Field Strength Calibration Techniques at the National Bureau of Standards

The techniques and instrumentation utilized by the National Bureau of Standards to calibrate field strength meters and standards in terms of absolute field strength at frequencies from 10 kHz to 1 GHz are described. Two basic techniques are used: (1) the standard-field method for loop antennas at frequencies from 30 kHz to 30 MHz and (2) the standard-antenna method for horizontally-polarized dipole antennas at frequencies from 30 to 1000 MHz. The accuracy and the limitations of each technique are discussed. Present loop antenna calibration uncertainties vary from 3 to 5 percent depending on the frequency. Dipole antenna uncertainties are presently 12 percent. Various other antenna calibration techniques that have been studied are mentioned, pointing out their limitations. The instrumentation used to calibrate the field strength receiver characteristics is described. These measurements include the calibration of the signal attenuators, calibration of the overall linearity of the receiver, and calibration of the receiver as a two-terminal rf voltmeter. Future plans of the National Bureau of Standards field strength and noise calibration services are discussed. Improved uncertainties and extended services are planned for antennas from 30 Hz to 1000 MHz. New noise calibration services are planned; these include the calibration of random noise generators at selected frequencies from 30 to 1000 MHz, and the calibration of impulse generators in terms of their spectral density.