Understanding how radio signals propagate in your home is the foundation for building a stable wireless network. Many users mistakenly believe that simply buying a router with the maximum number of antennas will solve speed issues. However, actual coverage depends on the complex interaction of numerous physical parameters, which can and should be calculated.
In this article, we'll explore the meaning behind those mysterious numbers in equipment specifications and why the stated 1000 mW doesn't always translate to a powerful signal in the farthest room.
First, it's important to understand the basic difference between transmitter radiated power and the effective radiated power of the entire system. This is a nuance that is often overlooked when selecting network equipment for large areas.
Basic units of measurement: mW vs. dBm
The first step to proper calculations is to abandon traditional linear units in favor of logarithmic ones. Transmitter power in routers is traditionally measured in milliwatts (mW), which is convenient for understanding power consumption, but extremely inconvenient for assessing signal strength over a distance.
Telecommunications engineers use a scale dBm (decibels relative to 1 milliwatt), as it allows for easy addition and subtraction of losses and gains, rather than multiplying complex coefficients. Conversion between these quantities is accomplished using the formula: P(dBm) = 10 * log10(P(mW)).
For example, a standard power of 100 mW on a logarithmic scale would be only 20 dBm, while increasing the power tenfold (to 1000 mW) would only add 10 dB, reaching 30 dBm. This logarithmic relationship helps quickly assess the impact of changes in the system.
When reviewing equipment specifications, always pay attention to the units of measurement, as marketing departments often use different standards to attract attention.
⚠️ Attention: Don't confuse dBm (absolute power) with dB (relative change). Antenna gain is always specified in dB, not dBm.
To quickly convert values, you can use the following conversion table, which will be useful when reading technical documentation:
| Power (mW) | Power (dBm) | Typical application |
|---|---|---|
| 1 mW | 0 dBm | Minimum level, standard |
| 10 mW | 10 dBm | Weak IoT transmitters |
| 100 mW | 20 dBm | Standard home router |
| 500 MW | 27 dBm | Powerful access points |
| 1000 MW | 30 dBm | The limit for many standards |
EIRP Formula: Calculating Effective Power
The key parameter that really affects the communication range is EIRP (Effective Isotropic Radiated Power) is the effective radiated power. It indicates the power that an ideal isotropic antenna would radiate to produce the same signal level in the direction of maximum intensity.
EIRP is calculated by adding the transmitter power and the antenna gain, then subtracting all cable and connector losses. The formula is: EIRP = P_tx + G_ant - L_cable, where all values should be expressed in dB or dBm.
Imagine this: you have a router with a power output of 20 dBm (100 mW) and an external antenna with a gain of 5 dB. If you connect them with a high-quality short cable with a loss of 1 dB, the total power will be 24 dBm. This is a significant increase compared to the standard antenna.
It's important to understand that increasing transmitter power without adequate antenna gain or using long cables may not produce the desired results. Sometimes it's cheaper to replace the cable than to buy a more powerful transmitter.
⚠️ Attention: The total EIRP power in the 2.4 GHz band in the Russian Federation must not exceed 100 mW (20 dBm) for access points. Exceeding this limit is a violation of radio spectrum regulations.
For the 5 GHz band, standards may vary depending on the specific sub-band and type of equipment, so always check with the regulator for the latest regulations.
The influence of antennas and cables on the final signal
The antenna does not create energy, but only redistributes it in space. Gain The gain of an antenna shows how much denser the radiation flux becomes in a given direction compared to a perfect sphere. The higher the gain, the narrower the radiation pattern.
Using a high-gain antenna (for example, 9 dBi instead of the standard 5 dBi) can significantly increase horizontal range, but will also flatten the signal, making it weaker on the floors above or below. This is critical for multi-story buildings.
Cable infrastructure also plays a role. At 2.4 GHz, the loss in the popular RG-58 cable is approximately 0.5 dB per meter, while at 5 GHz it can reach 0.8 dB or more. A seemingly small 3 meters of cable can absorb up to 3 dB of power, equivalent to a loss of half the signal.
When calculating the route, always take into account losses at the connectors (usually 0.2-0.5 dB per connection) and lightning protection, if it is installed outdoors.
