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nurul Antenna Theory Regular
Joined: 26 Apr 2011 Posts: 23

Posted: Wed May 04, 2011 12:19 pm Post subject: antenna polarization 


Hello,
how can we know the polarization of an antenna?
how to read / translate an axial ratio result of an antenna? there are some articles that make a plot of axial ratio vs frequency and if the ratio is less than 2dB it is said to be Circular polarized. but it the result is in polar form? how can we translate it?
thanks a lot for the reply.
Nurul 

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Schubert Antenna Wizard
Joined: 08 Apr 2009 Posts: 161

Posted: Wed May 04, 2011 10:30 pm Post subject: 


The best way to understand the polarization is to measure the antenna in an anechoic chamber. Or you can use simulation software and model it. Or if you understand the radiation mechanism you can figure it out that way.
Axial ratio is the ratio of two orthogonal Efield components. Axial ratio of 1 (or 0 dB) is a requirement for perfect circular polarization, but does not guarantee circular polarization (the components must all be 90 degrees out of phase with respect to time).
The "2 dB" rule is arbitrary. Someone else could say "close enough to CP" for 3 dB or 1 dB or whatever they feel like that day.
An axial ratio of 3 dB doesn't tell you which component of the Efield is stronger (that is, horizontal or vertical polarization). Hence, you can't tell a whole lot about the polarization from just axial ratio. A polar plot (versus theta or phi I assume) gives the axial ratio as a function of direction from the antenna.
Note that the polarization of an antenna is typically defined in the direction of peak radiation  the polarization generally varies for an arbitrary antenna as a function of direction from the antenna. 

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nurul Antenna Theory Regular
Joined: 26 Apr 2011 Posts: 23

Posted: Thu May 05, 2011 10:24 am Post subject: 


Schubert,
Thanks.
I modeled a square patch in CST design studio. Then, I truncate 2 of the edges to get them to be circularly polarized (this is a method to get CP out of a square patch, correct?  btw, how can this edge cutting change the antenna polarization from linear to circ?).
then, in order to check the axial ratio (as suggested), I chose the post processing results and plot the efield properties vs frequency. and if it is CP the value should be 1 or near to 1 or the 2dB/3dB cutoff limit. is this the right method?
'A polar plot (versus theta or phi I assume) gives the axial ratio as a function of direction from the antenna'  how to do this? is it polar plot at theta = 0 then versus polar plot at theta = 90? if the radiation matched  they are CP otherwise not?
thanks again for the reply. 

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bigSteve Antenna Wizard
Joined: 14 Mar 2009 Posts: 265

Posted: Fri May 06, 2011 8:06 pm Post subject: 


So...an axial ratio doesn't really tell you that you have circular polarization. It just says the Etheta and Ephi magnitudes are roughly in line. Hence, if you took a linearly polarized dipole (polarized along theta), and rotated it 45 degrees, it would be Etheta and Ephi polarized with an axial ratio of 1 (0 dB). However, this certainly does not mean the antenna is circularly polarized.
As for making a patch antenna CP, it is not easy. Basically you need to excite the two orthogonal modes  one along the length of the patch and the other along the width. The tricky part is that they need to be also out of phase by 90 degrees with respect to time. To do this you need the feed carefully chosen such that the relative impedance of the two modes gives the correct time delay.
CP patch antennas are typically only circularly polarized over a very narrow beamwidth and bandwidth. It isn't easy to do. 

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nurul Antenna Theory Regular
Joined: 26 Apr 2011 Posts: 23

Posted: Mon May 09, 2011 10:51 am Post subject: 


Thanks bigSteve.
for a patch antenna, can we know/predict its polarization by analyzing the current flow?
I see many papers on polarization include these (current flow diagrams) but I don't know what are they looking at
and thanks a lot for the explainations. 

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