A transmission beam control unit 8 changes a main beam direction of a radar transmission beam every predetermined number of transmission periods. The transition assembly having a clamping groove which engages, for example, the heads of a plurality of clamping screws upon seating of the transition assembly onto a spigot of the feed assembly. A drainage compartment communicates with a generally lower portion of and is disposed external to the storage cavity defined by the support wall for drawing melted ice away from the storage cavity and into the drainage compartment. A collector is installed inside each of the cathode channels and the anode channels, and a packing material is packed into the channels having the collector.