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Near-Field Antenna Measurements

An antenna's placement has a substantial bearing on its performance, which can vary across different frequencies and depends upon the shape of the object on which it is mounted.

Georgia Tech file photo At Fort Huachuca, Ariz., the U.S. Army constructed a compact radar range thought to be the world's largest. GTRI scientist Dr. R.C. Johnson invented the compact range in the late 1960s.

To determine the best antenna location for a particular situation, Dr. R.C. Johnson of the Engineering Experiment Stations (now Georgia Tech Research Institute, GTRI) invented the compact radar range in the late 1960s. The technique simulates a plane wave over the entire antenna under testing and involves two main elements: a parabolic reflector and a mechanical device that lifts and rotates the target antenna.

Although the compact range operates in a relatively small space — most are indoors — it can measure the radiation patterns of antennas as they would occur over long distances. Advantages of indoor compact ranges are security and the ability to operate regardless of weather conditions.

The characteristics of the compact radar range, now offered commercially by Scientific-Atlanta, are typical: Useful measurement area approximates a 4-foot cylindrical region — large enough for a 4-foot reflector — although actual useful area varies with frequency band and accuracy required. Antennas up to eight feet can be handled by the facility, which includes controlled access for security and a dedicated air-handling system for temperature stability.

Without the compact range, the alternative would have been a far-field range test site, which consists of two towers whose separation depends on the target size and frequencies studied. One tower holds the antenna under test while the other receives the signals. Given the size of vehicles and the high frequencies involved, far-field towers would have to be stationed several miles apart.

At Fort Huachuca, Ariz., GTRI designed an outdoor compact radar range, thought to be the world's largest, for the U.S. Army. The facility, which includes a 75-foot parabolic reflector and a lift capable of positioning objects up to 50 feet long and weighing as much as 70 tons, was built to measure antenna performance from 6 to 40 GHz on vehicles and helicopters.

For more information, contact Dr. Bob Trebits, Sensors and Electromagetic Applications Laboratory, GTRI, Georgia Tech, Atlanta, GA 30332-0859. (Telephone: 770-528-7915) (E-mail: bob.trebits@gtri.gatech.edu)

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Last updated: October 25, 1999