Antenna gain is a measure of the antenna’s directivity as well as a measure of the antenna’s efficiency. It’s a measure of how well the antenna can take an input power and transform that into an electromagnetic wave going in a specific direction.
Antenna gain is not like the gain on an amplifier. Gain for an antenna is more of a measure of how well the antenna can focus the energy rather than amplify the energy.
Antenna gain is measure in decibels and the gain measured could be in reference to two different theoretical sources: An isotropic antenna which is a theoretical antenna that radiates in all directions similar to how the sun radiates sun rays and a theoretical dipole antenna that transmits with nothing around it.
A reference dipole antenna is known to have 2.15 dBi of gain. You can easily convert back and forth from dBi to dBd by knowing this: dBi = dBd + 2.15. This is important to keep in mind when looking for an antenna.
So if you were to pick up an antenna and the antenna gain was 6 dBd that would mean it has the ability to focus the energy 6 dB more than a dipole antenna and 8.15 dB more than an isotropic antenna.
Focusing the Signal
We mentioned before that antenna gain is a measure of the antenna’s directivity. What does this mean exactly? Think about a maglite flashlight, if you turn the head it focuses the light into a narrower and stronger beam.
Antenna gain can’t be adjusted quite like a flashlight, but the idea that a higher gain antenna would have a narrower and stronger beam is the same. A lower gain antenna would have a beam that was a wider angle.
All antennas have radiation patterns. A radiation pattern is a picture of how the radio signal is transmitted. There are many different kinds of antennas that have similar and different radiation patterns.
Radiation patterns are characterized by beamwidths, sidelobes and directivity. The main beam is where the majority of the signal is concentrated. The sidelobes are areas around the antenna that are away from the main beam and have smaller gain.
The back lobes describe the radiation pattern behind the main lobe. Front to back ratio is how to determine how much signal will be picked up behind the antenna and is important for trying to figure out how to minimize interference. Front to back ratio is calculated by dividing the signal in the forward direction by the signal in the backward direction and taking the log of that.
A dipole antenna has an omnidirectional radiation pattern which means that the signal is transmitted the same 360 degrees around it perpendicular to the axis. A yagi antenna has a more directed radiation pattern which can change depending on how many elements are added or taken away.
Always check out an antenna’s radiation pattern to make sure it will do the job you need it to do. Also don’t be afraid to build your own and experiment! There is a lot of great software out there that can show you the radiation pattern of your antenna.
Do you need a high gain antenna?
So now we ask ourselves, is a high antenna gain really better? It depends on what is around your HAM shack. If you live on top of a mountain or a large hill or in a wide open space surrounded by vast nothing, then a high gain antenna would serve you quite well.
If you live in a rural area or a city or in a place with a lot of hills, then you might consider saving your money and opting for an antenna with a lower gain.
This is because a more directed radio signal isn’t going to be able to make its way around things the way a less directed radio signal would. Sure the signal won’t be quite as strong but it will have better odds of reaching its destination.
Antenna gain is a measurement of the antenna’s directionality and efficiency. It can be measured in dBi or dBd which has a 2.15 dB of difference.
An antenna that has more gain means that the beam is stronger and more directed. An antenna with a lower gain would have a weaker transmitting signal with a more broad directivity.
There are so many types of radiation patterns to choose from! Experiment with your antennas and don’t be afraid to have multiple.
It might not always be best to go with an antenna that is super high gain especially if you live around tall buildings or in a hilly area. Before you decide, think about your surroundings and what you plan to do with your antenna.