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I am thinking of let us say AM or FM radio being transmitted by a vertical antenna. Would the they be polarized vertically or horizontally and wouldn't the angle of the receiving antenna then determine the strength of the signal.

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    \$\begingroup\$ Hm. Is light naturally polarized? I think it is the function of how it is produced... \$\endgroup\$ – Eugene Sh. Jun 30 '17 at 21:23
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    \$\begingroup\$ @EugeneSh. natural light is usually pretty un-polarized, which is why a polarizing filter doesn't black out the whole world if you turn it the wrong way. Specular reflection selectively polarizes it, which is why a polarizing filter cuts "glare". You likely already know that, but there — rhetorical question, answered! \$\endgroup\$ – hobbs Jul 1 '17 at 5:09
  • \$\begingroup\$ @hobbs is right - natural light is all variations of polarity. Polarized sun glasses are always vertical, because when light (or any EM wave) bounces off water it reflects mostly just the horizontal component. (Feynman's QED explains why - it is all about those little clocks in each photon - but that's a subject for Physics SE) So in our experience, using vertically-polarized lenses blocks out most of that horizontally-polarized light, thus cutting glare. \$\endgroup\$ – SDsolar Jul 1 '17 at 20:32
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Radio waves emitted by an antenna have a specific polarization, and receiving antennas are generally sensitive only to a specific polarization. So in principle if the transmit antenna were strictly vertical and your receive antenna were strictly horizontal, you would receive nothing. But there are a couple of complexities:

  • Partially-aligned linear radio antennas can receive each other with modest losses.
  • A circularly polarized antenna can receive any linear polarization with modestly reduced efficiency, and vice versa.
  • Short-wave signals are generally received after bouncing off the ionosphere, which randomizes the polarization. Similarly, Wi-Fi and other 2.4/5 GHz signals are often bounced off buildings or walls, which tends to randomize the polarization.
  • Signals that are not narrow-band can have complex mixtures of polarizations, and polarization can change very rapidly with time.

The key difference between radio waves and visible light is that most of the radio signals we are familiar with are produced by coherent emission processes, which (usually) produce fully-polarized radio waves. More, almost all detectors of radio waves coherently detect just one polarization; radio astronomers usually use pairs of crossed dipoles so we can record both polarizations and reconstruct the input signal's polarization state.

Most of the visible light sources we deal with are incoherent and produce unpolarized light (an even mixture of polarizations) and our detectors mostly aren't sensitive to polarization anyway. Lasers are coherent and indeed are polarized, but unless the laser is designed to have a stable polarization, you tend to get random jumping around on very short time scales, averaging out to unpolarized. The human eye is in fact very slightly sensitive to polarization, though we don't usually pay attention, and there are processes - like reflection - that readily add polarization to light, hence the utility of polarized sunglasses (to preferentially block light reflected off horizontal surfaces).

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  • \$\begingroup\$ If I recall correctly: In radio applications, the cross-polarizaton penalty (horizontal to vertical or the other way around; 90° off) is about 20 dB in practice. Going from linear to circular or the other way around loses about 3 dB, regardless of the exact polarizations involved. Opposite circular polarization (left-hand circular to right-hand circular, or the other way around) loses about 20 dB. \$\endgroup\$ – a CVn Jul 1 '17 at 21:13
  • \$\begingroup\$ I'm not sure where the figure 20 dB comes from - I'm not an engineer at all, so to my more-theoretical mind, exact misalignment produces a complete failure to detect anything. The factor should be something like the square of the cosine of the angle between linear polarizations. But of course this is for ideal antennas with perfect polarization purity. Maybe 20 dB is a realistic number based on practical experience? \$\endgroup\$ – user2475529 Jul 2 '17 at 10:24
  • \$\begingroup\$ Like I said, if I recall correctly and all that. I don't have any citation handy for those numbers, but they are pretty widely circulated rules of thumb in amateur radio (for whatever that means). Still, -20 dB means you are only getting 1% of the signal strength you would if the polarizations matched; in my very personal experience, that's a reasonable value in practice. (It's still possible to pick up a reasonably strong signal even with cross-polarization, but it is much weaker than if you get the polarization right.) \$\endgroup\$ – a CVn Jul 2 '17 at 17:22
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Radio waves and light are both electromagnetic waves. The only difference between them is the wavelength.

The polarization is initially determined by the way they are produced

FM Broadcast radio is generally transmitted by circularly-polarized antennas in order to accommodate receivers in most any orientation.

AM broadcast towers are vertically-polarized.

The propagation tends to be by ground waves so retain their polarization for the most part. You may remember the old transistor radios used ferrite loop antennas that had a particular polarization, and you could move them around to find the best angle for reception at the best strength for the receiver's circuitry.

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    \$\begingroup\$ So. If radio waves are polarised, is there an equivalent radio polarising filter that will 100% block them by turning it through 90 deg? \$\endgroup\$ – Paul Uszak Jul 1 '17 at 0:37
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    \$\begingroup\$ That depends on a lot of things. If you are exactly 90 degrees you will get the minimum, yes. But if the waves have undergone rotations (like bouncing off the atmosphere or water) then they change. Water will always turn them horizontal. The sky (ionosphere) can do all sorts of things to them. \$\endgroup\$ – SDsolar Jul 1 '17 at 2:43
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The polarization of the emitted wave is a result of the antenna design.

Typically, antennas that primarily extent in vertical direction have vertical polarization. However, it's often hard to see the actual antenna inside the protective cases they come in, so, if you see an antenna thing that higher than wide, please don't assume vertical polarization. It might just be one case with many stacked smaller antennas inside, or a loop, or something else completely.

And yes, the polarization of the receiving antenna must match the polarization of the wave, or else you get worse reception.

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