confusion about vertical polarization and horizontal polarization


by Robismyname
Tags: confusion, horizontal, polarization, vertical
Robismyname
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#1
Jun23-12, 09:07 PM
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Could you please help me to understand some basic concepts about antenna polarization. The books I have are really confusing and do not have examples that I can learn from.

Please look at the attached E plane radiation pattern image I attached (figure 13).

The books image (figure 13) tells me that the attached image is a E plane radiation pattern which means that the E field is perpendicular to the earth (so vertically polarized).

The books image (figure 13) tells me that the attached image dipole is located on the line from 90 degrees to 270 degrees.

How can you have a vertically polarized antenna with a dipole positioned horizontally (90 degrees to 270 degrees)?

What am I missing here? Im sure the book is accurate I just dont know how to read the radiation patter. Please help

Figure 19 (attached) clearly correlates to horizontal polarized dipole but figure 13 says that it is a vertical polarized dipole.
Attached Thumbnails
Figure_13.jpg   Figure_19.jpg  
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sophiecentaur
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Jun24-12, 12:02 PM
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It all seems OK to me. Fig 13 tells you that, in the diagram, the dipole is along the X axis, which may seem an odd thing to do, but the maximum will be in the horizontal direction for a vertically mounted dipole. The way the radiation pattern happens to be plotted on a graph doesn't affect the way it will behave when it's in operation. The direction of maximum radiation is at right angles to the axis of the dipole - which is what you'd expect.
Robismyname
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Jun24-12, 10:59 PM
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Quote Quote by sophiecentaur View Post
It all seems OK to me. Fig 13 tells you that, in the diagram, the dipole is along the X axis, which may seem an odd thing to do
Can you explain why it is odd thing to have the dipole along the x axis?

sophiecentaur
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Jun25-12, 03:09 AM
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confusion about vertical polarization and horizontal polarization


I would say that it is odd, only in a paper that seems to be discussing polarisation, to show the dipole laying down. It works the same whichever way it is mounted, of course.
I hope that your initial question has been answered, though; the diagrams all seem correct and they just need to be read with care. HP and VP are probably not the best terms to use for such discussions, although they are used very commonly; they have managed to cause you some confusion,
Robismyname
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Jun25-12, 10:08 AM
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Quote Quote by sophiecentaur View Post
I hope that your initial question has been answered, though
To be honest I am still confused. No fault of your own. Here is the reason.

Figure 19 says that if the dipole is horizontal then the E field is horizontal.
Figure 19 says that if the dipole is vertical then the E field is vertical.

Figure 13 shows a horizontal dipole but I see a a vertical E field.

To me I see (conceptually) that if the dipole is horizontal that the E-field goes up/north and down/south from the dipole, what i believe to be vertical to the earth.

To me I see (conceptually) that if the dipole is vertical that the E-field goes right/east and left/west from the dipole, what i believe to be horizontal to the earth.

But what i think contradicts Figure 19 and this is where i get confused. Can you help untwist my thinking?
sophiecentaur
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Jun25-12, 10:52 AM
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Figure 13 shows nothing about the direction of the E field (it is assumed to be tangential to a sphere around the dipole in the far field and, of course is zero 'end on'). It is a radiation pattern, showing the magnitude of the E field. Those lines on the diagram show contours of equal field strength.
The label actually says "field strength: E plane', meaning the value of the E field in the plane containing the dipole. They are 'not lines of force'. Does that help?

If you take a horizontal dipole and go out along its axis, then move upwards, the (small) E field that you'll find there will have a component that is mostly vertical. You need to think 3D and not hang on to words like HP and VP for all cases. Those descriptions only apply in the direction of the maximum of the radiation pattern (the bit of the pattern that's usually of interest).

Imagine a sphere (the Earth) and look at the lines of longitude. When they are near the North pole, they lie almost parallel (!!) to the plane of the Equator and to the lines of lattitude. That's not how we normally think of them and it's the same with radiated EM fields
sophiecentaur
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Jun26-12, 04:05 AM
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Quote Quote by sophiecentaur View Post
Figure 13 shows nothing about the direction of the E field (it is assumed to be tangential to a sphere around the dipole in the far field and, of course is zero 'end on'). It is a radiation pattern, showing the magnitude of the E field. Those lines on the diagram show contours of equal field strength.
The label actually says "field strength: E plane', meaning the value of the E field in the plane containing the dipole. They are 'not lines of force'. Does that help?
That's rubbish but it's too late for me to edit, apparently. The lines are graphs of field strength - that's all.
dlgoff
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Jun26-12, 09:23 AM
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For a little fun with dipole antennas check out these applets.

http://www.amanogawa.com/archive/Dip...poleAnt-2.html

And there are plenty more.

http://educypedia.karadimov.info/ele...rfantennas.htm
Robismyname
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#9
Jun26-12, 10:47 PM
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Quote Quote by sophiecentaur View Post
That's rubbish but it's too late for me to edit, apparently. The lines are graphs of field strength - that's all.
Again, thanks for taking the time to help me out. It is much appreciated!

Ok I think I know what I was thinking wrongly about..........feel free to comment where needed.

