Radiation Patterns

Antennas generally don’t radiate their RF energy in all directions equally. The type and orientation of the antenna determines how well it will radiate in a certain direction.

The radiation pattern shows us the direction where the antenna will be most effective.

The radiation pattern of an antenna is often shown as plots as-seen-from-the side (vertical plane) and as-seen-from-above (horizontal plane) . Here are the plots for a Quarter Wave vertical:

Radiation pattern in the Vertical plane
(as seen from the side)
Radiation pattern in the Horizontal plane
(as seen from the side)

Another way to see the radiation pattern is by modelling the antenna in a 3D view. With some imagination, the radiation pattern for a ¼ λ looks a bit like a doughnut:

Quarter-Wave vertical ratiation pattern in 3DDoughnut

The pink doughnut screenshot above is taken from antenna modelling software called NEC.

We can learn from these radiation pattern plots that for a given height above the horizon this antenna radiates equally well in all wind directions (North – East – South West).

Furthermore, there is almost no radiated power going straight up from the antenna as the Vertical plane plot (the side-view) shows a dip of almost zero power going straight up.

We may also conclude that the maximum power is radiated at an angle of about 30 degrees above the horizon (at the 60 degree point in the plot, where the blue line is furthest away from the middle).

Another aspect to notice is the very little amount of power that is emitted towards the horizon at low angles (i.e. at 90 degrees as shown in the plots). For example, imagine two radios, fitted both with a quarter-wave vertical and separated maybe 2km away but one station is 200m higher than the other. The lower station will be able to ‘see’ the higher station, but the higher station’s antenna does not favour a radiation pattern that extends downwards.

This is where the dipole can make a difference.