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The Model 4391 is a multi-purpose Radio Freouency wattmeter designed around a microcomputer. A fairly extensive program stored in permanent memory controls the operation of the instrument at all times, permitting the detection and correction of various error sources and the refinement of the raw data produced by the directional detectors. Thus, the instrument can compute VSWR, amplitude modulation, and various decibel variables reducing the odds of error and making such measurements consistent or repeatable regardless of who is making the measurement. Other benefits include extended range using standard elements in some modes of operation, continuous monitoring monitoring of maximum and minimum readings, a peaking aid, and error messages. Because of its complexity, the proper use of the 4391 is not always obvious. For this reason it is strongly advised that this manual be read in its entirety before using the device.