How is minimum range calculated in relation to pulse width?

• A. Minimum range = pulse width
• B. Minimum range = 1/2 pulse width
• C. Minimum range = pulse width x 2
• D. Minimum range is independent of pulse width

Answer: (B)

Minimum range is set by how long the transmitter stays on (the pulse width) and the need for the return echo to travel to the target and back after the pulse ends. The earliest echo you can detect corresponds to a target at a distance where the round-trip travel time equals the pulse width. That gives R_min = (c × t_p) / 2, i.e., half the distance the signal would cover during the pulse. In other words, minimum range is half the pulse width expressed as a distance (using the speed of light). For example, a 1 microsecond pulse yields about 150 meters minimum range. The other options don’t account for the round-trip travel and the transmit/receive timing.