What is mW, dBm, and dBi, and how do they influence the signal’s range

One of the most common questions for those who are already in the FPV hobby is: How can we know the range of the video (or radio control) signal? And how can we improve the video or radio signal? This is what this article is about.

Wireless transmissions (over radio frequency) has several characteristics that influence both the range and signal quality. To keep this article as simple as possible, I will approach only these characteristics: Power and gain.

Range

The range of a radio transmission DOUBLES every 6dB (decibels).

But how do you know how many dB your system has? For this, you must know the power of your transmitter in dBm (decibel milliwatt), and the gain of your antennas in dBi (decibel isotropic).

Power

The power of the transmitters is usually expressed in mW (milliwatt), not in dBm. The easiest way to convert mW to dBm is by using this calculator .

Below is a small reference table with the most common values:

mW dBm
0,1 mW -10 dBm
1 mW 0 dBm
10 mW 10 dBm
25 mW 14 dBm
50 mW 17 dBm
100 mW 20 dBm
200 mW 23 dBm
400 mW 26 dBm
500 mW 27 dBm
600 mW 28 dBm
800 mW 29 dBm
1000 mW 30 dBm
1600 mW 32 dBm
2000 mW 33 dBm

Note that for every 6dBm increase, the transmitter needs to be 4x more powerful! With 25mW you have 14dBm, and with 100mW (4x more) you have 20dBm.

Antenna Gain

The antenna gain is measured in dBi (isotropic decibel). Antennas are NOT able to “amplify” the radio signal. All gain occurs because the antenna is more efficient in one direction and less efficient in another direction.

Low gain antennas (from 1 to 3dBi) are Omni-directional antennas. They can radiate the radio signal in almost all directions, with good efficiency in most of them and with few shadow spots.

Antennas with high gain (above 8dBi) are directional antennas, that is, antennas that need to be pointed to the origin of the signal. They have a lot of gain in specific directions, but in other directions the gain is very low.

The higher the gain, the more directional the antenna is.

Practical example

In a system consisting of a 25mW (14dBm) video transmitter, a 2dBi antenna on the transmitter and a 3dBi antenna on the receiver, you have a total of 19dB.

Let’s assume that with 19dB you get a distance of 400 meters.

To double this distance to 800 meters, you need to increase the capacity of your system by 6dB. For this you can increase the power of the transmitter, or use antennas with higher gain.

In the first case, you would need a 100mW transmitter (4x more powerful than the original 25mW).

In the second case, you would need a 9dBi antenna on the video receiver, and you would have to worry about keeping the antenna always positioned optimally.

Conclusion

Increasing transmitter power is not always an efficient way to increase transmission range. The increase in power leads to some undesirable effects, such as an expressive increase in the consumption of electric energy, an increase in the operating temperature, and often an increase in the physical size and weight of the transmitter. 25mW video transmitters tend to be much easier to install and accommodate in small aircraft models than 2W transmitters. In addition to consume less battery power.

The same applies to radio control transmitters: The more powerful, the faster it will deplete the radio’s battery, the more it will heat up and the heavier it will be.

Using high gain antennas may also not be a viable option as this type of antenna only works if it is properly pointed at the signal source, which is not a very simple task to perform without an automatic positioning system (Antenna tracker ).

It is necessary to find the ideal combination between transmit power and antennas for each specific use.

See also

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