Free space loss and obstruction

The signal radiates out from the transmitting antenna and is detected by the receiving antenna. The signal power attenuates (drops off) as it travels between the antenna. This is called free-space loss. Free-space loss increases with the distance between the antennae and the frequency. The power drops with the square of the frequency and the square of the distance. Doubling the distance or the frequency will cut power by a factor of four. This calculator computes free space loss.

Depending on the material, obstructions can attenuate or completely block radio waves. Even with a clear line of sight between the transmitting and receiving antennae, communication may be impeded. Trees, buildings or other obstructions can block radio waves if they are in the fresnel zone around the line of sight. ("Fresnel" is pronounced "fraynel"). The shape of the fresnel zone is determined by the distance between the antennae and frequency. This calculator gives the theoretical radius of the fresnel zone. As a rule of thumb, a link can tolerate up to around 20% blockage, but in practice, links must be field tested.

There are a couple of basic propagation tradeoffs:

• High frequency waves attenuate faster than low frequency waves. (That is why 802.11a falls off faster than 802.11b or g).
• Buildings, trees, storms, and other barriers have less effect on low frequency waves than high.
• Antennae for low-frequency waves are generally larger than for high-frequency waves.
• A high-power signal will travel further, but is more likely to cause interference than a low-power signal.
• All things being equal, a more sensitive receiver will cost more than a less sensitive receiver.

If the transmitter and receiver are far apart, the curvature of the earth becomes a problem -- the earth is in the fresnel zone. This calculator takes the heigth of antenna towers, the curvature of the earth and atmoshpheric conditions into account.