I have no background knowledge about antennas. I am trying to intuitively understand how an antenna transmits and receives a circularly polarized (CP) Electro-Magnetic (EM) wave.

  1. From the knowledge, I could gain after searching a lot about this on the internet is we can assume a transmitting antenna to have a "wave transmitter", that rotates continuously and keeps transmitting signals as it rotates. Based on the direction of rotation we can either an LHCP EM wave or RHCP EM wave gets transmitted. Is this intuitive understanding correct?

    Please see this animation to get what I am trying to say:


    (Image source as used on the Wikipedia page: Circular polarization)

  2. If I assume the above understanding is right. I can think of a receiver antenna to be something similar - in this case, instead of a rotating wave transmitter I would have a "rotating wave receiver".

  3. If we assume 2 is correct (at least intuitively), then the wave receiver could be rotating in one direction (i.e. it can have one particular CP) and the incoming EM wave might have the same or the opposite CP. Now, how can we prove that for a wave receiver with given CP - only the incoming EM waves having one particular polarization (LHCP or RHCP) get received, but the other one doesn't?

Please correct me if my understanding is wrong. I would be happy if you can also share some links where I can get to understand the above material better.

Thank you!

  • 2
    \$\begingroup\$ the antenna does not rotate mechanically ... qsl.net/sv1bsx/antenna-pol/polarization.html \$\endgroup\$
    – jsotola
    Nov 15 '20 at 3:04
  • 1
    \$\begingroup\$ Assuming that one "rotation" is completed every wave and knowing the wave length and speed of light, you can calculate the rotation speed. I think it won't be possible to rotate an antenna mechanically that fast. \$\endgroup\$
    – AJN
    Nov 15 '20 at 4:28

I'll admit the Wikipedia explanation makes my head spin too. Let's keep the antennas still.

Think of a pair of crossed dipole antennas just above the earth's surface. One oriented North-South, the other East-West. Connect the same transmitter to both, but use an extra piece of transmission line so the current in the E-W dipole is 90 degrees delayed relative to the N-S dipole. Presto, you have a circularly polarized transmitting antenna. Which polarization? Looking down onto the antenna, the signal is rotating from North to East (clockwise), but looking upward in the direction of propagation, it's counterclockwise or left-hand circularly polarized.

Now think of a geostationary satellite directly overhead, with a similar pair of dipoles oriented N-S and E-W (at this instant, anyway). To get the signals from the two dipoles to add up in phase at the receiver, you'll need that same extra piece of transmission line, but now it has to delay the N-S dipole relative to the E-W dipole to cancel out the delay. Otherwise, the signals will be 180 degrees out of phase. And that's why the antenna system would 'reject' opposite-circularly polarized signals.

While this sounds like the satellite antenna has the opposite circular polarization, that's not the case, because the direction of transmission/reception is downward, not upward. In that direction, it's the same left-hand circular polarization as the ground antenna.

I think...


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