2.4GHz
From ProPHOTO WIKI
Contents |
Introduction
The 2.4GHz ISM band is one of three regions of the spectrum originally allocated for non-commercial industrial, scientific and medical (ISM) purposes. As these bands weren't widely utilized for those purposes, many governments have reassigned them for use with unlicenced radio equipment such as wireless networking, cordless telephones and other common devices.
Under this new mandate, this band has seen explosive growth and is widely used in both residential and commercial environments. The popular IEEE 802.11b/g wireless networking protocols make use of these frequencies, as do other technologies such as Bluetooth.
Available Spectrum
Formally, the 2.4GHz ISM band consists of signals transmitted between 2400MHz and 2500MHz in the Microwave region of the spectrum. This naturally varies from jurisdiction to jurisdiction, so the exact limits on the available spectrum depend on where the equipment is being used. The following chart illustrates the location of this band relative to a number of other common services:
Applications
As the use of this band is unlicenced, there are a huge range of different devices that make use of it in very different ways. As such, use of several of these systems in range of one another may cause reduction in performance. A few of the more common technologies have been listed below:
IEEE 802.11b/g
One of the major standards that pushed the use of this band, WiFi is now widely used in wireless networks all over the world. This relatively high bandwidth application makes full use of the available spectrum, allowing three to four networks to cooexist within proximity of one another.
The 2.4GHz variants of the IEEE 802.11 standard divide this spectrum into up to 14 individual 5MHz channels (depending on local regulations) with 10MHz of padding on each end. As each network requires approximately 22MHz of bandwidth, each network occupies five channels centred on the users selection. Please see the following diagram for an illustration of this arangement:
Bluetooth
A short range wireless technology used to connect devices to one another, Bluetooth is widely used on many cellular handsets and handheld computers. While it doesn't provide the bandwidth of WiFi networks, it is much simpler to implement making it suitable for tasks that have modest bandwidth requirements.
Cordless Phones
Many modern cordless phones make use of this band to communicate with their base stations. As bandwidth requirements are relatively modest for this purpose, interference caused by these devices is generally avoided by simply changing the channel.
Microwave Ovens
As this band is a resonant frequency for water, Microwave ovens use high-power energy in this band to heat food. While these emmisions are typically well contained within the oven, when running some lower quality models may interfere with 2.4GHz devices in proximity.
Power Levels
Each jurisdiction has their own set of rules governing the use of this spectrum. The following sections highlight the regulations in several major markets. Note that rules do change over time, so users verify the limitations in their jurisdiction before pushing the limits.
This is an advanced topic, please see the articles on antenna gain and EIRP for background on these limits. Mainstream WiFi equipment is generally not powerful enough to reach these limits so an understanding of these regulations is typically only needed when using amplifiers or high-gain antenae.
North America
The maximum power output of a 2.4GHz ISM radio (before the antenna) is 30dBm (1 Watt). Additionally, anntena gain can be used to boost the radiated power to the equivalent of 36dBm (4W). That is, if a user had a 30dBm radio, they could use antenae with gain as high as 6dBi. Alternately, if using a 17dBm radio the user could use an antenna as large as 19dBi.
Point-to-Point
An additional exception is provided for radio connections using high-gain directional antenae to communicate with two discrete points. When using well focused transmissions over long-hauls, the FCC allows an increase of 3dBi of antenna gain for every 1dBm the radio is below the 30dBm limit. If, for instance, the radio output power was set to 24dBm than a 24dBi antenna (6+(6*3)) could be used to generate an EIRP of 48dBm (~63 Watts).
Examples
| Output Power | Maximum Antenna Gain | |||
|---|---|---|---|---|
| Multipoint | Point-to-Point | Multipoint | Point-to-Point | |
| 30dBm (1W) | 6dBi | 6dBi | 36dBm (4W) | 36dBm (4W) |
| 29dBm (794mW) | 7dBi | 9dBi | 36dBm (4W) | 38dBm (6.3W) |
| 28dBm (631mW) | 8dBi | 12dBi | 36dBm (4W) | 40dBm (10W) |
| 27dBm (501mW) | 9dBi | 15dBi | 36dBm (4W) | 42dBm (15.8W) |
| 26dBm (398mW) | 10dBi | 18dBi | 36dBm (4W) | 44dBm (25.1W) |
Europe
While much simpler than the North American regulations, European rules limit power output to substantialy lower levels. Radios operating on the 2.4GHz ISM band in Europe are limited to an EIRP of 20dBm (100mW). As such, building long range wireless networks on this band can be difficult.
