Clemens Moroder

Attenuation data from microwave links can be used to derive important near-ground precipitation estimates which complements observations from rain gauges and weather radars. Wetness on the cover of the microwave link antennas can, however, lead to additional attenuation, resulting in an overestimation of rainfall rates. To investigate the impact of this wet antenna attenuation (WAA) effect, a versatile microwave field experiment was built. It is based on a two-port vector network analyzer and it is equipped with six dual-polarized commercial microwave link antennas at different frequencies between 18 GHz and 40 GHz. A measurement frequency range of 150 MHz and range gating enables simultaneous measurement of the antenna reflection coefficient and the total path attenuation. An optical camera is integrated for visual inspection of the antenna covers. Appropriate housing and active temperature control makes the device weather-proof and enables all-season operation. Several supplementary meteorological instruments at the installation site provide information about the actual weather conditions. They can be used as the ground truth for evaluation. The presented instrument is a unique setup to study the WAA effect, by simultaneously providing measurements of the total path attenuation together with reliable WAA estimations, directly derived from measurements of the antenna reflection coefficient. Its data will give insights into WAA magnitude and temporal evolution in relation to meteorological variables. This will help developing improved WAA estimation algorithms, which will result in improved rainfall estimation from microwave links.

This letter presents an analysis of wet antenna attenuation (WAA) from an electrodynamic perspective. The WAA is a relevant part of the path attenuation determined from the received signal level (RSL). In the case of extracting rain rates from path attenuation, as it is done with data from country-wide commercial microwave link (CML) networks, the WAA is a significant error source and leads to overestimation of rainfall. To handle this effect, a model, based on the three antenna parameters, reflectivity, efficiency, and directivity, is developed. The model allows the variation of those three parameters depending on the degree of wetness and shows that WAA is composed of variations of antenna impedance, antenna efficiency, and antenna directivity. The suitability of the theoretical model was assessed by a full-wave numerical simulation of a pyramidal horn antenna at 18 GHz with homogeneous water film on the surface of the antenna cover. The result of this simulation is a quantification of these three subeffects. For verification of the simulation, an antenna, similar to the antenna in the simulation, was manufactured and a controlled sprinkler device to generate a water film was built. The adjustable water flow rate determines the thickness of the water film. The parameters were generated by measuring the near field and the reflectivity of the antenna in an anechoic chamber, with a near-field scanning system and a vector network analyzer, under different flow rates equaling different water film thicknesses. From these results, the reflectivity, directivity, and efficiency were computed and compared with the results of the full-wave simulation. It is found that the reflectivity is linked to water film thickness in a such way that the WAA can be deduced from monitoring the antenna reflectivity.