8.7.2 Calibrated Measurement Antennas
Frequency-selective systems commonly use calibrated antennas whose antenna factor, gain, polarization, impedance, and directional response are known over a specified frequency range. The antenna factor and associated cable and receiver corrections allow the measured terminal voltage or power to be related to the incident field quantity.
Dipoles are useful as well-characterized narrowband reference antennas. Biconical antennas provide broad coverage at lower VHF frequencies; log-periodic antennas provide wider-frequency directional measurements through VHF, UHF, and adjacent bands; and horn antennas provide stable gain and directivity at microwave and millimeter-wave frequencies. These descriptions are general: the usable range and accuracy of any antenna are those established by its calibration and the measurement method.
Directional antennas can help identify a source and reject signals arriving from other directions, but their response depends on pointing and polarization. Measurements of an unknown or varying field may require rotation, orthogonal polarizations, or systematic spatial scanning. An antenna calibrated under far-field conditions should not be assumed to provide the same accuracy in a strongly nonuniform near field.
8.7.3 Selecting And Positioning A Sensor
Sensor selection should account for the quantity to be measured, frequency range, field region, expected level, required spatial resolution, polarization, directionality, dynamic range, and permissible uncertainty. The entire signal path, including cables, adapters, preamplifiers, and software corrections, must be included in the calibration and uncertainty evaluation where applicable.
During measurement, the sensor should be positioned and oriented as required by the assessment method. The operator, support structure, and connecting cable should be kept from materially perturbing the field. Adequate spatial sampling is required to identify localized maxima and to perform any prescribed spatial averaging. Section 8.9 examines these practical measurement effects in more detail.