Radiation patterns - an overview

An overview of how radiation patterns are generated, and how they relate to each other.

Radiation patterns

Each of the antenna designers in this site offer the display of one or more radiation patterns:

For any particular antenna, each of these radiation patterns shows a different aspect of the same thing: namely, how the RF signal energy from the antenna at a particular frequency spreads out, or propagates, from the antenna, and in the far field of the antenna: that is to say, in the region around the antenna where the E- and H- fields form radiating waves; typically at r ≳ 2D²/λ from the antenna of size D (for a 20-meter band dipole, this would be greater than about 10 meters distance from the antenna).

In this section we will attempt to explain how each of these patterns relates to the others, and to the whole. Some of these relationships are particularly easy to comprehend; others perhaps less so.  Let's start with the pattern which shows the most information first: the 3D radiation pattern.

The 3D radiation pattern

For this discussion, we show a simplified version of a 3D radiation pattern: one for a 40-meter EFHW antenna (a multi-band antenna) being used on one of its' resonant bands, the 10-meter band.  This antenna is set out in an east-west direction, so that the antenna wires are in the 0° - 180° vertical plane.

We have replaced the normal color information (which would normally be used to show the signal strength at a particular point on the 3D figure) in the 3D patterns diagrams in this discussion, with coloration according to height only, and to highlight certain details in the following sections.

Fig.1 - 3D pattern - basic
Fig.2 - 3D pattern - basic with tooltip

We present here just static images of a 3D pattern, but in a designer page, the 3D figure is capable of being rotated, panned and zoomed.  As the second image shows, the figure can also show a tooltip containing information on the point under the mouse pointer.

The azimuth and elevation radiation patterns

The azimuth and elevation radiation patterns each represent "slices" and projections made from the basic 3D pattern for an antenna at a particular frequency.  Each of these two patterns is related to the other, with the program deciding how or where the "slicing" is to take place.

Fig.3 - Azimuth radiation pattern diagram, with notations

Fig.3 shows an azimuth radiation pattern diagram for this antenna, and we see that the blue outline of the radiation pattern has a shape resembling a horizontal cross-section through the 3D pattern as displayed in Fig.1 - we will see later that this is not in fact the case.  This diagram is annotated to highlight certain features in the diagram:

Fig.4 - Elevation radiation pattern diagram, with notations

Here are two views of the 3D pattern after the vertical plane "slice" has been made:

Fig.5 - 3D pattern sliced at 42° azimuth angle, from above
Fig.6 - 3D pattern sliced at 42° azimuth angle, oblique view

Note in Fig.6 the outline of the edge (highlighted in red), formed by slicing through the 3D pattern, represents the elevation radiation pattern displayed in Fig.4 above.  This is how such elevation radiation patterns are calculated and defined.

When the program is in "automatic" elevation angle mode - i.e. when the user has NOT chosen a particular elevation angle from the "Choose reference elevation angle" dropdown provided - then the program chooses a point at which BOTH azimuth and elevation angles are set to the angles corresponding to the maximum gain in both azimuth and elevation.  Figs. 3 and 4 show the results when the choice is made automatically.

If, however, the user wishes to know what kind of performance the antenna has at an elevation angle OTHER than that chosen automatically by the program, then they can use the "Choose reference elevation angle" dropdown to set this angle manually.   Why might the user decide to do this? - reasons for this might include:

...and so on.

Deriving the azimuth radiation pattern from the 3D pattern

Before we go any further, let's see just how the azimuth pattern is obtained from the 3D pattern - it's not an immediately intuitive process, as we hinted at earlier. Let's take a look at the 3D pattern one more time, this time after "slicing" it according to the elevation angle as chosen automatically by the program - in this case 21°:

Fig.7 - 3D radiation pattern diagram, sliced at constant 21° elevation angle

Here, we need to imagine the solid black line, from the antenna center C through the point P on the 3d pattern exterior, as acting like a rotating knife with its' point fixed at the center C, kept at a constant elevation angle θ = 21°, and completing a full 360° in azimuth Φ.  This action describes a cone with apex angle α = 2 × (90°-θ) cutting away the top portion of the 3D pattern, and leaving the bottom portion intact.  The "edge" or "path" revealed, where the 3D pattern has been "cut" or "sliced" in this way, represents the azimuth pattern as shown in Fig.3 above.

If we were to examine this view of the 3D diagram in the program, we would see that the tooltip shows the elevation angle of 21° at all points along this top edge of the "sliced" diagram, even though this edge looks uneven - it's not at all horizontal!:

Fig.8 - as Fig.7, with tooltip

Let's now look at a more extreme example: the same antenna, same band, and same 3D pattern as above, but this time with elevation angle set to a much higher angle of 60°.  We use the "Choose reference elevation angle" dropdown to set this angle, and click on the "Display plots" button to show the new azimuth and elevation patterns:

Fig.9 - azimuth pattern, with elevation angle set to 60°
Fig.10 - elevation pattern, with elevation angle set to 60°

We see the radial axis in the elevation pattern diagram (Fig.10) set to our choice of 60°, and also that the program has set the azimuth angle to 0°, which is the direction of maximum gain in this configuration.

