Electromagnetic Theory

Dead Zone

Q: How do you calculate the minimum range of a pulsed radar?

The minimum detectable range of a monostatic pulsed radar is R_min = c times tau divided by 2, where tau is the transmitted pulse width and c is the speed of light. The factor of two accounts for the round trip. During the pulse the transmit/receive switch protects the receiver, so any echo returning before the pulse ends is lost. A 1 microsecond pulse gives a dead zone of 150 meters; a 100 nanosecond pulse shrinks it to 15 meters. Add the receiver recovery time and any T/R limiter recovery before the receiver is usable. Pulse compression keeps a long transmit pulse for energy while the dead zone is governed by the uncompressed pulse width, which is why short-range modes use short pulses or a separate high-pulse-repetition-frequency waveform.

Q: What causes the dead zone in an OTDR measurement?

An OTDR launches an optical pulse and measures backscatter versus time. The strong Fresnel reflection from the launch connector saturates the photodetector, and the detector needs time to recover before it can resolve weak backscatter again. The attenuation dead zone (where loss can be measured) is typically 2 to 10 meters and the event dead zone (where two reflective events can be distinguished) is shorter, around 0.5 to 3 meters, both scaling with pulse width. The one-way distance per unit time is v_g divided by 2 = c divided by (2 times n_g), so with a fiber group index near 1.468 each nanosecond of pulse corresponds to about 0.1 meter of fiber. Launch fibers or pulse suppressors are used to push connector events out of the dead zone.

Q: How does MTI radar create a velocity dead zone, and how is it removed?

A moving target indicator cancels clutter by subtracting successive pulses, which also cancels any target whose Doppler phase shift between pulses is a multiple of 2 pi. These blind speeds occur at v_blind = n times lambda times PRF divided by 2 for integer n. At X band (lambda = 3 cm) with a 2 kHz pulse repetition frequency the first blind speed is 30 m/s and every multiple of it is also blind, producing notches in the velocity response. Staggering the PRF from pulse to pulse moves the blind speeds to the least common multiple of the individual PRFs, pushing the first effective blind speed far higher. FMCW and pulse-Doppler waveforms with multiple PRFs are also used to fill these velocity dead zones.

/ded zohn/

The blind interval in range or velocity over which a pulsed sensor cannot detect a target. In a monostatic pulse radar the receiver is protected while the transmitter is on, so any echo arriving before the pulse ends is eclipsed; the minimum detectable range is R_min = cτ/2, equal to 150 m for a 1 μs pulse and 15 m for a 100 ns pulse. The same mechanism creates the launch-end dead zone in an OTDR, where the connector reflection masks the first few meters of fiber, and the velocity dead zones (blind speeds) of an MTI radar, where Doppler phase shifts that are multiples of 2π per pulse are cancelled along with clutter.

Category: Electromagnetic Theory

Radar R_min: cτ/2 (150 m per μs)

OTDR event DZ: 0.5 to 3 m typical

Where the Blind Range Comes From

Any sensor that shares one aperture between transmit and receive must protect its receiver while the high-power pulse is on the line. In a pulsed radar a transmit/receive switch and a T/R limiter blank the receiver for the duration of the pulse plus a short recovery interval. An echo that returns during this window is partly or wholly lost, an effect called eclipsing. The result is a near-range hole: the radar simply cannot report a target closer than the range corresponding to the round-trip time of one pulse width. This minimum range is the most common meaning of dead zone in microwave engineering, and it is set entirely by the uncompressed pulse width rather than by signal-to-noise ratio.

Pulse compression complicates the picture. A long transmit pulse carries the energy needed for detection range, but the dead zone is governed by the physical length of that transmit pulse, not by the compressed range resolution. A radar that transmits a 10 μs chirp has a 1.5 km dead zone even though its compressed range resolution may be a few meters. For this reason multifunction radars interleave a short-pulse, high-pulse-repetition-frequency waveform to fill the near range, accepting reduced energy on those returns because nearby targets are strong anyway.

The optical analogue is the OTDR. A reflectometer fires an optical pulse into a fiber and times the Rayleigh backscatter. The Fresnel reflection from the launch connector is many orders of magnitude stronger than the backscatter and saturates the photodiode, so the receiver needs recovery time before it can resolve weak signal again. The result is an attenuation dead zone of a few meters and a shorter event dead zone over which two reflective features cannot be separated. Both scale with pulse width, and both are pushed off the fiber under test by inserting a launch fiber or a pulse suppressor.

Governing Relationships

Radar minimum range (monostatic):
R_min = c × τ / 2 ≈ 150 m per μs of pulse width

OTDR dead-zone distance:
d = v_g × τ / 2 = c × τ / (2 n_g) ≈ 0.1 m per ns (n_g ≈ 1.468)

MTI blind speeds (velocity dead zone):
v_blind = n × λ × PRF / 2 (n = 1, 2, 3, …)

Where c = 3 × 10⁸ m/s, τ = pulse width, v_g = group velocity, n_g = fiber group index, λ = wavelength, PRF = pulse repetition frequency. Example: X band (λ = 3 cm) at PRF = 2 kHz → first blind speed = 30 m/s.

Dead Zone by System Type

System	Dead zone	Root cause	Typical value	Mitigation
Pulsed radar	Near range R_min	Receiver blanked during TX (eclipsing)	15 m (100 ns) to 1.5 km (10 μs)	Short-pulse high-PRF fill waveform
FMCW radar	None inherent	Continuous wave, no TX/RX gating	~0 m (TX/RX leakage limited)	Good TX/RX isolation
OTDR	Launch-end masking	Connector Fresnel reflection + detector recovery	0.5 to 3 m event; 2 to 10 m atten.	Launch fiber, pulse suppressor
MTI radar	Velocity (blind speeds)	Doppler phase = multiple of 2π per pulse	n × 30 m/s at X band, 2 kHz PRF	PRF stagger, multiple PRFs
Bistatic radar	Baseline / forward-scatter	Direct-path masking near the TX-RX line	Geometry dependent	Multistatic geometry, gating

Common Questions

Frequently Asked Questions

How do you calculate the minimum range of a pulsed radar?

The monostatic minimum range is R_min = cτ/2, where τ is the transmit pulse width; the factor of two is the round trip. A 1 μs pulse gives 150 m of dead zone, a 100 ns pulse gives 15 m. Add the receiver and T/R limiter recovery time. With pulse compression the dead zone is still set by the uncompressed transmit pulse, which is why short-range modes use short pulses or a separate high-PRF waveform.

What causes the dead zone in an OTDR measurement?

The strong Fresnel reflection from the launch connector saturates the photodetector, which then needs recovery time before it can resolve weak Rayleigh backscatter. The event dead zone (separating two reflections) is typically 0.5 to 3 m and the attenuation dead zone is 2 to 10 m, both scaling with pulse width. Because d = cτ/(2n_g), each nanosecond maps to about 0.1 m of fiber. A launch fiber moves connector events out of the dead zone.

How does MTI radar create a velocity dead zone, and how is it removed?

An MTI processor subtracts successive pulses to cancel clutter, which also cancels any target whose Doppler phase shift between pulses is a multiple of 2π. These blind speeds fall at v_blind = nλPRF/2; at X band with a 2 kHz PRF the first is 30 m/s and every multiple is also blind. Staggering the PRF moves the blind speeds to the least common multiple of the individual PRFs, pushing the first effective notch far higher.

Related Terms

← Dead Time Deauthentication →