Friis' formula for calculating free-space attenuation
Attenuation (dB) = 20*log10(d) + 20*log10(f) + 32.44, where d is the distance in km and f is the frequency in MHz. This formula shows that doubling the distance weakens the signal by 6 dB, and doubling the frequency also weakens it by 6 dB.
Therefore, when assembling a system with remote antennas, the choice of high-quality low-loss cable (for example, LMR-400) is often more important than buying the most powerful antenna.
Regulation and power limitation standards in the Russian Federation
The use of the radio frequency spectrum is strictly regulated by the state. In Russia, the primary document governing these issues is the decision of the State Commission on Radio Frequencies (SCRF). Equivalent radiated power limits are established for the 2.4 GHz and 5 GHz bands.
For the 2400–2483.5 MHz band (standard 2.4 GHz Wi-Fi), the maximum EIRP should not exceed 100 mW (20 dBm). This means that if your router outputs 20 dBm, you cannot use an antenna with a gain greater than 0 dBi (isotropic) unless you reduce the transmitter power using software.
However, most modern routers have software limitations that prevent the legal limit from being exceeded, even with powerful antennas. In the 5 GHz band, the limits can be stricter (often up to 200 mW EMI for some sub-bands, but with mandatory use of DFS).
⚠️ Attention: Radio frequency usage regulations are subject to change. Before installing high-power equipment in a commercial property or apartment building, be sure to check the current regulations on the official Roskomnadzor website or in the equipment documentation.
Violating these regulations can lead not only to fines, but also to interference with critical services, as Wi-Fi frequencies border military and satellite bands.
Practical calculation of coverage and signal attenuation
For real-world network design, knowing only transmitter power is not enough. Signal attenuation when passing through obstacles must be taken into account. Walls, ceilings, and even people significantly reduce signal strength.
A rule of thumb is that the signal attenuates by 6 dB for every doubling of distance in free space. However, indoors the situation is more complex. A plasterboard wall can attenuate the signal by 3-5 dB, a brick wall by 10-15 dB, and a reinforced concrete wall by 20 dB or more.
To calculate the approximate signal strength at the receiving point (RSSI), use the formula: RSSI = EIRP - PathLoss - WallLoss. If you have 20 dBm at the router's output, the loss at a distance of 10 meters in free space will be about 40 dB. Therefore, the signal level will be -20 dBm (which is very high), but one brick wall will add another 12 dB of loss, giving us -32 dBm.
☑️ Check before calculating power
For receiver sensitivity (smartphones, laptops), the normal level is considered to be -60...-70 dBm. If the calculation shows values below -80 dBm, stable operation will not be possible, and either increasing the power (within the specified limits) or adding a repeater/access point is required.
Tools for measuring and verifying calculations
Theoretical calculations are good, but practice often makes its own adjustments. To fine-tune the network, you need to use specialized software. Programs like WiFi Analyzer or NetSpot allow you to see the real picture of the ether.
When taking measurements, pay attention not only to the signal strength (RSSI) but also to the signal-to-noise ratio (SNR). A high signal strength is useless if a powerful microwave oven or a neighbor's router is operating on the same frequency nearby.
A signal level above -65 dBm with an SNR of at least 25 dB is considered ideal. If your measurements show worse results, recalculate the link budget, taking into account these new factors, or reposition the antennas.
Remember that power is only one side of the coin. The client device (phone) has a small antenna and low transmit power, so it can "hear" a powerful router, but the router won't hear the phone's weak signal in response.
How to convert dBm to mW without a calculator?
Remember the basic points: 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1000 mW. Every +3 dB doubles the power, and -3 dB halves it. For example, 23 dBm is 20 dBm (100 mW) + 3 dB (x2) = 200 mW.
Why is a router with 5 antennas not always better than 3?
The number of antennas often indicates MIMO (multi-channel) technology support, not power. Three antennas can provide speeds for three streams, and five antennas for five, but their transmit power may be the same and limited by law.
Is it possible to increase the router's power programmatically?
In stock firmware, this option is often hidden or region-specific. Installing alternative firmware (OpenWrt, DD-WRT) can remove this restriction, but this will void the warranty and may result in exceeding EIRP limits and interference.
Does the installation height of the router affect the power?
The transmitter power itself won't change, but the installation height affects signal propagation. The higher and more central the router is, the fewer physical obstacles (furniture, people) there are in the signal's path, improving the actual reception level.