Looking now at the 3D pattern for this configuration, we see in Fig.11 the 3D pattern sliced at 0° azimuth angle as viewed from the top. In Fig.11, the pattern has been rotated to display an oblique view, where the edge resulting from this slicing has been highlighted in red.

Fig.11 - 3D pattern sliced at 0° azimuth angle, from above
Fig.12 - 3D pattern sliced at 0° azimuth angle, oblique view

Again, as in the previous example, the red-highlighted edge reveals the same elevation pattern for this configuration as in Fig.10.

So far, so good - but what about the azimuth pattern in Fig.9?  Here we show the 3D pattern after the 60° slicing has been applied to it.

Fig.13 - 3D pattern sliced at 60° elevation angle, from above
Fig.14 - 3D pattern sliced at 60° elevation angle, oblique view

The first figure, Fig.13, reveals the same profile at the cut edge as is displayed in the azimuth radiation pattern Fig.9.

So we see the fundamentally different means by which the azimuth and elevation radiation patterns' profiles are generated:

  • azimuth patterns are produced by "slicing" the 3D pattern using a cone described by a line at a constant elevation angle θ, rotated through 360° about the antenna support which is coincident with the Z-axis at the center of the coordinate system used here;
  • elevation patterns are produced by "slicing" the 3D pattern using a vertical plane running through the Z-axis of the coordinate system, and at a pre-defined azimuth angle Φ.
Polarization radiation patterns

Polarization radiation patterns are plotted in azimuth, and represent the degree to which the signal is polarized in different azimuthal (compass) directions around the antenna.  A single polarization patterns diagram displays three curves at once:

for a chosen frquency.

So, what is meant by "polarization"? - put simply, it is by convention taken to be the orientation of the electric field (the E-field) of an electromagnetic wave (the RF signal) as it propagates from an antenna; the magnetic field of the wave (the B-field) being here ignored.

Let's take a look at the polarization pattern for the antenna from the previous example, where we chose an elevation angle of 60°:

Fig.14 - polarization radiation patterns diagram for elevation angle 60°

We see in this diagram that the horizontal and vertical polarization patterns have very different shapes, each describing the signal strength in their particular polarization, and in different azimuth (compass) directions around the antenna. The antenna itself is erected in the plane 0° - 180°, i.e. east to west.

In this example, we see that, in directions toward the east (0°) and a little less so toward the west (180°), the vertical component is stronger (> 10dB) than the horizontal, but in directions orthogonal to the antenna plane, the horizontal component wins out, but only just.  Those with sharp eyes may already have spotted that the dotted curve representing the combined horizontal and vertical components of the EM field is the exact same shape and size as the azimuth radiation pattern as seen in Fig.9 above.  This is, of course, entirely to be expected, since each represents how the whole signal is distributed in azimuth.

Let's take a look at another couple of examples. In the first, we'll look at the polarization patterns diagram for the configuration used for the figure Fig.3 above, where we let the program decide which elevation angle it would use (here, 21°):

Fig.15 - polarization radiation patterns diagram for elevation angle 21°

As in the previous example, the vertically polarized component of the radiation is at a maximum in the eastern (0°) and western (180°) directions, but in other directions the situation is mixed, with the horizontal component having several maxima stronger than the vertical.  The combined horizontal and vertical components form the dotted "Summed" curve here, which is exactly the same curve as in Fig.3 above.

One more example, this time using a different antenna altogether. Here, we use an inverted-vee dipole for the 20-meter band only, with apex angle of 70° (full apex angle 140°), with the apex at 7m AGL.

Fig.16 - polarization radiation patterns, 20-meter inverted-vee dipole

Again, we see the directions of maximum vertical polarization of signal as being along the east-west line, i.e. around the vertical plane of the antenna.  But isn't this somehow counter-intuitive? Haven't we all often heard that signals from a dipole are always horizontally polarized, and exclusively so?  Indeed, many of us have heard this, but it's a myth - except for dipoles set up VERY FAR above the ground, or situated in "free space."

For dipoles erected at heights AGL typical of portable set-ups - a few meters above the ground - the situation is in fact just as we observe in this example.  To get a rough idea of why this would be so, imagine we have set up this antenna on level ground somewhere.  We're able to walk around the antenna, viewing it from all azimuth (compass) directions.  If we view the antenna from the north or the south (azimuth 90° and 270°) directions, we see the antenna strung out as a long wire, we fully expect the signal to be horizontally polarized in these directions, and in directions close to the north-south line as in Fig.17; and we would be right to expect this.

Fig.17 - 20-meter inverted-vee dipole viewed from the side
Fig.18 - 20-meter inverted-vee dipole viewed from nearly inline

If, however, we walk around the antenna to a position near to the plane of the antenna - i.e. nearly inline with the antenna as portrayed in Fig.18 - the antenna will look very foreshortened, almost like a couple of near-vertical elements.  It's in these directions that we observe the maximum vertical polarization from the antenna, almost as if the antenna DID consist of near-vertical elements.  Imagining how the antenna looks from different azimuth angles can help us to understand - at least in part - how and why the polarization changes around the compass.