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Wireless Interference

Types of RF (Radio Frequency) Interference & Antenna Placement
Direct interference
Other 802.11b/g devices. Access Points nearest to each other should be set to non-overlapping channels (unless bridged). The theoretical non-overlapping channels are 1, 6, and 11. Any channels in between will overlap with one of these 3, causing interference and reduced bandwidth. North America uses only Channels 1 through 11 (2.412Mhz 2.462Mhz)
Indirect interference
Non-802.11b/g devices such as microwave ovens, 2.4 cordless phones and baby monitors. Again, most Wi-Fi systems use channel 1, 6 or 11 as the default. Channel 6 is usually the most common default, so try channel 1 or 11. If that doesn't work, try switching the channel to 7 or 10 to avoid the channels used by the competing non-802.11b/g devices.
Path interference
Fish tanks, water heaters, hot tubs, filing cabinets, foil-backed insulation, metal bookcases, tinted windows that use metal-based films and aluminum window screens. Reflection, Refraction, Diffraction, Scattering
Line of Sight interference
Signal absorption from interfering objects such as dense building materials, walls, furniture, filled cinder block, brick, rock fireplace walls, adobe or stucco construction, and trees.

The location of your access point (AP) is important. Pick a place near the center of your building. Keep the AP out of the line of sight of microwaves, cordless phones, refrigerators, and other appliances that contain metal. Avoid putting the AP low to the ground by placing it on a table or a shelf. If you have an 802.11b or g network, consider getting one of the newer 5GHz cordless phones. An older 900MHz phone won't interfere with either type of network. Your computer can cause interference, so keep the AP away from it. Bluetooth-enabled devices can be a source of interference, as well.

Power Output & Antenna Gain & Health Effects

The power output of the radio is in dBm and the gain of your antennas is in dBi.

Gain is term used for measuring of increase in signal power, described in decibels (dB) which is a logarithmic measure of relative power. Gain is typically measured in dBi (decibels relative to isotropic radiator - a theoretically "perfect" antenna) and is based on decibels. The dBi is computed by comparing the output of the antenna to a theoretical isotropic radiator (antenna) with a dBi of 0: the higher the dBi measurement, the higher the power level of the antenna.

Antennas are designed to raise the power in the wanted direction and reduce it in unwanted directions.

To convert milliwatts to dBm :(0 dBm = power dissipated by a 1 mV signal into a 50W load)

dBm = 10 * (log (1000 * P))
P = Power in Watts
1000mW = 1 Watt

To convert dBm to milliwatts:

P = 0.001 * (10**(dBm/10))
P = Power in Watts

Calculator to convert dBm to watts and milliWatts

A 3 dBi gain antenna is equivalent to raising transmit power from 10 mW to 20 mW.
5 dBi ~ 33 mW,
8 dBi ~ 66 mW,
12 dBi ~ 160 mW,
15 dBi ~ 320 mW,
18 dBi ~ 640 mW.

EIRP (pdf), Equivalent Isotropic Radiated Power, or (ERP, effective radiated power) is the total effective transmit power of the radio system which includes the gain of the antenna and any losses from the antenna cable.

WiFi antennas emit RF electromagnetic energy that can harm humans. Because of that, there are standards that limit EIRP.

Section 15.247 (2006) of the Federal Communications Commission rules defines power limitations for license-free radio emissions. The rules rely on effective isotropic radiated power (EIRP), which represents the total effective transmit power of the access point (including gain provided by the antenna and loss provided by the cable). The FCC requires EIRP to be 1 watt (+36 dBm) or less for omni-directional antennas that have 6-dBi gain, and for directional antennas the EIRP can increase by 1 dB for every 3-dBi gain provided by the antenna.

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Revised: 02/28/2008