As a wireless signal propagates through the air, its strength is diminished as a function of distance. To calculate the free space loss formula you can go to sites such as swisswireless.org or zytrax.com and figure out the range of a device.
While that range might be of interest in an outdoor application or in a large conference hall where you have direct line-of-sight throughout the device’s range, those formulas aren’t useful for indoor purposes.
You’ve measured signal strengths and done an RF site survey (if you took our advice) simply because it is so difficult to accurately predict the factors that diminish your signals.
When a wireless signal is forced to propagate through denser material, it suffers a greater loss of signal, called attenuation, as it passes through the material. The strength of a signal as measured in milliwatts is attenuated by a certain amount as it is output from the transmission medium.
Attenuation is measured in decibels (dB), the same unit as sound strength, and is the logarithm of the power input to the medium divided by the output power over a specified distance.
Thus if you input 300 milliwatts into a wall and the output is 150 milliwatts, you have attenuated the signal by +3dB. A repeater that doubled the power would have an attenuation of –3dB.
An omnidirectional antenna with a 6dB rating therefore improves the performance of a signal by four times. As a signal is attenuated and diminishes, you reach a point where the signal strength won’t decode cleanly at the receiving end.
That forces the two endpoints in the transmission to have to request and resend the part that doesn’t error check correctly, which lowers the effective throughput of your connection.
Thus not only do you lose range during attenuation, but you lose throughput. Below a certain point, the wireless connection drops out completely. When maximum attenuation is calculated, a formula called EIRP (Equivalent Isotropically Radiated Power) is used.
It is important to understand that attenuation isn’t a linear function. The further away from the wireless source that you put a barrier, the more effect it has on dropping the signal strength.
Thus, a single wall close to the source will have considerably less impact on the signal strength than the same wall would have a couple of rooms away. Of course, what the wall is made of is important, too. The following estimates of attenuation are offered for common building features:
- Plasterboard: 3dB
- Office window: 3dB
- Concrete or cinder block walls: 4dB
- Glass wall in a metal frame: 6dB
- Metal door: 6dB
- Metal door in brick wall: 12.4dB
The type of wireless protocol you are using affects the attenuation of a signal through the same media. The higher frequencies and larger ranges of the 802.11g devices you might use suffer more degradation of their signal than an 802.11b device.
The essential issue in setting up a wireless bridge is one of distance and the resulting signal loss. Both are affected by the nature of your wireless devices and the path from one device through the other.
You can greatly affect your connection by selecting the right kind of antenna and positioning your devices correctly. If you are connecting two sides of the same house together, an omnidirectional or semidirectional antenna might be enough to create a strong link.
For a separate guest house on the property a more directional antenna might work, at one or better yet both wireless network nodes. Finally, for very long distances where you have direct line-of-sight, a high-gain antenna such as a can or waveguide, or a parabolic reflecting antenna would be the choice.
These highly directed signals are very sensitive to anything in their path, even leaves of trees. Whatever equipment you use, the goal is to calculate the strength of your direct signal.
It should be more than 20dBm above your access point’s sensitivity. Less than 20dBm results in signal losses that degrade the signal too much. You can calculate the power of the received signal with this general formula:
Received signal = Input power - total losses + total gain
where both Received signal and Input power are measured against a reference level such as dBm (1 milliwatt over a 600ohm impedance) and the total losses and total gain are in dB.
Your measurements also need to consider any signal loss over wiring such as coaxial cabling or connectors on each side of the bridge. A consideration of signal loss over distance will give you some appreciation of the needed equipment. For example, the calculated signal loss over a one-mile connection is 104dB.
