Review: Point-to-Point Systems: Page 13 of 15

Fresnel Zone: The amount of clearance needed for a radio wave to clear an obstacle--building, hill, the ground--such that the obstacle does not cause attenuation of the signal. At least 60 percent of the diameter of the calculated Fresnel Zone must be clear for the signal to minimize the attenuation caused by the obstacle.

Frequency: The range of frequencies that a radio can operate at. For instance, the 5-GHz ISM frequency band includes frequencies from 5.725 GHz to 5.850 GHz.

Gain: An increase in the power of a wireless signal. Antennas can add gain (measured in dBi) by focusing a wireless signal in a particular direction. Amplifiers can also add gain (measured in dBm) by adding power from an external power source. Radios will have amplifiers in them to add gain to a signal. Both dBi and dBm can be used to derive the overall gain of a link in dB.

Interference: Interference is generated when two RF devices operate on the same frequency. Interference can cause an RF signal to be impaired such that it cannot be understood by the radio on the other end.

Line of sight: An unobstructed path between two points in a wireless link is termed to have "line of sight." However, just because you can see between the two points in the link, doesn't mean there is enough clearance for the radio link to operate reliably. You must calculate the Fresnel Zone of the link to ensure that enough clearance has been budgeted around any obstacles.