Spherical Near Field Antenna Measurement Facility in VHF Range – Investigation on System Design
Résumé
Recent enhancements in military telecommunication systems for monitoring and tracking in low VHF
range (30-80 MHz) implies the use of specific antenna measurement facilities to characterize either the
antenna alone or the antenna mounted on a supporting structure which can be heavy and bulky.
Conventional outdoor far-field antenna measurement ranges used at UHF frequency bands and above
become poorly effective down in VHF frequency band due to ground effects and electromagnetic
pollution. Other implementations such as indoor far-field or compact range are not suitable at these
very large wavelengths.
The multi-sensor near-field approach shows benefits in terms of compactness and measurement speed.
However this approach involves considering and treating high levels of reflectivity of anechoic
chambers, positioners and structures at these frequencies. The implementation of such an antenna
measurement range thus requires identifying and characterizing the effects of each contributor on the
measurement performance and accuracy.
In a first step, the proposed paper focuses on the feasibility of an indoor measurement facility at VHF
frequencies with reflectivity analyses of each constituting elements of the multi-sensor near-field
range (anechoic chamber, mast, positioner and measurement probes). Performances of the ferrite tiles
as a low frequency absorbing solution covering anechoic chamber walls are estimated through 3D EM
simulation and analytical model. A global measurement error budget is established for different
antenna directivity classes. In a second step, activities are dedicated to an outdoor near field range
including analysis of reflectivity (floor, probes, arch, tent …) and estimation of system performance.
Mechanical aspects to obtain a removable and transportable system are discussed and considered in
the global budget error. Finally both indoor and outdoor multi-sensor solutions are compared in terms
of performances to conclude on the feasibility of such facilities and to identify key points.