Antennas are something that every active HAM radio enthusiast uses. They are necessary for your radio transmissions. It’s important to know how they work and which direction you are transmitting.
Every antenna is transmitting it’s signal but not necessarily in every direction. If you are experiencing trouble receiving signals or sending signals to a specific station in a specific direction, then you might want to investigate the radiation pattern of your antenna.
Electromagnetic radiation (radio signals) is actually a dipole radiation. Dipole means two ways and because of this there is no antenna sending a radio signal that is capable of radiating in all directions at the same time.
To calculate antenna gain, every antenna is compared to a theoretical antenna that does radiate in every direction. This is called an isotropic antenna and does not actually exist. To help visualize the radiation pattern of the isotropic antenna, it radiates the same way as the sun which is in every direction on every plane.
You might hear people talk about radiation patterns in terms of zenith and azimuth. These are just terms that specify direction. The zenith is an imaginary line that goes straight up from a particular location. The azimuth is the angle from the zenith down to a particular point in the sky.
Knowing the azimuth for the destination of where you want to transmit will be very beneficial if you are using an antenna with a more directional radiation pattern. There is tons of free software online that can help you create a map from where you are anywhere in the world.
Types of Radiation Patterns
Every radiation pattern has a main lobe, side lobes and a back lobe. The main lobe is the strongest and the ratio of front to back lobe is called front to back (F/B) ratio. The side lobes radiate less and the side lobe level is defined as the difference between the main lobe and the side lobes in decibels.
When looking at a radiation pattern on a graph, you can clearly see the lobes and it should be obvious which is the main lobe. An omnidirectional radiation pattern has a main lobe that is almost the same size as the side lobes making it’s radiation pattern look almost like a figure eight.
The most common HAM radio antennas are the dipole and the monopole. A monopole is an antenna that sticks straight up into the air and a dipole also sticks straight up into the air but has a “T” shape to it.
Monopoles and dipoles both have the same radiation patterns which are omnidirectional. Omnidirectional means that the signal is going to radiate in every direction only along the plane that is perpendicular to the antenna. So to help visualize, if you have a monopole antenna sticking straight up into the air imagine putting a giant donut around it in the center. This is the radiation pattern of a monopole antenna.
Radiation patterns are three dimensional so it is always helpful to look up a three dimensional image of the radiation pattern that your antenna has. There are many antennas that have more directed radiation patterns and it’s helpful to see these images when pointing the directed antenna.
When looking at a radiation pattern in 2D, there is usually a side view and a top view so you can get an idea of what is going on. These can also be called horizontal and vertical patterns.
A yagi antenna is an example of an antenna that has a more directed radiation pattern. When looking at a horizontal pattern you can see that the main lobe is significantly longer than the side lobes or the back lobe.
Every antenna has a radiation pattern and it’s important to know the radiation pattern of the antenna you are using to have maximum signal when transmitting or receiving.
The zenith is an imaginary line sticking straight up into the air and the azimuth the angle between the zenith and a point in the sky where you might want to broadcast to. Knowing the azimuth of your signal’s destination will help you to know which way to point your antenna.
The most common radiation pattern is omnidirectional which is the radiation pattern of a dipole and monopole antenna. The three dimensional omnidirectional radiation pattern looks sort of like a donut.
When dealing with a more directed antenna, the radiation pattern is going to have a much longer main lobe. It’s especially important to know the radiation pattern of a directed antenna because that is going to make or break your transmissions.