Why are some PtP antennas and radio units connected by two feed lines -- one for horizontal polarization and one for vertical polarization?

I know the difference is in the orientation of the electric field, but why do we transmit/receive using two polarities?

Does it have something to do with MIMO?

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    \$\begingroup\$ A link to the source of your question is really needed. \$\endgroup\$ – Andy aka Nov 12 '20 at 11:42
  • \$\begingroup\$ Andy aka please see Mimosa, Ubiquiti, Cambium networks. \$\endgroup\$ – Noob_Guy Nov 12 '20 at 13:06
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    \$\begingroup\$ I'll put it on my Christmas list. \$\endgroup\$ – Andy aka Nov 12 '20 at 13:54
  • \$\begingroup\$ Andy aka you are asking for source??? Are you not familiar with wireless telecoms?? \$\endgroup\$ – Noob_Guy Nov 15 '20 at 13:35

Does it have something to do with MIMO?

Yes, it could do. MIMO (multiple-input multiple-output) can use different polarizations to transmit two data streams in the same space. Lowest link loss occurs when the transmitting and receiving antennas are aligned to the same polarization. At 90° the signal strength is reduced (theoretically to nothing, but in practice by a bit less than 20 dB) so the two signals can easily be distinguished even when on the same frequency.

Another reason for using two antennas is simply to relax the need for a specific orientation. This is commonly done in portable devices and other things that cannot maintain a fixed antenna orientation (eg. drones). In this case the same signal is sent through both antennas, and the receiver uses whichever signal is best.

  • \$\begingroup\$ Bruce Abbott no, not two antennas. Only one antenna (e.g. dish) but with two feed lines; one labeled "Horizontal Pol", and one labeled "Vertical Pol". \$\endgroup\$ – Noob_Guy Nov 15 '20 at 13:39
  • \$\begingroup\$ @Noob_Guy Each line is connected to a different antenna. If they plugged into the same antenna there would be only one polarization. \$\endgroup\$ – user1850479 Nov 15 '20 at 13:57
  • \$\begingroup\$ @user1850479 you mean only one feed line is used at a time? \$\endgroup\$ – Noob_Guy Nov 16 '20 at 2:26
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    \$\begingroup\$ Yes. en.wikipedia.org/wiki/Parabolic_antenna#Polarization "To increase the data rate, some parabolic antennas transmit two separate radio channels on the same frequency with orthogonal polarizations, using separate feed antennas; this is called a dual polarization antenna." \$\endgroup\$ – Bruce Abbott Nov 17 '20 at 2:11
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    \$\begingroup\$ Look closely at the RD-3G26 antenna feed and you will see the two connections are at right angles. This suggests that there are two feed antennas inside the tube, at right angles to each other. Other MIMO antennas consist of flat plates or separate Yagi style antennas eg. iskra.eu/en/MIMO/Mimo-Antenna-P-30-UNICOM \$\endgroup\$ – Bruce Abbott Nov 17 '20 at 2:27

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