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Active input impedance

 
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the_unforgiven
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Joined: 23 Jun 2009
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PostPosted: Tue Jun 23, 2009 9:28 am    Post subject: Active input impedance Reply with quote

Hi all, i have a question fou you.
I need a formulation for the antenna input active impedance for a simple quadripole....

i mean when the two port are simultaneously feeded...

sorry for my english Smile

Thank you
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admin
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Joined: 03 Jan 2007
Posts: 210

PostPosted: Tue Jun 23, 2009 10:30 pm    Post subject: Quadrupole Antenna Reply with quote

I believe the quadrupole antenna is the same as the crossed dipole antenna or "crossed dipoles". I assume you mean basically 4 monopole antennas forming a plus sign (+) in this analysis.

For half-wave dipoles, the impedance is roughly 72 Ohms if they are shortened to be resonant at about 0.49 lambda. If you feed two orthogonal dipoles with the same source, that is basically having two 72 Ohm antennas in parallel, so that the input impedance would be roughly 36 Ohms.

To first order, the antennas are orthogonally polarized (or completely mismatched in terms of polarization - 1 dipole is vertical and the other dipole is horizontal). So from a far-field perspective, they won't influence each other to a huge degree.

That said, I am not sure what the exact solution would be. For a back of the envelope calculation, use Zactive = Z_dipole/2
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