Topics

• Wi-Fi Fundamentals
• Antennas
• Wi-Fi Range
• dBm to Watt Conversion

Wi-Fi Range

The range that can be achieved when connecting to a Wi-Fi hotspot is affected by numerous factors including Wi-Fi device power output, antenna type and gain and characteristics of the surroundings. These factors are discussed following.

Wi-Fi Devices

Wireless networking (Wi-Fi) at 2.4 GHz is a two way system. Each device must be capable of both sending and receiving a signal equal distances. Think of it as two people, a substantial distance apart, throwing a ball to each other.

Person A has to be strong enough to throw the ball to person B. Person B also has to be strong enough to throw the ball back to person A. If neither Person A nor B can throw the ball that distance they will not have much of a throwing game.

Access points and wireless routers (as shipped from the manufacturer) have an advantage over laptop because they have a higher output power and therefore have the ability to send a signal further than most laptops. When a higher gain antenna and Wi-Fi device are installed, the output power is now increased closer to the output level of the access point or wireless router thereby equaling the two devices. You will need to replace your Wi-Fi device and antenna if the distance you are attempting to achieve is greater than the capabilities of the laptop when using the built-in antenna that came with your laptop.

Laptops typically put out between 35-50mW of power. In a coffee shop, this may be completely adequate, but when distances exceed 100 feet, connectivity may be lost or impossible to get.

Access Point To Client Device
Output power of client devices with factory antennas are less than that of access points. A signal from an access point will travel farther than that of the client device. While it may be shown as an available access point in a laptop, the access point cannot be connected.		A high power Wi-Fi device and high gain antenna solution extends the range of the client's laptop, increasing its power to equal or exceed that of the access point.

Note that extending the range of your Wi-Fi 802.11b/g/n device is only a piece of the whole puzzle. At 2.4GHz (the frequency that Wi-Fi operates at), Line-Of-Sight is an important factor. Please see below regarding Line-Of-Sight.

Line-Of-Sight

802.11b/g/n at 2.4GHz requires unobstructed visual Line-Of-Sight (LoS). Unobstructed Line-Of-Sight means just that; there should not be trees, terrain, buildings, or structures between your two antenna points. Basically both antennas should physically be able to see each other. The radio waves at this high frequency will not penetrate metal, steel, concrete, cement, stone, brick, etc. very well, if at all. Wood and water will absorb the signal.

Surrounding the visual Line-Of-Sight is the Fresnel zone (LoS, image 1). Any obstructions that come into the Fresnel zone, although not obstructing the visual Line-Of-Sight, may hinder and effect your signal. The radio waves may deflect off of those obstructions. This is called Near Line-Of-Sight (nLoS, image 2). Although you may see a slight signal with nLoS situations, your data transfer rate may decrease. You may find you are incapable of accessing the internet. An obstruction that cuts across the visual Line-Of-Sight and prohibits an optical visual between the two antennas in your bridge is considered Non-Line-Of-Sight (NLoS, image 3). Any signal, in this case, will be minimal or non-existent.

Line Of Sight (LoS)


Near Line Of Sight (nLoS)


Non Line Of Sight (NLos)

You may find in your situation that your antenna and that of the access points can visually see each other through spaces and breaks in an obstructing tree or tree line. Please note that tree branches that cross the visual Line-Of-Sight will move with the wind. This movement will disrupt and have an effect on a vertically polarized Wi-Fi signal, such as an omni-directional antenna.

Most 802.11b/g/n antennas on the market today are linear (or vertically) polarized. This includes the small, "rubber ducky" antennas that ship from the factory with most wireless devices.

A radio wave travels through the air about the size of a pine needle. If the antenna is vertically polarized the pine needle must remain vertical, as sent. If the signal hits an obstruction the signal will flip or rotate into multiple positions as it gets to the receiving radio's antenna where it will be seen as noise. The vertically polarized antenna will not capture that signal.