The apparatus has a switchable generator of electromagnetic pulses which operates over a predetermined period of time. The optical display (10,11) is activated according to the receive signal pulse amplitude and may be analog and/or digital. The display (11) is for the maximum signal amplitude and may be supplied via a store (6) for oscillations of the receive signal over a period shorter than the transmit period. In addition the peak valves of the receive signal are stored in respective parts (S1...Sn) of a store showing the variations of the signal. A comparator (9) compares the peaks of successive receive pulses of which the earlier is stored for comparison with the current. The store is unloaded between two successive receive signals. The apparatus may also include an acoustic output.
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