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It is claimed in blogs and other unverified internet sources that circular polarized RF signals are better for penetrating structures, such as walls.

An example here, there are many others when you search. All of them unverified, or not clearly explained.

More scientific information and papers tend to be about optics and polarization of light. While comparable to a certain extent, it does not explain the claims in regards to RF, both Amateur bands, such as 70/23cm nor consumer WiFi.

A few years ago, I took those claims and built myself some RHCP "clover" antennas and helix antennas for home WiFi distribution. Living in a period house, with 3 ft thick walls, my signal attenuation declined from room to room, and therefore WiFi reception improved throughout the property. Practically confirming the claims made.

Over the years I have built various circular polarized antennas and experimented on many different frequencies (70cm, 23cm, as well as consumer WiFi)

Throughout time, I have shared my practical solution with many, and others have reported back similar results. Again practically confirming the claims.

None of this without any real explanation why this is.

However a few days ago someone asked me... why ?

And other than explaining E-planes, random reflection of building materials with "unknown properties", and the "chance" that "some" signal is not reflected, I could not explain this phenomenon clearly.

My questions:

  • Do circular polarized signals have better penetration properties compared to linear polarized signals?

  • and if so: why?

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Does circular polarized signals have better penetration properties compared to linear polarized signals?

In general, no. Maxwell's equations don't give us any indication why that should be the case.

At least, that is, for homogeneous media:

and if so: why?

If this is an observed phenomenon, then it might have to do with the structure of obstacles being penetrated: a vertical steel rebar might interact with vertically polarized waves stronger than with circularly polarized ones, for example.

I'd be a bit careful about comparing your home-made clover with the linear antennas that came with your WiFi-equipment:

  • Might simply be luck; the indoor WiFi scenario is one based on multipath propagation including reflections, and it's impossible to predict performance of two antennas in the same (linear) polarization plane. I could imagine that making a difference for wavelengths larger than the distance of steel armaments in concrete. I doubt it makes a lot of difference in WiFi – you don't need a lot of concrete to attenuate that by 60 dB, and whether there's appropriately polarized conductors inside that won't do much.
  • If we're talking about rooms with specular reflectors (e.g. sheet metal etc), we might get periodic deep fades in places where the reflected wave "meets" the original wave for linear polarization; not sure it happens in the same manner for circular; not an attenuation, but a resonator phenomenon if that's the case!
  • Might simply be that your antenna is better in the sense that it focuses more energy in the right direction, or is more efficient (I doubt efficiency makes a huge difference here). There's a lot of indications the directivity is the real difference here: the amount of angular range covered by a Wifi stick or patch antenna is definitely larger than that of any significantly helical helix antenna, and probably still larger than that of a clover antenna, so you might simply be covering less space with the same amount of energy. (potentially breaking EIRP regulations underway)
  • WiFi performance is often dominated by interference; since your interferers would be linearly polarized, they'd lose a couple dB. (but that usually won't make a huge difference). That effect would diminish if all WiFi devices were to use the same circular polarization
  • You're comparing SISO systems. Modern WiFi systems are often MISO or MIMO, and for these, diversity vs uniformity considerations might lead to a preference of linear antennas in orthogonal planes.
  • confirmation bias of people who've rebuilt your antennas and spent much work on that

That is not to say that your observation isn't right – I thoroughly believe it could be – but I could imagine that what you're observing for WiFi is more of a multipath/interferer environment and directivity effect than an absorption phenomenon.

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  • $\begingroup$ All valid points, including the last "bias" remark, which is why I posted the question to remediate "bias" including my own. Thank you for your answer, good info to consider. $\endgroup$ – Edwin van Mierlo May 30 '18 at 10:08
  • $\begingroup$ While you answered the question that there is no scientific formula which allows to answer the question, it is surprising that "through wall radar systems", such as this one is using circular polarized systems. Which could indicate that there possibly is a greater penetrating property of such signals. $\endgroup$ – Edwin van Mierlo May 31 '18 at 7:01
  • $\begingroup$ I didn't read the specifics of that system, but in object identification systems, polarization is often chosen for reflective properties of the target, or for considerations of not wanting to have two differently polarized receiver antennas (assuming it's rare that penetration and one-way reflection revert the direction of circular polarization) (this is a bit similar to the "in mimo applications, you might want two linearly polarized antennas", just the opposite argument) $\endgroup$ – Marcus Müller May 31 '18 at 7:06
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    $\begingroup$ True, a RHCP signal reflected will be a LHCP signal (on a true, flat, ideal, reflecting surface), right ? sot the must have two antenna's or an antenna which can handle both (that'll be something). As the reflected signal from the target/image is probably no longer RHCP. I am not saying you are wrong in your answer (in fact you are right), I am just trying to figure out if there is a non-anecdotal answer to my question. It seems to me that there still might be an answer, if just may not be answerable with the information readily available. If no other answers appear today, I will mark yours. $\endgroup$ – Edwin van Mierlo May 31 '18 at 7:12

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