The 4391 is an updated version of the 4391. The two are visually identical.
Model 4381
Measures forward/reflected power from 450 kHz MHz up to ~2.3 GHz.
Power range from 100 mW to 10 kW, depending on the elements used.
Bird 4391 / 4391A
Extended frequency range of 450 kHz to ~2.7 GHz.
Power range of 100 mW to 10 kW using Bird plug-in elements.
