Antennas how important is polarization

Hi all,
I am new to antenna engineering, currently I am studying Balanis book on antennas and there is a big part that discusses polarization and PLF (polarization loss factor) when you capture less energy when polarizations do not much.


At the same time my background is on radio propagation and higher layers where typically one will not see polarization being discussed. Even if it is GSM, Bluetooth or WiFi you will find ready modules that do not discuss polarization at all.

When polarization is important?


The second part of my question is when one measures antennas inside chambers. I have seen measurements where horizontal and vertical polarization are being measured. Those two then are being summed up to give the total gain of the antenna. This is the gain that one typically will see being "advertised", I am not sure why though we assume that the antenna under test will be able to receive horizontal and vertical polarization signals as ideally as the summation denotes.

Any comments on that?
I would like to thank you in advance for your reply.
Regards
Alex

Re: antennas how important is polarization

Very important. Theoretically, a horizontally polarized antenna would not receive a vertically polarized signal (and vice versa). Most of the modules that you speak of do not discuss polarization, because they usually have "diversity" antennas, or two antennas arranged orthogonal to each other. The receivers (the chips contain at least two separate receivers) continually sample each antenna to see which one has the higher received power and switches to that one. This makes reception possible under most conditions.

As for summing both the H and V gains, I've never seen that - usually the gain of the intended use is published. For instance, an antenna with a V gain of 2.15dBi and an H gain of -30dBi would still have an approximate overall gain of 2.15dBi.

Re: antennas how important is polarization

Generally very important; but.

Instead of vertical and horizontal polarizations, you will also see (hear about) circular polarizations- left handed and right handed. Most of the transmissions from satellites are circularly polarized.

The nature of the polarization depends on the antenna (as you have already learnt) but during propagation it tends to get "depolarized" but I do not know the detailed mechanisms.

Again, the gain of the receiving antenna depends on the polarization but as you have also pointed out it is not accurate to add the horizontal and vertical gains (to make an overall gain look impressive)- but I frankly do not know the reasons.

Your dish antenna, say, is supposed to receive left-circularly-polarized signals. The signal after one reflection will be right-circularly-polarised (mirror image) and will be ignored by your antenna (multipath problems) and that is one example.

Re: antennas how important is polarization

if you use the wrong polarization you can lose 15 dB or more system gain, which will kill most systems.

if your antenna has poor crosspolarization, your system might end up self jamming, OR you might fail some govenrnement regulatory body test.

and on and on.

I guess polarization is not that much a topic in "GSM, Bluetooth or WiFi" because orientation of the device (and resulting polarization of the built-in antenna) varies a lot.

A big thanks to all for the replies I got. I was sick and I was not able to reply back in time.

SLK001 said:

... Most of the modules that you speak of do not discuss polarization, because they usually have "diversity" antennas, or two antennas arranged orthogonal to each other. The receivers (the chips contain at least two separate receivers) continually sample each antenna to see which one has the higher received power and switches to that one. This makes reception possible under most conditions.....

As for summing both the H and V gains, I've never seen that - usually the gain of the intended use is published. For instance, an antenna with a V gain of 2.15dBi and an H gain of -30dBi would still have an approximate overall gain of 2.15dBi.

Click to expand...

What I have seen is that for bluetooth zigbee application one can use a patch or a bended monopole antenna, or a ceramic antenna. Antennas that will need very small space to fit in. I have been working in simulating some of those in CST software. There I never found polarization (in terms of the simulation) to be discussed. In real products I have seen for example antennas of this type
https://www.farnell.com/datasheets/...82.462133079.1512030810-1340748487.1494250717
to be used (see that there is nowhere the term polarization used)

Now regarding your last sentence " For instance, an antenna with a V gain of 2.15dBi and an H gain of -30dBi would still have an approximate overall gain of 2.15dBi" I am not sure I understand it. Why a V gain of 2.15dBi and an H gain of -30dBi give an average of 2.15dBi?

Thanks again for your time

volker@muehlhaus said:

I guess polarization is not that much a topic in "GSM, Bluetooth or WiFi" because orientation of the device (and resulting polarization of the built-in antenna) varies a lot.

Click to expand...

