dBm, dBi and mW - Briefly explained

Understanding WiFi Units: mW, dBm, dB, and dBi Explained

In our offers and technical descriptions, you often stumble upon the mystical abbreviations dBi, dBm, mW, or Watt. What do these terms mean, and why are they important? It’s simple: they are the key to specifying the performance of your WiFi equipment.

The golden rule first: The higher the specified value (whether dBi, dBm, or mW), the better the transmit power or gain of the respective component.

1. Milliwatt (mW) - Transmit Power in Numbers

Milliwatt (mW) is the unit used to state the actual transmit power of your active WiFi components - meaning your WiFi router, Access Point, or USB adapter.

Since WiFi devices only transmit at low power (to comply with legal limits), we measure in milliwatts, the smaller, handier unit.

Commercially available WiFi components in Germany typically operate with a transmit power of 50 to 100 mW.

2. Decibel (dB) - The Simple Math Master

Decibel (dB) is a logarithmic measure. That sounds complicated, but it's super practical for those who are mathematically lazy! Instead of having to multiply and divide, you can simply add or subtract the values.

dB is used to describe the ratio of amplification or attenuation (power loss). For example, the attenuation of a long antenna cable or a connector is specified in dB. To calculate the total performance of your system, you just add up all the dB values.

3. Decibel Milliwatt (dBm) - Absolute Power

The unit dBm (Decibel Milliwatt) is an absolute power specification that is always relative to one milliwatt (1 mW). It is essentially a logarithmic representation of the Watt value.

dBm can also be used to specify the transmit power of a WiFi component. The usual 50-100 mW corresponds to 17-20 dBm. This logarithmic representation is especially useful because the power differences in the radio frequency range are often massive.

4. Antenna Gain (dBi) - The Focusing Power

dBi (Decibel isotropic) describes the gain of your antenna. This is not an absolute transmit power but a relative value.

The reference point is the so-called isotropic radiator (0 dBi): a theoretical, lossless antenna that radiates energy perfectly evenly in all directions.

Real antennas focus the signal. Imagine squeezing a water balloon - the water squirts out in a specific direction! By focusing the signal in a specific direction (defined by the antenna's beam width), you increase the gain relative to the isotropic radiator.

In short: An antenna with 9 dBi focuses the signal more strongly than one with 5 dBi, thereby increasing the effective range in the desired direction.

🚨 Super Important: The Legal Situation (EIRP)

In Germany (and large parts of Europe), the maximum Effective Isotropic Radiated Power (EIRP) is strictly limited! This power is the sum of your router's power (dBm) and your antenna's gain (dBi).

The limits depend heavily on the frequency band and are regulated as follows:

• 2.4 GHz Band: The EIRP must not exceed 20 dBm (100 mW). This is the fixed value you correctly pointed out.
• 5 GHz Band: Higher power levels are generally allowed here, typically up to 23 dBm (200 mW) indoors, and under certain, stricter conditions (e.g., DFS mechanism) even up to 30 dBm (1000 mW).
• 6 GHz Band (WiFi 6E): Rules are complex. Power is typically limited to 23 dBm (200 mW) for indoor use.

Note: The strict limit of 20 dBm (100 mW) primarily applies to the highly congested 2.4 GHz band. When upgrading to 5 GHz or 6 GHz, different rules apply, but compliance is still mandatory!

Always check your math: If your router transmits at 10 dBm in the 2.4 GHz band, your antenna can have a maximum gain of 10 dBi (10 dBm + 10 dBi = 20 dBm EIRP). Anything above that is illegal and can lead to problems!