Measuring Radome Panel high Frequency Properties

Considering the influence of radome properties on radar performance, DWD is conducting stringent radome acceptance tests. Among others, the electrical properties of radome panels are required to be homogeneous within and identical between panels. In this context, measurements of the complex relative permittivity (Re(εr) and Im(εr), or modulus εr and loss tangent tan d) have been devised and accomplished by the Technische Universität München on several plane foam panels of different make (gelcoat and tedlar coating type). Measurements of wave impedance and propagation constant in a waveguide filled by a material sample as well as resonator measurements of quality factor and resonance frequency have been excluded by the non-destruction requirement. In free space transmission/reflection measurement, beam focusing would be weak at a wavelength of 5.3 cm and a panel size of 2 m, leading to edge diffraction effects, and precluding intrapanel measurements. Therefore, a twoport setup has been chosen where the panels were clamped between open waveguide flanges to measure reflection and transmission. With this setup it is also possible to probe various positions the panels in order to detect undesired inhomogeneities. The S- Parameters have been measured using a vector network analyzer calibrated to the waveguide ports by the thru-reflect-line method and the intrapanel as well as interpanel variabilities have been considered.

However, the S-parameters (reflection and transmission) measured in between waveguide flanges are not identical to the properties at perpendicular incidence of a plane wave in free space. Hence, a simple transmission line model does not hold. An invertible electromagnetic model of the measurement setup has therefore been developed to numerically calculate reflection and transmission at reference planes (the wave guide flanges) in dependence on geometry (thickness) and electrical properties (permittivity and loss factor), considering also the open structure. This parameterized model has been iteratively optimized in order to solve for those material parameters that best represent the measurements.

The relative permittivity at transmitting frequency f = 5.6 GHz turned out to be εr = 1.56, tan d = 0.006. No measurable variation could be found between the panels which may thus be considered to be identical and homogeneous within the given accuracy limits. Specifically, this permits to combine panels of different type into one radome. A complexity-reduced benchmark model (extended TEM horn with obstacles inside its aperture) indicates the presence of radome metal screws in the antenna near field to be more influential, increasing main beam cross polarization to -40 dB.