Wi-Fi Transmitter Power at 20 dBm: How Many Meters Will the Signal Reach?

The question of how many meters a wireless signal will reach with a transmitter power of 20 dBm is a common one among users looking to improve their home network coverage. The figure of 20 dBm, equivalent to 100 milliwatts, is often found in the specifications of standard routers and mid-range access points. However, a simple conversion from decibels to meters is impossible without taking into account many variable factors, such as frequency range, wall material, and the presence of interference.

In practice transmitter power — this is only one side of the coin that determines communication range. Many people mistakenly believe that increasing signal power will solve all the problems with "blind spots" in an apartment or house. In reality, even a strong signal can be useless if the client device (smartphone or laptop) isn't sensitive enough to "call" back to the router.

In this article, we will examine in detail the physics of radio wave propagation, calculate theoretical and practical distances for different conditions, and also consider how WiFi standards affect the final range. You'll understand why manufacturer claims often differ from reality and learn how to correctly assess the potential of your equipment.

Converting dBm to milliwatts and signal physics

First, you need to understand the units of measurement to understand the power your equipment operates at. 20 dBm is a logarithmic value, which corresponds to 100 mW on a linear power scale. This is a standard, fairly high power rating for household routers, permitted by regulators in many countries without requiring a license.

However signal level Doesn't remain constant over distance. Radio waves attenuate with distance from the source, and this process is described by the law of free space. The higher the signal frequency, the faster it loses energy. Therefore, the 2.4 GHz band will always have a greater range than 5 GHz with the same transmitter power.

⚠️ Attention: Legislation in various countries limits the maximum permitted radiated power in the 2.4 and 5 GHz bands. Using amplifiers that output power beyond 100 mW (20 dBm) or 200 mW (23 dBm), depending on the region, may be illegal and cause interference to nearby equipment.

It's also important to consider that router antennas act as a beamformer. They don't create new energy, but rather focus existing energy in a specific area. Therefore, antenna gain (measured in dBi) directly affects how far a signal will travel in a particular direction.

Formula for converting dBm to mW

To convert, use the formula P(mW) = 10^(P(dBm)/10). For 20 dBm, the calculation is 10 to the power of 2, which gives 100 mW.

Theoretical range in open terrain

If we imagine ideal conditions—an open field without trees, buildings, or precipitation—then calculating the range becomes a mathematical exercise. At 20 dBm and using a standard omnidirectional antenna, an 802.11n or 802.11ac signal can be reliably received at a distance of up to 100-150 meters.

However, "reliable reception" is a broad concept. For stable operation of modern apps, video calls, and streaming video, a high signal level (RSSI) is required, typically no worse than -65 dBm. At the receiver's sensitivity limit (around -85...-90 dBm), a connection may be formally present, but the speed will drop to a minimum.

The difference in range also depends on the channel width. A narrow channel (20 MHz) has greater penetration and range than a wide channel (40, 80, or 160 MHz), which provides higher speed but fades faster.

The influence of obstacles: walls and ceilings

In a real-life apartment or office, the signal encounters obstacles that absorb or reflect radio waves. Each material has its own impact on the final range. A 25 cm thick brick wall can attenuate the signal by 15-20 dB, which, with an initial signal strength of 20 dBm, will effectively reduce the range several times.

Metal structures, mirrors, and reinforced concrete are particularly critical for WiFi signal strength. Water contained in living plants or even in walls after rain also actively absorbs radiation, especially at the 5 GHz frequency. Wooden partitions and drywall are the least problematic materials.

Below is a table of approximate signal attenuation when passing through various obstacles:

Thus, in a typical apartment with load-bearing concrete walls, a 20 dBm signal can reliably cover only 1-2 adjacent rooms. Penetrating two load-bearing walls often renders the signal unusable.

Band Difference: 2.4 GHz vs. 5 GHz

With the same transmitter power (20 dBm), signal behavior in different frequency ranges differs dramatically. The 2.4 GHz range is characterized by a lower frequency and a longer wavelength, which allows it to better bypass obstacles and experience less attenuation in space.

The 5 GHz band, which provides high speeds, has a shorter wavelength. This makes it more susceptible to attenuation. At the same distance and under the same conditions, a 5 GHz signal will be approximately 10-15 dB weaker than a 2.4 GHz signal. This means that the range of 5 GHz will be approximately 50-60% of that of 2.4 GHz.

If you need to cover a large area or multiple rooms with thick walls, the 2.4 GHz band with a power of 20 dBm will perform significantly better. However, it's important to remember that this band is highly noisy due to neighboring routers and household appliances.

⚠️ Attention: Router interfaces may vary depending on the firmware version and device model. The location of the transmitter power settings may differ from that described below. Always consult the official documentation from the manufacturer of your equipment.

Communication asymmetry: the problem of the "deaf" client

One of the most common mistakes when planning a network is ignoring power asymmetry. A router with a power of 20 dBm (100 mW) is very loud, and your smartphone may show full signal strength (full bars) even through three walls. But will your smartphone be able to respond to the router?

The transmitter power in a mobile phone or tablet is usually significantly lower—around 15-17 dBm (30-50 mW), and their antennas are tiny and inefficient. As a result, the device sees the network but is unable to establish a connection or experiences constant drops.

This phenomenon is called link asymmetryIncreasing the router's power beyond 20 dBm won't help in this situation; it will only exacerbate the problem, creating the illusion of good coverage where there's no real connection. The effective range of a network is always limited by its weakest link—the client device.

To solve this problem in larger homes, a mesh system or repeaters are used, which are placed closer to client devices, reducing the "last mile" distance.

Practical methods of measurement and adjustment

To accurately determine how many meters your signal penetrates under specific conditions, relying on theoretical calculations isn't enough. You need to take measurements using specialized software. Android apps are great for this. WiFi Analyzer or WiFi Man, on iOS - AirPort Utility (in scanning mode) or Network Analyzer.

The measurement process is as follows: stand next to the router, record the signal level (it should be around -30...-40 dBm), and then begin moving in the desired direction. Once the signal level drops to -70...-75 dBm, you can consider the coverage area to be over.

To configure the power level, go to the router's web interface. Typically, the path looks like this: Wireless → Advanced Settings → Transmit PowerHere you can select "High," "Middle," or "Low." For a power of 20 dBm, select "High" or "100%," if available.

# Example command to check signal strength in Linux (iwlist)
sudo iwlist wlan0 scan | grep -E "Signal|ESSID"

Keep in mind that maximum power doesn't always mean the best results. In apartment buildings, an excessively strong signal can interfere with its own reflected signal (multiplexing), resulting in a drop in speed.