On-site antenna pattern measurement for operational weather radars in Germany

Due to its favored position on top of a pre-alpine mountain in southern Germany, DWD‘s Hohenpeissenberg research radar qualifies as a potential test range for measuring the antenna radiation diagram. A check of this pattern is important especially when the antenna is being manipulated, e.g. for polarization upgrades, as the antenna is probably the key element in a polarization scheme. For instance, "squinting" main beams would introduce artificial polarization differences, and even the impact of non-overlapping side lobes may screen the small desired polarization effects. Considerable effort has therefore been put to an on-site measurement of the antenna diagram.

A stationary, calibrated RF transmitter has been scanned by rotating the radar antenna through 360 degrees creating PPIs that indicate the azimuthal antenna pattern at a given elevation. However, the elevation diagram can be obtained by RHIs between -2 ... +90 degrees only. To overcome this limitation, a scheme has been devised to rotate the antenna dish around its symmetry axis, RX and TX feeds included. Thus, the elevation antenna diagram can be measured via azimuthal scan. Similarly, cross sections through any arbitrary angle of the 3D antenna pattern may be taken.

This measurement has been conducted prior to the antenna modification and will be repeated after the polarization upgrade this summer. It will allow to assess a possible antenna degradation as well as the effect of post-adjustments. In addition, the suitability of the test site can be checked comparing elevation diagrams obtained by real elevation scan with those obtained by azimuthal scan after the rotation mentioned above.