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Minimum Range
Recall from NEETS, Module 11, Microwave Principles, that the DUPLEXER alternately switches
the antenna between the transmitter and receiver so that only one antenna need be used. This switching is
necessary because the high-power pulses of the transmitter would destroy the receiver if energy were
allowed to enter the receiver. As you probably already realize, timing of this switching action is critical to
the operation of the radar system. What you may not realize is that the minimum range ability of the radar
system is also affected by this timing. The two most important times in this action are PULSE WIDTH
and RECOVERY TIME.
This timing action must be such that during the transmitted pulse (pulse width), only the transmitter
can be connected to the antenna. Immediately after the pulse is transmitted, the antenna must be
reconnected to the receiver.
The leading edge of the transmitted pulse causes the duplexer to align the antenna to the transmitter.
This action is essentially instantaneous. At the end of the transmitted pulse, the trailing edge of the pulse
causes the duplexer to line up the antenna with the receiver; however, this action is not instantaneous. A
small amount of time elapses at this point that is referred to as recovery time. Therefore, the total time in
which the receiver is unable to receive the reflected pulse is equal to the pulse width plus the recovery
time. Note that any reflected pulses from close targets returning before the receiver is connected to the
antenna will be undetected. The minimum range, in yards, at which a target can be detected is determined
using the following formula (pulse width and recovery time are expressed in microseconds or fractions of
microseconds):
For example, minimum range for a radar system with a pulse width of 25 microseconds and a
recovery time of 0.1 microseconds is figured as follows:
Most modern radar systems are designed with such small recovery times that this figure can often be
ignored when figuring minimum range.
Maximum Range
The maximum range of a pulse radar system depends upon CARRIER FREQUENCY, PEAK
POWER of the transmitted pulse, PULSE-REPETITION FREQUENCY (prf) or PULSE REPETITION
RATE (prr), and RECEIVER SENSITIVITY with prf as the primary limiting factor. The peak power of
the pulse determines what maximum range the pulse can travel to a target and still return a usable echo. A
usable echo is the smallest signal detectable by a receiver system that can be processed and presented on
an indicator.
