Calibration of antenna factor at a ground screen field site using an automatic network analyzer

Two approaches are described that permit automatic stepped-frequency data acquisition and rapid calculation of several antenna parameters. These parameters include antenna factor, intrinsic gain, realized gain, antenna input impedance, voltage standing wave ratio and site attenuation. The effects of antenna impedance on the meaning and accuracy of antenna measurements are emphasized throughout. The technique now used at the National Institute of Standards and Technology for calibrating the antenna factor at frequencies from 25 to 1000 MHz uses a standard open-circuit half-wave receiving dipole to measure the electric field strength. This approach is compared with a three-antenna method that uses an accurate automatic network analyzer with 120 dB dynamic range to measure insertion loss between the transmitting and receiving antennas. A field site having a 30*60 m ground screen, which acts as a good reflector, is used. Thus, the effects of ground reflection are calculated and compensated for. The new insertion loss technique permits faster measurements with greater repeatability and reduction in calibration uncertainty, especially at frequencies above 75 MHz.<
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