear line-of-sight path between the two points in order to operate effectively. Such systems have difficulty penetrating obstructions due to reflected signals, which weaken the propagating signal. Reflected linear signals return to the propagating antenna in the opposite phase, thereby weakening the propagating signal. Conversely, circularly-polarized systems also incur reflected signals, but the reflected signal is returned in the opposite orientation, largely avoiding conflict with the propagating signal. The result is that circularly-polarized signals are much better at penetrating and bending around obstructions.

Multi-path: Multi-path is caused when the primary signal and the reflected signal reach a receiver at nearly the same time. This creates an «out of phase» problem. The receiving radio must spend its resources to distinguish, sort out, and process the proper signal, thus degrading performance and speed. Linear Polarized antennas are more susceptible to multi-path due to increase possibility of reflection. Out of phase radios can cause dead-spots, decreased throughput, distance issues and reduce overall performance in a 2.4 GHz system.

Line-of-Sight: When a line-of-sight path is impaired by light obstructions (i.e. foliage or small buildings), circular polarization is much more effective than linear polarization for establishing and maintaining communication link

*Atmospheric Conditions: rotation of the signal,

Circular polarization is more resistant to signal degradation due to atmospheric conditions. These conditions can cause changes in the rotation of the signal, and  will more adversely affect linear polarization than   circular polarization.

Higher link reliability

There is higher link reliability since there is a low risk of misalignment, and encountering interference.  Faraday’s effect will not affect transmission with circular C-band, so there will be no need to readjust the  alignment. Finally, because transmission is sent and/or  received at different frequencies, interference (cross  polarization) is less of a concern.

The Faraday effect

The Farraday’s rotation is caused by earth’s magnetic field and the atmosphere (the ionosphere acts like a plasma) charged with free electrons. Rotation of EM vectors (as by this  Faraday  effect)  deals with the interaction between transmitted electromagnetic signals  (including the light) and the magnetic fields. The effect is higher for receiver near the poles (higher latitudes) where the earth magnetic field is stronger. This may be even more important (have more significant effects) when we need to cover the areas close to the earth magnetic poles.

It is a result of a ferromagnetic resonances when the material permeability is a tensor, generating a doubling of waves in two circular polarized ones, doing a propagation at different velocities (circular birefringence), and  recombining  later when its arrive to the medium interface, producing a wave with a rotation in the polarization plane.

The direction of polarization rotation depends on the properties of the material through which the light is shone. A full treatment would have to take into account the effect of the external and radiationinduced fields on the wave function of the electrons, and then calculate the effect of this change on the refractive index of the material for each polarization, to see whether the right- or left-circular polarization is slowed more.

In a material, the electric field causes a force on the charged particles comprising the material (because of their low mass, the electrons are most heavily affected). The motion thus effected will be circular, and circularly moving charges will create their own (magnetic) field in addition to the external magnetic field.

There will thus be two different cases:

*the created field will be parallel to the external field for one (circular) polarization, and in the opposing direction for the other polarization direction – thus the net B field is enhanced in one direction and diminished in the opposite direction. This changes the dynamics of the interaction for each beam and one of the beams will be slowed more than the other, causing a phase difference between the left- and right-polarized beam.

*When the two beams are added after this phase shift, the result is again a linearly polarized beam, but with a rotation of the polarization vector.

It affects linear polarized signal , but has  no effect on circularly polarized signals.   It is a less known but probably more important factor is the sensitivity of linearly polarized signals

The effect decreases rapidly with the  frequency increment, that is,  the effects are appreciate more severe at lower frequencies, such as C-Band, and not noticeable, practically negligent at higher ones, such as Ku-Band. It is doing the use of  linear polarization in C-band rather risky.  Ku-Band is at a high enough frequency that Faraday’s effect is not a factor. In ground RF transmissions the reflections in the ionosphere from linear polarized antennas, is rather unpredictable, for 435 MHz (UHF) the rotation is about 1.5 complete rotations, and for 1.2GHz is clse the 1/4 of a complete rotation.

.

Figure 3: Faraday’s Effect: The Linear Polarized Signal Twists in transparent materials.

Where:, for transparent materials:   d: length of the path [m] of diamagnetic material,   B is the magnetic fulx density Fields [T] ,  E Transmitted electric field , ν :Verded Constant of Material [rad/(T.m)] where the polarized Electric Field with the magnetic field have interaction , β: Angle [rad] of Received electric  Field Rotation produced by thje faraday’s  effect.

β=ν.B.d

When it is depending of the free electrons , is determined by:

λ: wavelength
RM: is the Rotation measure in function os the following variables:
ne(s) is the density of electrons at each point s along the path.
B(s) is the component of the interstellar magnetic field in the direction of propagation at each point s along the path
e is the charge of an electron;
c is the speed of light in a vacuum;
m is the mass of an electron;
ϵ0  is the vacuum permittivity;

where

β=RM(ne(s),B(s),e,c,m,ϵ0) λ2
β= e3/2∙π∙m2c4).INT((0,d,ds)),ne.B λ2
β= e3/(8∙π2ϵ0m2c3)INT((0,d,ds),ne.B) λ2

IMPLEMENTATIONS.

 The most common forms of polarization utilized are linear polarization and circular polarization.

In the satellite chart, the majority of C-band transponders with circular polarization, while  Ku-band  operate in Linear or circular  but the majority of transponders and higher frequencies operate with linear polarization (vertical or horizontal), Ka-band can also be either: Linear or circular polarization

Once the decision to purchase satellite capacity or services is made, the ranking well behind bandwidth, power, and price in the decision process, however a determining factor  that  must be recognized  and considered  is that of frequency polarization  which usually have a preference relatively low.

