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
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Probe Positioning System for Antenna Range
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Overview
The white paper discusses a novel probe positioning system designed for antenna measurements and testing of landed assets, particularly in the context of rover applications. Traditional antenna measurement techniques face significant challenges when applied to in-situ environments, especially on rovers in simulated terrains. The desired measurement data typically covers a hemisphere above the antenna, which is difficult to achieve with conventional methods that rely on movable supports that tilt and rotate.
To address these challenges, the proposed system utilizes a probe suspended by three or more non-conductive cables, which are controlled by winches. This setup allows for precise positioning of the probe within the area enclosed by support towers, enabling the simulation of moving sources and the measurement of antenna patterns without the need for large, rigid structures. The technology has previously been used for flying cameras but has not been applied to antenna measurements until now.
The system's design emphasizes flexibility and precision. By adjusting the lengths of the cables, the probe can be positioned accurately, and an external tracking system can enhance this precision further. The use of non-conductive cables is particularly advantageous in RF applications, as they minimize interference with RF fields and improve safety by not conducting electricity.
The paper also discusses the importance of tower height in relation to the maximum desired probe height, suggesting that a height of 1.5 to 2 times the maximum probe height is optimal. This consideration helps manage cable tension and movement speed. Additionally, the system can be adapted for indoor deployment, allowing the RF payload to be maneuvered over test articles in a controlled manner.
Power for the probe can be supplied through various means, including batteries or solar cells, and control signals can be transmitted via optical fibers or wireless methods. The system can support multiple cables, allowing for greater motion range and control over the probe's orientation, potentially achieving six degrees of freedom.
Overall, this innovative probe positioning system represents a significant advancement in antenna measurement techniques, offering a practical solution for in-situ testing on rovers and other applications in aerospace and beyond.
