You will find quite a few diagrams throughout these web pages. These diagrams are called polar plots and are meant to be intuitive. Polar plots are a very good way to describe how an antenna performs. I've been asked a few times to explain how to interpret them.

In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a fixed point and an angle from a fixed direction. The fixed point (analogous to the origin of a Cartesian system) is called the pole, and the ray from the pole with the fixed direction is the polar axis. The distance from the pole is called the radial coordinate or radius, and the angle is the angular coordinate, polar angle, or azimuth. [1]

Lets look at a polar plot more closely. (Figure 1)

The first thing that we see is some number on the outside of the big circle, from which straight lines are going into the center of the circle. These lines and numbers represent a direction and are expressed in decimal degree, the likes we see on a compass, from 0° to 360°.

Inside the big circle we see smaller circles which have also a number associated with them. These are the gain rings/circles and are expressed in dB.

In each corner of the plot you see some additional information. In the top left corner you'll see the Horizontal gain in dBi (Hor-gain [dBi]) at a Frequency of 7 MHz (Freq=7). The top right corner of the plot shows you the plane of the plot, in this case it is a Horizontal plot (Horizontal plane).

In the left bottom corner we have the NEC output file (VK1HW LAZY-DELTA-LOOP.out) and more importantly the elevation angle where maximum gain occurs (Elev.=80) and last but not least, in the right bottom corner we can see the minimum and maximum gain figures (<-999 <4,94)

Lets start by analyzing Figure 1. This is a horizontal plot of my Lazy-D antenna [2].

The blue line, the line that looks like a figure 8, as seen at Figure 1 shows us a representation of the antennas horizontal radiation pattern. The so called horizontal polar plot of an antenna. As can clearly be seen, this plot shows us that the intensity of the radiated power goes into two directions, 180° and 0°/360°. This pattern is frequently called a bi-directional radiation pattern.

The two green lines at the plot,indicate the so called -3 dB bandwidth of the antenna. This means that at these angles, 225° and 135°, the radiated power has dropped to half of the maximum power (-3 dB). Which means that the 3 dB bandwidth or the half-power bandwidth is 90° (225° - 135°) wide with approx. 1dB of horizontal gain. Remember we are looking at the horizontal plot.

The red line shows us that the maximum horizontal gain is either at 180° or 0° direction and that the radiated power, or the received signal from this direction, is approx. 4.9 dB stronger/lauder then a signal coming from the 120° direction. The plot also shows us that there are two strong so called nulls at 90° and 270°. Nulls, because the antenna does not radiate nor does it receive well at these directions. The gain here is way below the 0 dB mark and is approx. 60° wide.

The red triangle in the centre of the plot represents the modelled object in the current plane, in this case my Lazy-D.

To wrap it up, Figure 1 represents a horizontal polar plot of the modelled signal strength in dB (4.94 dB) as a function of the azimuth angle at the maximum elevation angle of 80°. (i.e. 4.94 dB @ 180/360° az @ 80° el)

The next diagram shows us a vertical polar plot of the same antenna.

to be completed soon