You often hear “higher gain antenna has longer range”, but don’t be fooled! Antennas doesn’t amplify radio signals, but rather just changes the radiation pattern, which is a trade-off between coverage and range. Antenna gain is a reflection of the maximum range you can get.
Firstly, we have omni-directional antennas which have a low gain between 0dB to 3dB, and can radiate signal evenly in almost all directions.
Then there are directional antennas, which focus its signal into one direction in order to achieve further range and therefore have much higher gains.
For basic overviews of FPV system and antenna, make sure to check out these guides first:
- The complete guide to FPV
- How to choose the best FPV antenna
- How polarization affects antenna performance
How to Increase Range?
There are normally 2 ways to increase the range of your FPV system: increase video transmitter power, or use higher gain antennas.
As RC hobbyists, we should avoid blindly increasing the power of video transmitter to achieve longer range, simply because it might not be legal, and it will consume more power and generate more heat. Always start with upgrading your antennas when trying to improve range 🙂
Understand Antenna Gain
Antenna gain is the measure of antenna power in decibel (dB), which is equal to 10*log(Pout/Pin).
Don’t worry about it, just remember that the higher the gain, the more directional an antenna is – more range, narrower beam width.
Every 6dB increase in antenna gain, should in theory, doubles the range. However, as mentioned, directional antennas are not amplifiers. It’s only focusing all the energy into a narrower beam.
A water balloon is a good example.
The volume is the total power of your VTX (or think of it as the signal coverage), and it doesn’t change no matter how you squeeze it or pull it, it’s still the same amount of water in there.
By using an antenna of different gain and radiation pattern, merely changes the shape of the balloon. The balloon might become longer, but the width becomes narrower, and so the total volume is still the same.
A different analogy would be a light bulb vs. a torch, which represent an omni-directional antenna and a directional antenna.
Although signal reception becomes narrower with increased antenna gain, a common technique is to use a diversity receiver. It allows multiple antennas working together, constantly checking signal strength of each antenna and pick the one with the strongest signal. This way we have longer range and wider coverage.
Every antenna has a different radiation pattern, and you can clearly see the effect of antenna gain in these examples.
A radiation pattern of the hypothetical isotropic antenna at 0db gain. It’s a nearly perfect sphere in both vertical and horizontal axis.
This is a standard omni-directional 3dB rubber duck antenna. Notice it has significant signal loss on the top and bottom (90/270 degree). Similar radiation pattern applies to most omni-directional FPV antennas out there.
In 3D this radiation pattern looks like a doughnut 🙂
And here we have a directional antenna of 8dB (a patch antenna). As you can see, majority of the signal are focus on one direction to the right (0 degree) just as we expected from a directional antenna. But we are not getting much signal on the other direction (180 degree).
It’s a common practice to use an onmi-directional antenna and a directional antenna together on a diversity receiver. Always point the directional antenna at your aircraft so you can get good range. If you decide to fly behind yourself you are still covered by the omni-directional antenna.