Thursday, May 22, 2014

How Antenna Gain affects Range in FPV

How Antenna Gain affects Range in FPV

To increase the range of the reception range of the radio system, there are two ways: either to increase the power of the transmitter or the power gain of the antenna.
As RC hobbyist we usually try to avoid modifying the transmitter to increase the power, or use a high power one, because that would add more weights to the plane. So that left us with the second option – increasing the gain of the antenna. However there are always misunderstanding about the concept of antenna gain.

Understand Antenna Gain

The gain or loss of antenna is measured by decibel (dB), which is equal to 10*log(Pout/Pin). Every 3dB increase in the gain, it doubles the range of the Antenna. However, here is the thing, it does not really “double” the range but more like taking the range from other direction and focus on the desired direction.
To make it easier to understand, imagine we have a balloon and the amount of air in it is like the power of the system (the range) which is constant. The reason we are getting a larger range is because we are “pressing the balloon” and the shape of the balloon is changed (the increase of intensity of the radio signal toward certain direction). This way we sacrifice coverage of some direction to increase the range of other direction, but the total covered area is the same.
antenna gain
Although the range becomes narrower when increase the gain, we can put a multiple channel diversity controller to put more antenna in. This way we have more signal coverage and the system will pick the antenna that has the strongest signal to use.

Radiation Pattern

That was a very rough description of the idea. The actual shape of the radio frequency signal coverage changes irregularly with the antenna gain.
A radiation pattern of the hypothetical isotropic antenna at 0db.
This is a standard omnidirectional 3 dB rubber duck antenna.
8dB Patch directional antenna


By increasing Antennas Gain do not magically create extra power but simply focus the radiated radio frequency into narrower space, so the signal coverage can reach out further in the required direction. Remember that by gaining distance the effective angle will be lost.
So when choosing antenna, unless you know what you are doing, you should choose a low dB omnidirectional antenna since the airplane is moving around in all directions, so you don’t lost connectivity when turning. Alternatively if using high db antenna, also use a diversity controller and add more antenna to cover more space. Check out my post about how polarization affects the performance of an antenna.
On the receiver side, you can consider using a patch antenna which allows you fly further in one direction, and you can turn it around to face your airplane to maximize the reception. If you plan on flying around yourself, a 3dBi omnidirectional or something similar would be safe.