Near-field radiation patterns are useful in diagnosing antenna array defects, measuring far-field antenna patterns where the far-field is prohibitively far, and locating field concentrations in high power microwave applications, which could lead to material breakdown. There are two categories of near-field measurements: direct and indirect. In a direct measurement, the field from the antenna-under-test (AUT) is directly measured by a probe whereas, in an indirect measurement, the field is inferred from the scattering off of a probe that is placed in the near-field.
A simple direct measurement method is to use an open-ended waveguide connected to a network analyzer as the receiver. The near-field is measured by spatially scanning either the AUT or the receiver. Reflections off of background objects can be filtered out in the time domain. For array applications, antennas that are smaller than the open-ended waveguide must be used to achieve spacing less than a half wavelength. An array of time-domain sensors has been demonstrated using loop antennas. The metal transmission line from the probe can perturb the very-near-field making the direct measurement approach less favorable than the indirect measurement approach.
In an indirect near-field measurement, a probe is placed in the near-field and the near-field at the probe is inferred from the scattering off of the probe. These reflections can be measured in either a monostatic or bistatic configuration. In a monostatic configuration, where the transmitting and receiving antenna are both the AUT, the received signal is proportional to the square of the gain of the AUT. Conversely, in a bistatic configuration, where the transmit antenna is the AUT and the receive antenna has a known antenna pattern, the received signal is proportional to the gain of the AUT after normalizing by the antenna pattern of the known receiver. If the scattering strength of the probe is modulated, then the reflected signal off of the probe is amplitude modulated and can be isolated from the unmodulated background signals. The probe can either be modulated mechanically, electrically, or optically. Mechanical modulation is not preferred because it may cause ambiguities in polarization measurements and has a physically limited modulation frequency.
When a probe is electrically modulated, the modulation signal is delivered to the probe on bias lines. These bias lines can cause the maximum amplitude response of the probe to shift in frequency and must be accounted for in the probe design. In an optical modulation system, the active element (typically a photodiode) in the probe is modulated by a laser through a fiber optic cable. The photodiode in an optically modulated probe can operate without a bias voltage, which removes the need for bias lines. Additionally, the fiber optic cable that couples the modulation signal onto the probe has no metal, which reduces the perturbation on the near-field by the probe compared to an electrically modulated probe with metallic bias lines.
To decrease measurement time, the modulated probes can be assembled into arrays. Both electrically and optically modulated probe arrays have been demonstrated. Each probe could be modulated at a different frequency to measure the field at each probe simultaneously to further reduce measurement time.
A block diagram of a monostatic, near-field, measurement system with an optically modulated scatterer (OMS) probe is shown in the accompanying figure. An on-off modulated light source delivers an optical signal to the photodiode in the OMS probe that modulates the radar cross-section (RCS) of the OMS probe. The AUT transmits a signal from an RF source into free-space. The return signal consists of a large, unmodulated signal with a small, modulated signal due to the modulation of the RCS of the OMS probe. The unmodulated signal consists of reflections off of objects in the room, including support structures, and from the impedance mismatch between the AUT aperture and free-space.
The combined modulated and unmodulated signal is down-converted and a narrow-band filter eliminates the unmodulated component of the signal. The modulated signal is recorded by a digital lock-in amplifier at the modulation frequency. The amplitude of the measured signal is proportional to the square of the gain of the antenna because the signal is passing through the AUT twice. The OMS probe is raster scanned in a plane in front of the AUT to record the spatial field distribution at a set of discrete points.
This work was done by Mark Patrick, Surface Electronic Warfare Systems Branch, Tactical Electronic Warfare Division; Meredith N. Hutchinson, Photonics Technology Branch, Optical Sciences Division; and J. Brad Boos, Electromagnetics Technology Branch, Electronics Science and Technology Division for the Naval Research Lab. NRL-0071
This Brief includes a Technical Support Package (TSP).
Development of an Optically Modulated Scatterer Probe for a Near-Field Measurement System
(reference NRL-0071) is currently available for download from the TSP library.
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