Polarization : Linear vs. Circular
An important factor in the decision to purchase satellite capacity is what polarization will be used. The preference for polarization is typically low on the preference scale, coming after bandwidth, power and price. Satellites are typically either polarized linearly or circularly. In the coming paragraphs, we will detail both of these approaches and discuss advantages of both.
Linear Polarization is defined as polarization of an electromagnetic wave in which the electromagnetic radiation is confined to a given plane along the direction of propagation. Satellite typically utilizes either Vertical or Horizontal Linear Polarization. In Vertical Linear Polarized signals, the electric field is perpendicular to the Earth's surface. In Horizontal Linear Polarized signals, the electric field is parallel to the Earth's surface.
Figure 1 : Linear Polarization
Circular polarization is defined as a polarization of an electromagnetic wave in which at each point, the electric field of the wave has a constant magnitude but its direction rotates with time at a steady rate in a plane perpendicular to the direction of the wave. . A circularly polarized signal (or wave) can be either right circular polarization where the electric field vector rotates in a right-hand sense with respect to the direction of propagation, or left circular polarization where the vector rotates in a left-hand sense.
Figure 2 : Circular Polarization
Switching from circular to linear polarization is done by replacing the feed of the antenna. There is a price difference and depending on the antenna, it is almost negligible, typically $200.00 in the case of a VSAT antenna. In the case of an earth station, the difference is more but that is offset by circular polarity's advantages.
Operationally, the feed horn separates the two polarities of a given signal removing unwanted signals from the opposite polarity. For a linear polarized feed horn, the feed must be precisely aligned to the exact satellite's signals. Circular feed horns do not have this requirement.
The Faraday effect
The Faraday deals with the interaction between light and magnetic fields. It affects linear, but not circular, polarized signals and the effects are more severe at lower frequencies such as C-Band and not noticeable at higher ones, such as Ku-Band. As signals pass through the atmosphere, they become de-polarized causing undesirable reception of the opposing polarity. Linear Polarized feeds are aligned in such a way to compensate for the Faraday effect and with the help of a tracking device, corrections can be made by rotating the feed system or using adjustable polarizers within the feed system. This can be time consuming because the alignment must be exact. The result of incorrect alignment is increased interference. Circular polarity does not suffer from the Faraday effect since there is no need for exact signal alignment, and Ku-Band is at a high enough frequency that it is not affected by the Faraday effect.
Figure 3 : The Faraday Effect
Circular polarization is more resistant to signal degra- dation 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. The effect of a high frequency signal passing through rain can cause signal attenu- ation 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).
Ease of Installation
Circular polarization is easier to install since there is no need to adjust the polarity on the feed. The only requirement is ensuring that the antenna is aimed in the correct direction on the satellite; simply point and transmit. This allows for circular feeds to be set up quicker, and there is less of a risk of being misaligned.
Circular Polarized links typically have a 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.