Unpolarized Beams
Also does anyone have an idea how to obtain or assign unpolarized excitation in MWS?
Hi, irfan!
Could you please explain whats "unpolarised" means?
I'm bit confused, EM field has vector character thus E field should have always a direction..
Tnx
eirp
Actually Randomly polarized would be more appropriate term. Lets take a plane in the direction of propagation. And let E -Field be parallel to this plane. Unpolarized or randomly polarized means that direction of E-field at any point in this plane is random. I hope this clarifies my point.
Hi irfan1,
This is somewhat confusing: any electromagnetic wave propagating in a certain direction does have a fixed polarization. Everything else would be non physical. The polarization is just the E-field vector and the E-field vector can not just change its direction without interaction with any material?
F.
The only condition for a plane wave is E-field,H-field, and propagation vector to form an orthogonal coordinate system. So I beleive the situation that I described does not violate this condition.
Hi, irfan1:
I post some basic definition about the polarization from balanis "Antenna Theory".
Polarization of an antenna in a given direction is defined as ?the polarization of the wave transmitted (radiated) by the antenna. Note: When the direction is not stated, the polarization is taken to be the polarization in the direction of maximum gain.? In practice, polarization of the radiated energy varies with the direction from the center of the antenna, so that different parts of the pattern may have different polarizations.
Polarization of a radiated wave is defined as ?that property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric-field vector, specially, the figure traced as a function of time by the extremity of the vector at a fixed location of space, and the sense in which it is traced, as observed along the direction of propagation.? Polarization then is the curve traced by the end point of the arrow representing the instantaneous electric field. The field must be observed along the direction of propagation.
The polarization of a wave can be defined in terms of a wave radiated (transmitted) or received by an antenna in a given direction. The polarization of a wave radiated by an antenna in a specified direction at a point in the far field is defined as the ?the polarization of the (locally) plane wave which is used to represent the radiated wave at that point. At any point in the far field of an antenna the radiated wave can be represented by a plane wave whose electric field strength is the same as that of the wave and whose direction of propagation is in the radial direction from the antenna. As the radial distance approaches infinity, the radius of curvature of the radiated wave?s sphere front also approaches infinity and thus in any specified direction the wave appears locally as a plane wave.? This is a far-field characteristic of waves radiated by all practical antenna. The polarization of a wave received by an antenna is defined as the ?polarization of a plane wave, incident from a given direction and having a given power flux density, which results in maximum available power at the antenna terminals.?
In general, the polarization characteristic of an antenna can be represented by its polarization pattern whose definition is ?the spatial distribution of the polarizations of field vector excited (radiated) by an antenna taken over its radiation sphere. When describing the polarizations over the radiation sphere, or portion of it, reference lines shall be specified over the sphere, in order to measure the tilt angles of the polarization ellipses and the direction of polarization for linear polarization. An obvious choice, though by no means the only one, is a family of lines tangent at each point on the sphere to either the θ or φ coordinate line associated with a spherical coordinate system of the radiation sphere. At each point on the radiation sphere the polarization is usually resolved into a pair of orthogonal polarizations, the co-polarization and cross-polarization. To accomplish this, the co-polarization must be specified at each point on the radiation sphere.?
?For certain linearly polarized antennas, it is common practice to define the co-polarization in the following manner: First specify the orientation of the co-polar electric field vector at a pole of the radiation sphere. Then, for all other directions of interest (points on the radiation sphere), require that the angle that the co-polar electric field vector makes with each great circle line through the pole remain constant over that circle, the angle being that at the pole.?
?In practice, the axis of the antenna?s main beam should be directed along the polar axis of the radiation sphere. The antenna is then appropriately oriented about the this axis to align the direction of its polarization with that of the defined co-polarization at the pole.?
Best Regards,
two more cents about the "random polarization".
We think about the far field of any antenna, if the observation far from the antenna enough, we can treat is as plane wave, the principle of the EM wave propagating is the engery transformation between E and H. As my understanding,
one the starting postion and the properties of the EM wave, and all the surrounding boundary condition are known , it must follow the Maxwell's Equation to propagate no matter in what kind of medium. So I can say the polarization is a deterministic properties.
Any way, even if we can imagin such kind of beam, the polarization pattern is random, then what kind of application can be provided by that? According to the transimitter, we must have receiver to retrive back the signal, then pls consider the polarization mismatch between transimitter and receiver. Assume tranmitter is random polarized, how can you make the receiver to follow its' polarization pattern to retrieve back signal?
Best Regards,
Actually this is the point. Lets say you can make a device which does not care about the polarization of the incoming beam but still can measure it someway.
Hi Irfan1 ,
I still don't get it
This is correct. However, this condition is not a "local" effect. This condition has not only be fulfilled at a particular point, but for the entire plane surrounding this point. All points are connected via the Integral Equations. Just have a look at Gauss?s Laws. The E-field integrated over a close surface in free space (no charge) has to be zero. This has to hold for any closed surface. For a ?random? E-field, which changes its direction from point to point, this condition certain does not hold!
In optics, an "un-polarized Beam" is a beam who randomly changes its polarization in time. Maybe this it what you want?
F.
that would do I guess
Hi again,
In this case it sounds more like a statistic investigation. At any point in time, the incident plane wave has a certain state of polarisation (circular or linear or something in between...) A random polarized beam will then change its polarization state with time. There will never be an abrupt change in the polarization. Instead this change happens slowly over several oscillation periods and therefore at a much smaller frequency then the wave itself.
My first approached to this would be, to run 2 simulations with a perpendicular polarisation. As a post processing step, one could then superimpose the simulation results of both simulations with a ?random? amplitude and phase. I guess one could then do some statistics on the results.
Just my thoughts, any comments are welcome..
F.
thank you very much RFSimulator. Your comment makes a nice starting point for the analysis.