AboutLuis Emiliani Expertise Strengths
- Satellite link design and analysis
- Vsat network design and analysis
- Propagation aspects for terrestrial and satellite links
- ITU-R P recommendations
Can not help with:
- Questions related to TV receivers, TV kits, or specific networks such as DirecTV or DISH
- Antenna pattern synthesis, RF MoM analysis, S-parameters.
Experience My experience is in satellite communication networks and terrestrial microwave links.
Expert: Luis Emiliani Date: 6/22/2008 Subject: polarisation
Question What is the exact definition of polarisation ?
Which polarisation is better ? either linearly , circular or elliptical & why? Where it is used either in ku band or c band ? what is the effect of transmiting the RF FWD PWR by using these polarisation ?
What is the range of various polarisation in degree ?
Answer Hi,
The polarisation of an electromagnetic wave describes the position of the tip of the electric field vector, as a function of time, as it propagates. it can be seen as the inclination of the tip of the electric field vector of the electromagnetic wave with respect to a horizontal line (0 degrees) including the direction of propagation.
the selection of a particular polarization for a satellite link depends on the effect that the ionosphere and troposphere will have on the propagating wave.
If you are working in frequencies between say 30 MHz and 3 GHz, the ionosphere will cause the wave to change its polarization as it propagates, a phenomenon known as faraday rotation. To minimize the impact of said rotation, it is recommended to use circular polarization (Left hand or right hand)
faraday rotation stops being a big problem after say more or less 4 GHz, and other phenomena such as scintillation and rain attenuation begin to be more important. besides attenuation, rain drops and ice also cause depolarization, so it is recommended to select a polarization that minimizes the impact of these two phenomena.
in C band you normally use circular whilst in Ku band you would go for linear polarizations.
anything in between is an elliptical polarization (technically speaking, all polarizations are elliptic, and only the phase between the propagating fields dictates which particular case you will have)
so you can't really say a a polarization is better than another, you can say a polarization is better suited than another for your particular link...
you have to match the antenna/feed/waveguide to the polarization selected for the link. one that is fixed you can not change polarization (from linear to circular) without changing the assembly..you might use adapters from linear to circular and vice versa, but you must change the antenna feed..
