Antenna Radiation Patterns Explained

[alexxs88]

Hi,

Been struggling to find information that I find to be obviously missing on how to interpret antenna radiation patterns. For example have a look in the datasheet for my chip antenna: https://www.johansontechnology.com/datasheets/2450AT42A100/2450AT42A100.pdf. I can understand the three 2D radiation pattern graphs, one for each plane, what I don't understand is what the Horizontal and Vertical plots mean in the context of an antenna. I know it refers to the polarization of the RF waves transmitted/received, but couldn't find a decent source explaining exactly how this information can be used correctly.

Thanks!

[radiolistener]

I can understand the three 2D radiation pattern graphs, one for each plane, what I don't understand is what the Horizontal and Vertical plots mean in the context of an antenna. I know it refers to the polarization of the RF waves transmitted/received

Radiation pattern shows how transmitter power is distributed at different directions around antenna.
But it only make sense for the far field region of antenna (at distance more than lambda/2 from antenna).
If you use antenna at near field region (at distance shorter than lambda/2 from antenna), radiation pattern won't works.

For example when you use flashlight, it emit main part of power in forward direction. The same thing happens for antenna, power distribution depends on the direction and radiation pattern shows this power distribution.

It works the same for transmit and receive. If your antenna transmit more power at some direction, it will also receive more power from the same direction.

[alexxs88]

Thanks for the clarification in that regard, but could you please reply to the other part of my post as well? That's the thing I'm struggling with. I'm referring to why the graph for each plane has two plots, one for vertical polarization and one for horizontal. The horizontal one makes sense, as it's the defined plane, but don't understand how the vertical plot isn't just the same as the YZ graph further down.

[coppercone2]

a more advanced antenna plot shows you whats going on in 3d , you are taking 2d contours based on information that is really 3d

it might look something like this in its original form (as far as complexity but not similarity goes)



And that is only possibly the simulation of the pattern. (dish antenna)

What vertical and horizontal might mean is that the receiver is screened for vertical and horizontal polarization, not the antenna feed.

Keep in mind for directional antennas its wishful thinking, you end up having to do boresight adjustment with reception being measured and tuning for optimal performance.

Obviously 3d measurement of antenna gain is very expensive, compared to rotating the thing on a platter with the receiver in front of a non reflecting screen. This is the practical measurement for most things and it serves its use for most stuff beyond radio physics experiments.

[cdev]

As radiolistener and coppercone both touched on, there are also multiple kinds of antenna polarization, both linear and circular, two groups that within each group, the other polarization does not function well with its opposite, in fact, they null one another out.

So if you are examining vertically polarized radiation, horizontally polarized signals will be much reduced, and vice versa. Similarly with circular polarization. If you have an antenna that responds or radiates right hand circular polarization (RHCP), left hand circular polarized (LHCP) signals will be quite substantially reduced.

This property is also useful in allowing you to receive some signals with less noise.

[hagster]

It's all about polarization mismatch. The image in Coppercones post shows "Total" radiation. This is the sum of the horizontal and vertical pattern. It shows what you could receive if you had an exact polarization match at every point on a sphere. Any mismatch will reduce the apparent gain. This is a common way for representing 3D antenna patterns as showing the actual vertical and horizontal plots is a bit confusing.

The plots you linked to show 2D cuts with a receive antenna in either horizontal of vertical polarization.

As discussed representing 3D gain plots with polarization is a bit mind bending. There are also several possible ways to do it. The polarization are normally represented as phi and theta. You can read more here
https://www.nsi-mi.com/images/Technical_Papers/2007/AMTA07-0092-GFM_SFG.pdf

See page 7 for descriptions of the Ludwig Definitions.

[radar_macgyver]

For each of the plots of the Johanson antenna, when measuring the radiation pattern, they measured it with a dual polarized reference antenna, showing the output power in V and H respectively. The XZ plane shows isotropic radiation for vertical polarization, and some deep nulls in the horizontal polarization; this tells me that if you're designing this antenna into a product, position the antenna so that the XZ plane intersects the other station, assuming the other station is transmitting vertically polarized. This is often the case for WiFi and other 2.45 GHz applications. If your radio has a 'diversity' receiver, add a second antenna with the XY plane intersecting the other station, since its 'vertical' pattern is higher overall than the pattern in the YZ plane.

Also compare the XY-H and YZ-V plots. From the geometry, they should both be pointing at the 'top' of the antenna (marked with the dark square) at zero degrees, and in both cases, the gain is about -25 dBi. The change in polarization results from the 'twist' of the antenna-under-test with respect to the receiving (reference) antenna. Similarly, XY-V and YZ-H have the same value of about -10 dBi at 0 degrees. You can do similar comparisons between the other plots as well to help understand the pattern in 3D, and the relationship between the patterns in two polarizations.

Hope this helped. Also, I agree with hagster, the plots in 3D just look cool; to me the 2D cuts convey a lot more information.

[radiolistener]

That's the thing I'm struggling with. I'm referring to why the graph for each plane has two plots, one for vertical polarization and one for horizontal. The horizontal one makes sense, as it's the defined plane, but don't understand how the vertical plot isn't just the same as the YZ graph further down.

If you're talking about blue and red plots from the datasheet. It just show radiation pattern for different polarization. Some part of power is radiated with horizontal polarization and some part of power is radiated with vertical polariazation. And these plots just showing you radiation patterns for different polarization.

Horizontal polarization and vertical polarization are not the same. They have different orientation in 3d.

[kensmart]

A couple of things, it’s a small chip antenna doing the best it can, polar plots make an average antenna look better, so they are always on datasheets.

To possibly help with the Vertical and Horizontal interpretation. The antenna is essentially linear and as it is shown on page two, polarised horizontally. That is; when the e-vector (electric vector) is horizontal (in the plane of the long dimension) the maximum amount of energy is coupled either to or from the antenna. This is shown in the third plot YZ cuts, with maximum energy 0dB at 90 and 270. (although Y and Z seem transposed in relation to the antenna coordinate system for this diagram compared to the others).

When you stand the antenna up on its end (Plot 2) now when the e-vector is vertical the maximum amount of energy coupled either to or from the antenna will be in the vertical plane. Almost 0dB all the way around. Less energy couples in the orthogonal, horizontal plane

For the first plot neither the horizontal nor the vertical are aligned for maximum coupling so they are both somewhat reduced. As others have said the antenna is not ideal so some of the energy gets coupled into the other plane. If you didn’t know what polarization to expect this orientation might give you a bet each way. If you inserted the antenna into the hole in a doughnut so it’s wearing it like a hat (Fig 2)  this “roughly” describes the shape of the pattern in 3D.

There are lots of ways to interpreted and display the same antenna pattern data or data from the same antenna.

Good start point if you don’t want to jump straight into Ludwig polarization basis’.
https://www.usna.edu/ECE/ee434/Handouts/EE302%20Lesson%2013%20Antenna%20Fundamentals.pdf