This is straight from the Humminbird site:
AS 21RD4KWAccessories408560-1.4 KW Radar
Rugged, 21-inch radome easily connects to your Humminbird® Ethernet Network.
•Up to 4kW transmit power delivers accurate target detection
•Adjustable VRM and EBL functions provide precise range and bearing information for any target.
•Shaded target intensity helps users judge weather severity and differentiante between large and small vessels.
Also available in a 12 inch 2kW model.
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It's a big succer, but it does work with your 1198.