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Cross polarization lobes

 
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Ali
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Joined: 21 Jun 2015
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PostPosted: Sun Jun 21, 2015 10:29 am    Post subject: Cross polarization lobes Reply with quote

Hi,
I want to ask why the cross polarization lobes are formed in antenna radiation pattern especially in parabolic antennas??
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curlew
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Joined: 12 Jul 2015
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PostPosted: Sun Jul 12, 2015 9:20 pm    Post subject: Re: Cross polarization lobes Reply with quote

Ali wrote:
Hi,
I want to ask why the cross polarization lobes are formed in antenna radiation pattern especially in parabolic antennas??

If the dish is very deep, with low f/D, then if it was an optical mirror it would distort the image. Parts of the image away from the centre would look curved. This is called spherical aberration. If we use, say, a dipole feed, the image of the dipole in the reflector is similarly distorted, and in fact the current flow in the reflector follows curves like lines of longitude. This means that current is not only flowing H or V, as wanted, but the curvature means that there are current components in the other plane of polarisation. On axis, the radiation from these cross polar currents cancels, but off axis in the 45 degree planes it does not. The resulting lobes, which may only be 20dB or less than the main lobe, are called Condon lobes. They were of great importance 40 years ago when microwave links were important.
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