If the dipole is positioned vertically (y axis), on the directivity plot the radiation pattern of the E-field would be in the direction of 90 degrees and 270 degrees. I see now that the E-field in this situation is indeed vertical. Before I "wrongly" thought the E-field was horizontal because as I looked at the radiation plots of a vertical oriented dipole that since the E-fields were extending outward toward 90 and 270 this meant that the E-fields were horizontal.I didnt realize I was wrong until I saw the radiation pattern as a "donut" for lack of a better word, then I was able to see the "donut" sliced and projected or superimposed on the polar format chart. Now I know that when you see a directivity plot of the radiation of the E-field on the polar plot its only a slice or cross section of the "donut" representing the E-field radiating in the plane.

When measuring dipole antenna E-field radiation I see now that you can do so if the antenna is "physically" positioned horizontally(x axis) or vertically(y axis). For some strange reason I use to "wrongly" think that the dipole had to be positioned vertically only to obtain E-field directivity plots. And I use to "wrongly" think that when the dipole was orientated horizontally that the radiating field you are measuring are magnetic fields.

Since you can place a dipole in the horizontal position why would someone deliberately utilize a horizontally positioned dipole because it seems like your putting radiation in areas that arent going to be in a location that is most usable.

So since I can measure E-field radiation when the dipole is vertical or horizontal how do you obtain the Magnetic Field radiation? Or is it just always understood that Magnetic Fields are circular as when current travel through a wire. Explaining why every time you see a H plane plot it looks like a full circle.

One last thing now that I have a better understand of a E field radiation directivity plot how would you read the actually plot I attached? For instance looking at the radiation at lets say 30, 60, 90. Looks like to me the more elevation you have have the less is your radiating strength. Please confirm.
Attached Thumbnails
Screen shot 2012-06-26 at 11.42.02 PM.png  
sophiecentaur
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Jun27-12, 04:47 AM
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You still seem to be confusing the directions of the field vectors with the lines on a field strength plot. Read all of the following carefully and don't try to race ahead with your own pictures in your head - that has been your problem, I think.
Once you are in the far field, both the E and H field vectors are at right angles to a line from the antenna - i.e they are directed around the surface of a sphere. (The wave is a transverse Electric and Magnetic Wave: TEM) The terms 'horizontal' and 'vertical' do not apply here, yet. They only become relevant or convenient when you actually put an antenna near the ground and when you are receiving the signals at a site that is also near the ground. The E field is parallel with the axis of a dipole in the equatorial plane and then follows the sphere all the way round. The H field is always directed round 'lines of latitude', all over the sphere.

The 'H plane' of a dipole is the equatorial plane. For a dipole, the field strength shows circular symmetry in this plane - so it should be no surprise that plots of it are circles - but this is also true for the E field strength, too. For more complicated arrays (where dipoles are placed side by side), the field strength pattern is not omnidirectional in the 'H plane'. AS IT HAPPENS, the H field lines lie parallel to the lines on the field strength plot for a dipole but this is not particularly relevant.

The E plane of a dipole is any slice through the poles and the field strength (E or H) will be maximum at right angles to the dipole and zero on its axis.

If you actually mount a dipole vertically near the ground then the E vectors will be vertical at zero elevation and we say it's VP. It will be omnidirectional in the horizontal plane. The H vectors will always be horizontal.
If you mount it horizontally, the E vectors will be horizontal, wherever you stand but the field strength pattern will not be omnidirectional. The H vectors will always be vertical.

As soon as you look from anywhere else but the ground, the polarisations can no longer be described as HP or VP; there may be both vertical and horizontal components of fields. This can be confusing (particularly for satellite transmissions, where the transmitter may be vertically above the receiver) and it is common to use circular polarisation which resolves the problem.
sophiecentaur
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Jun27-12, 05:11 AM
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One last thing now that I have a better understand of a E field radiation directivity plot how would you read the actually plot I attached? For instance looking at the radiation at lets say 30, 60, 90. Looks like to me the more elevation you have have the less is your radiating strength. Please confirm.
Two comments, here. Firstly, the E field directivity pattern is exactly the same as the H field pattern (EM waves have the same E/H ratio everywhere, once they are 'far field').

Secondly, antennae are designed to produce a pattern which directs most of the power where it's needed. This may or may not involve directing a maximum in a horizontal direction*.
Many antennae consist of several elements, to produce a range of radiation patterns. HF broadcast antenna ('curtain arrays') are always HP, which, because of the presence of the Earth, gives a null in the (unwanted) horizontal direction and a maximum which points upwards, slightly, in order to launch a skywave towards the ionosphere. TV transmitting antennae (HP and VP) on high masts may or may not be omnidirectional in the horizontal plane (depending on the wanted service area) but the vertical pattern is often tilted slightly downwards, to avoid spilling power over the horizon with a tailored pattern so that receivers the edges of the service area get the same strength as those nearby.

*If you mount a single horizontal dipole above the ground, the resulting pattern (because of the ground reflection) will have a zero in the horizontal and can be directed Upwards or at an angle depending on the chosen height.


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