20 dBm range in meters how high

WiFi antennas

Antennas bring both transmission and reception gains by bundling electromagnetic waves. With WLAN antennas of various designs you can therefore influence the range and direction of WLAN devices and improve the area of ​​the free wireless network with appropriately selected and positioned antennas: The antennas of commercially available WLAN end devices (such as WLAN routers, laptops, etc.) allow 30 to Expect a range of 100 meters in open space, but outdoor devices or better antennas can sometimes achieve over 10km, so they can reach everything within sight. With the latest technology, even 80 meters can be reached in closed rooms.

Better WiFi hardware should allow the connection of an external antenna. With external omnidirectional antennas, 100 to 300 meters can be bridged in the open with visual contact.

The frequency use of the radio bands in Germany is determined by the Federal Network Agency. There are two regulations on the subject of WLAN that describe the permitted usage framework:

A publication and the document Short Term Frequency Assignment important general assignments from the Federal Network Agency also names the most important points.

There it says:

WLANs work in the frequency ranges 2.400 GHz - 2.4835 GHz, 5.150 GHz - 5.350 GHz and 5.470 GHz - 5.725 GHz. The Federal Network Agency has allocated these frequencies for use by the general public. With WLAN radio connections, different properties can be connected to one another without the obligation to register. There is no specific range prescribed. This is determined exclusively by the maximum radiated power of the radio system and the surrounding conditions such as buildings, forest cover, terrain, etc. In the frequency range 2.400 GHz - 2.4835 GHz, the maximum radiated power must not exceed 100 mW (EIRP). A maximum of 200 mW (EIRP) is permitted in the 5.150 GHz - 5.350 GHz frequency range, while a maximum of 1 W (EIRP) may be emitted in the 5.470 GHz - 5.725 GHz frequency range. No specific antennas are prescribed for WLAN radio applications. The maximum radiated power must not be exceeded and the manufacturer's declaration of conformity of the radio system must not be violated by changes to the antenna. With the declaration of conformity, the manufacturer certifies the conformity of the technical properties of the radio system with the requirements of a technical standard. If changes of any kind are planned to the system, a specialist dealer or the manufacturer should be consulted beforehand.

The details of the transmission power in mW (EIRP) can be converted into dBm with the Radiated Power Calculator (which is often the unit of measurement used in the driver software / firmware) and the maximum transmission power is obtained

  • 20dBm for 2.4GHz
  • 23dBm for 5.150GHz to 5.350GHz (only indoor use allowed - only with aTPC - otherwise max. 20 dBm - DFS must be activated)
  • 30dBm for 5.470GHz to 5.725GHz (only with aTPC - otherwise max. 27 dBm - DFS must be activated)

For the technical basics of DFS and TPC, reference is often made to EN302502.

The transmission power (in dBm) of a WLAN device is calculated from:
+ Transmission power (dBm)
+ Gain amplifier (dB) (if present)
- Cable attenuation (dB)
- Attenuation plug (dB)
- Attenuation of lightning protection (dB)
+ Gain antenna (dBi)
─────────────────────────────────────
= Total transmission power

Only the transmission path is calculated here. The legislature has not imposed any restrictions on the route of reception.

There is also an antenna computer at heise.de and at Freifunk Augsburg you can have the expected signal quality between two WLAN stations approximately calculated.

Some WLAN devices are also capable of antenna diversity modes, in which the errors caused by interference are reduced by using two antennas at the same time for reception or alternately for transmission. The two antenna connections can also be used strictly separately for sending and receiving. This has the advantage of being able to use an antenna for receiving that would exceed the permissible transmission power.

Special coaxial connectors for high-frequency applications are used to connect a WLAN device to an associated antenna. With WLAN, these are mainly the otherwise rarely used connectors RP-TNC and RP-SMA.

Attenuation in the 2.4GHz band

The IEEE 802.11b / g standards use up to 14 channels (11 in the USA, 13 in Germany, 14 in Japan) in the ISM 2.4GHz spectrum, which is divided into 5MHz steps (channel 1 starts at 2.412GHz and channel 11 is therefore at 2,462 GHz) at a wavelength of 12.5 cm.

Signals in this band are easy to disrupt (understanding and mitigating the impact of RF interference on 802.11 networks), which is why good antennas are very important:

  • Due to the wavelength and the selected step size, there are only four orthogonal channels (1, 5, 9, 13) that have no overlap (i.e. interference) with their neighboring channels.
  • There are other technologies in the band: cellular phones, Bluetooth, Zigbee (IEEE 802.15.4) embedded devices, RFID tags and more, which can still interfere with each other even though they are supposed to coexist (such as 802.11 and Bluetooth).
  • (Old, commercial) microwave devices also significantly interfere with this band (Microwave oven interference on wireless LANs operating in the 2.4 GHz ISM band)
  • Lightweight walls reduce the range by attenuating the signals, but individually are not an obstacle; on the other hand, steel and concrete are not penetrated, but can be used experimentally as a reflector wall outside to "reflect out" dead spots. Trees, especially leafy ones, are also obstacles to WiFi connections.
  • The Fresnel zone is much larger here than in the 5 GHz range. Objects such as trees are penetrated better by 2.4 GHz frequency signals than signals with 5 GHz.

The 5GHz band

WLAN according to 802.11a / n works in the 5 GHz band, in which a larger frequency range is available than in the 2.4 GHz range. Many disadvantages such as microwave interference at 2.4GHz are eliminated here, but the band is close to satellite connections and radar devices that must not be disturbed. (TPC) and "Dynamic Frequency Selection" (DFS) must use (ETSI EN 301 893). DFS is particularly important throughout Europe, as otherwise weather radars that use the same frequency ranges will be disrupted. If the device software cannot handle automatic TPC, the maximum transmission power is 3 dBm lower. These techniques are described in the European standard ETSI EN 301 893 (currently version 1.7.0) and apply to all of Europe.

Overall, the 5 GHz band should always be used if possible, as this is used much less than the 2.4 GHz frequency band and thus more stable radio connections are possible, even over long distances.

The 60GHz band

Data transmission is permitted in this band and permitted for the general public. There is no standard like WiFi in this area and therefore the technology is a bit more exclusive and expensive.

This tape offers the following advantages:

  • high permissible transmission power (40 dBm; 100W)
  • high bandwidth, up to 2GHz
  • little interference from other frequency users

But there are also the following disadvantages:

  • high device costs
  • short range (by damping the air) up to 1000m

With special directional antennas, several kilometers can be bridged with visual contact. Here, records are sometimes set with connections over several hundred kilometers, in which no other active amplifiers are used apart from the antennas. However, this only works between high mountains; on the sea, visual contact ends after about 30 km due to the curvature of the earth.

German-language instructions and lists:

Instructions in English:

French-language instructions:

No WiFi antenna, but useful for wireless communication from routers over long distances:

  • RONJA (web archive), an optical through-the-air straightening section in the free and unregulated gigahertz band (red light) to build yourself. 10 Mbit / s net, connection via 10BaseT-FD and AUI.