I try to understand antennas in what one would call electrically small antenna and it might be indeed that polarization is not being discussed since devices change so much orientation that one has to @budget@ also for extreme polarization losses.

It is a quite exciting topic this to me. I would also try to understand many more

see that there is nowhere the term polarization used

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Look sharp. It's annotated by the letters V and H for the radiation characteristics and gain values in the linked Johanson chip antenna data sheet.

Now regarding your last sentence " For instance, an antenna with a V gain of 2.15dBi and an H gain of -30dBi would still have an approximate overall gain of 2.15dBi" I am not sure I understand it. Why a V gain of 2.15dBi and an H gain of -30dBi give an average of 2.15dBi?

Click to expand...

The comment is dedicated to your suggestion to sum up H and V gain. But as SLK001 mentioned, it's no commonly used method. If at all, it could make sense for a circular polarized wave.

Regarding preferred polarization for certain radio services, you'll notice that GSM generally uses vertical (simply look at a GSM tower), also WiFI routers have their antenna rods mostly vertically oriented.

alaios said:

Now regarding your last sentence " For instance, an antenna with a V gain of 2.15dBi and an H gain of -30dBi would still have an approximate overall gain of 2.15dBi" I am not sure I understand it. Why a V gain of 2.15dBi and an H gain of -30dBi give an average of 2.15dBi?

Click to expand...

Remember what a decibel is. A gain of 2.15dBi is an equivalent gain of 1.64 absolute. A gain of -30dBi is an equivalent gain of 0.001. If you add them up, you get 1.641 for an absolute gain, or 2.151dBi - not a significant increase in gain.

Since you cannot do addition in the logarithmic domain, you must first convert back to absolute values.

Since you cannot do addition in the logarithmic domain, you must first convert back to absolute values

Click to expand...

And if you do, you are essentially getting a geometric mean; you need to consider the specific application. In this case the geometric mean will bias the result too much towards the smaller number.

On the other hand, arithmetic mean is unbiased (terms and conditions apply) and for this specific case, arithmetic mean is more relevant.

- - - Updated - - -

notice that GSM generally uses vertical (simply look at a GSM tower), also WiFI routers have their antenna rods mostly vertically oriented...

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True but during propagation the waves have a tendency to depolarize (I do not know how; reflection is perhaps one of them) and the plane polarized radiation becomes somehow elliptically polarized and/or uniformly polarized. (somewhere in the setup, you can see the strength of the received signal and if you turn the receiver antenna- I am talking about the wifi antenna- you can see the change in the intensity)

A free-space propagating wave doesn't really depolarize - there is no mechanizm for that and multipath signals are something to be avoided at the receiver, since they are usually destructive. Most mobile devices (for instance, a laptop with WIFI) have diversity receivers - meaning "more than one", and each has its own antenna (they are usually situated orthogonal to each other so as to always have an adequate signal available to the receiver). The receiver samples the received signal from each antenna and switches to the receiver that has the highest signal strength. It does this a couple of times a second. The access point also does this sampling and switching. In the access point's case, it also uses the highest signal power channel to transmit on. Both antennas are not used to transmit on simultaneously - to do so would result in significant dead areas of destructive interference.

Static equipment can get by with only one antenna, since once it has been set up, things don't change much.

It might be helpful if you find a pair of polarized sunglasses. They are designed to pass a particular linear polarization and block the orthogonal component. They are not perfect but will give you a view of the situation. Go outside and look at a scene where you have strong reflections ( water, big sheets of glass, asphalt roads work too) and notice how the image changes as you rotate the glasses or tip your head a bit sideways. If you have two glasses you can block most of the light by placing the lenses at 90 degrees to each other. You will get a sense of what polarization is all about.

Think of a rope being shaken in the vertical plane as another example. The wave on the rope will pass through a picket fence between the slats. Now shake the rope in the horizontal plane and the wave will be blocked. This is another demonstration of polarization.

An antenna always has some preferred polarization and is always blind to the orthogonal polarization. Polarization is not usually constant in all directions and the big entertainment is trying to figure out how much power is transferred between two antennas, particularly when their polarizations are not precisely matched

As I recall Stutzman has a very useful book devoted to the subject. Understanding this feature is essential in designing a functional antenna.

Regards,

Azulykit