In situ measurements of antenna patterns on rovers in a simulated terrain are difficult to make with conventional antenna range techniques. The desired pattern data covers a hemisphere above the antenna of interest, which is close to the ground. This is incompatible with traditional measurements that place the antenna under test on a movable support that tilts and rotates.
The solution is to suspend the probe from three or more flexible cables attached to computer-controlled winches that are supported above the test volume. By varying the length of the cables, the position of the probe can be moved anywhere in the test volume. A separate metrology system can be used to increase the accuracy of knowledge of the probe position and orientation.
The probe carrier, at the junction of the suspension cables, has actuators to alter the probe orientation. Power is supplied by passing current through the suspension cables, or by battery power on the probe carrier. Multiple sets of cables and probe platforms can be used to simulate multiple orbiting assets. The probe can be a transmitter, a receiver, or both, and can be at any frequency that is needed for the test scenario. A calibration reference, for phase and/or amplitude, can be transmitted to the probe carrier by RF, optical, or wired means.
This technology has been used to “fly” a video or film camera over a football stadium, but has never been used for antenna measurements or simulation of moving sources over an area. For low-precision applications, the lengths of the cables can be mathematically transformed into the position of the probe. An external tracking system, using optical or RF means, can be used for higher precision. Closed loop control can be implemented from the tracking system to the winch controllers to allow very precise and repeatable control of the position of the probe.
The cables can be made of any reasonably strong material, although materials with low elasticity (e.g. Kevlar or Spectra) are preferred for precision positioning. Steel cables have been used in the commercial flying camera applications. However, a non-conductive cable has significant advantages for the antenna and RF application, since it does not perturb the RF fields as much as a conductive cable; however, with suitable probe design, a conductive cable may be acceptable. Non-conductive cables improve safety, since the cables won’t conduct dangerous voltages or currents, as from an inadvertent contact with some energized conductor or lightning.
Power for the probe is provided either by batteries, solar cells, or other means. If it is desired to use conductive support cables, they can be used to carry power. In this case, the addition of suitable RF absorbing materials may be needed to reduce the effect on the measurement accuracies. Control and telemetry signals from the moving probe can be carried by an optical fiber or other wireless means. While three cables are the minimum to provide the desired positioning, the use of more cables can provide a wider range of motion, and can allow positioning of the orientation of the probe.
This work was done by James P. Lux of Caltech for NASA’s Jet Propulsion Laboratory. NPO-44090
This Brief includes a Technical Support Package (TSP).
Probe Positioning System for Antenna Range
(reference NPO-44090) is currently available for download from the TSP library.
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