Some customers, considering the  linear polarization to be superior, if only because the specific equipment costs are marginally less, also  feeling  that circular polarization is not as desirable as linear polarization, and that since Intelsat is the only satellite provider offering C-Band, the availability of antennas could become an issue.

Some customers consider Linear polarization can be found in both C-Band and Ku-Band (see Table 1).

However,  It is importand know the benefits or the price sensitivity towards equipment. The  circular polarization increased reliability in signal strength, resistance to weather conditions, and ease of installation outweigh the expense of the feed horn.

Covered Area: The providers decide what area they want to cover.

If the area of interest has a big chance of having bad weather condition (rain, snow) they would rather choose C-band which  is less sensitive to bad weather conditions than Ku-band.

If the is located at high latitudes (what means,  near the poles and with  longer paths through clouds), choose also  C-band which  is less sensitive to bad weather conditions than Ku-band, and  because the C-band is sensitive to the Faraday’s effect, the circular polarization is a better choice.

If the area is located in medium latitudes and the dish dimension is of major concern (like in big European cities), Ku-band would probably be the choice. Since we do not have to worry about the Faraday’s effect here, then the linear polarization will make it easier to provide the end users with high performance LNBF’s.

In general uses, the C-Band  is implemented with Circular polarization and  Linear polarization (but affected by the Farraday’s rotation) . The  Ku-Band  implemented  in circular polarization but mainly with linear  polarization.  The Intelsat  fleet  use only Linear polarization for the Ku-Band  (does not have any  with circular polarization) (see Table 1).

The polarization choice  is always about ensuring the highest reliability of reception.

 Availability on the Satellite provider Fleet

In some cases the user need follow the avialibility of the provider, who usually design the satellite according the more convenient conditions for transmition in the coverage areas. Circular polarized C-band is available on key roles on the Intelsat fleet.

Table 1: Breakdown of Polarization on Intelsat satellites

Circular Polarization:

C-Band: IS-601, IS-602, IS-603, IS-605, IS-701, IS-702, IS-704, IS-705, IS-706, IS-707, IS-709, IS-801, IS-802,  IS-901, IS-902, IS-903, IS-904, IS-905, IS-906, IS-907, IS-10-02

Ku-Band : None

Linear Polarization:

C-Band : G-3C, G-4R, G-9, G-10R, G-11, G-12, G-13, G-14, G-15, G-16, G-23, G-25, G-26, G-27, G-28,  Horizons 1, APR-1, IS-1R, IS-2, IS-3R, IS-5, IS-6B, IS-7, IS-8, IS-805, IS-9, IS-10 and IS-12

Ku-Band:  All Satellites

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The light and the Polarization in Cameras:

Other way to see more clear the effects of the polarization in our daily life is in the filters of the camera Lents.

What is Polarization?

Here it is the behaviour of light waves that are hitting your lens. Light waves travel in different ways and the easiest way to visualise this is to take a physical example. By taking a piece of string and attaching it at one end to something you can replicate waves. Move the string up and down and the light wave produced is vertically polarised. Move the string from side to side and the wave is horizontally polarised. These two are referred to as linear polarisation. Spinning the string around in a circle can lead to either right-handed or left-handed polarisation. this is referred to as circular polarisation.

The term polarisation basically means that light is oriented in one direction, making it predictable and not random.

Polarisers are one of the most misunderstood filters on the market and photographers often question the difference between the linear and circular versions.

There is a common misconception that ‘circular’ refers to the shape of the filter. This is in fact not the case as both circular and linear polarising filters can look exactly the same. Here we aim to give you a better understanding of polarising filters and what they do.

 Linear Polarizing Filters 

Linear polarizers are comprised of two elements that can be twisted to alter the direction of the light waves that are allowed through. So at one angle it will allow the passage of horizontal waves but when rotated by 90 degrees would only allow vertical waves.

This is ideal when wanting to suppress one direction of light more than the others. In reflections, especially from water and partially reflective surfaces such as windows there will be one direction of light that is stronger than the others and by suppressing that, you’ll lessen the amount of reflection vastly.

Polarizers are also known for darkening the sky by blocking light rays from the sun or certain polarisation and saturating colours by lessening the reflection of less reflective things like leaves. Atmospheric haze is also caused by scattered light and so a polarizer can help to lessen this too.

 Circular Polarising Filters

Circular polarisers are a little more complicated to explain. They are sensitive to both linear polarisation and the left and right-handed circular polarisation. A circular polarizer consists of two elements; a linear polarizer just like above, and a quarter wave plate which is stuck to the back of the linear polarizer with a specific orientation so that the light emerging from the quarter wave plate is circularly polarised. This is where the name comes from.

So, why do we need circular polarizers if the provide the same photographic results that a linear polariser would? Well, it’s all down to the camera kit you have. If you have a DSLR, then the likelihood is that it has a partially reflecting mirror. Because of this partial reflection, metering errors can occur when using linear polarizers with DSLRs that have partially reflecting mirrors. Using a circular polarizer can eliminate these issues.

Even if your camera doesn’t need a circular polarizer, you can still use on and it won’t make a difference to how it affects the image.

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file:///storage/emulated/0/Android/data/com.android.browser/pages/polarization2.pdf/index.mht

https://docs.google.com/gview?embedded=true&url=http://www.tele-audiovision.com/TELE-satellite-0611/eng/polarization.pdf

Circular or Linear Polarization. Peter Miller

https://docs.google.com/gview?embedded=true&url=http://www.intelsat.com/wp- ontent/uploads/2013/02/Polarization.pdf

http://www.intelsat.com/network.

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