Page 69 - Radar Technology Encyclopedia

attenuator, capacitance [capacitive] AVAILABILITY 59

attenuator is used to increase the stability of microwave tubes due to the reflected signal received by the antenna. The main
by reducing the influence of the tube capacitance, which is characteristic of an autodyne is its sensitivity S, which is
achieved by using capacitance bridge circuits. IAM given by the formula
Ref.: Popov (1980), p. 124. U
S = ------------------ h h
t r
A cutoff attenuator is based on a transmission line, the cross P P
¤
f t
section of which is less than is critical for the propagating
wave. It is usually a portion of cutoff waveguide of a regu- where U is the amplitude of the reaction voltage in the auto-
lated length with a capacitive probe or a coupling loop at the dyne; P and P are the transmitted and received powers; and
r
t
end of the waveguide to receive the attenuated wave. The h and h are the transmit and receive antenna efficiencies.
t
r
maximum attenuation in decibels is directly proportional to The radar range equation for a point target with radar cross
the length of the waveguide section. To widen the passband in section s at wavelength l takes the form
cutoff attenuator circuits, the inputs are matched with absorb-
2
ing elements. IAM R = SlDF s
2
--------------------------
Ref.: Sazonov (1988), p. 153; Lavrov (1974), p. 353. 4pp U
where D and F are the directivity and antenna pattern in the
A reflection-type attenuator uses the reflection of a wave
direction of the target.
from a controlled surface. The simplest reflection attenuator
A specific feature of coherent autodynes is their low
is a p-i-n diode shunting the transmission line, or a diode
range capability, due to which they are used primarily in
module placed across the waveguide. A PIN diode attenuator
short-range radar systems.
controls power up to hundreds of watts, introducing attenua-
The active elements in an autodyne may be practically
tion due to reflection of up to 20 dB.
any of the known electronic devices: microwave tubes, field-
Drawbacks to such simple reflection attenuators are the
effect and bipolar transistors, and semiconductor diodes (tun-
large standing wave ratio and the change in phase of the
nel, IMPATT, and Gunn diodes, etc.). IAM
reflected wave due to the complex conductivity of the diode.
Ref.: IEEE (1993), p. 66; Kogan (1973), p. 70.
IAM
Ref.: Gassanov (1988), p. 140. A doppler autodyne is one that measures the doppler shift of
the signal reflected by the target. It is an oscillator, the energy
A waveguide attenuator is “a waveguide component that
from which is radiated into space and, after reflection by a
reduces the output power relative to the input, by any means,
moving target, returns shifted in frequency by the doppler fre-
including absorption and reflection” and is typically used in
quency. After frequency conversion, the low-frequency dop-
waveguide transmission lines. The construction of the attenu-
pler signal is extracted and processed. An example is an
ator depends on the type of waveguide. For metal waveguides
autodyne used to control the motion of an automatic transport
with various cross sections, the attenuator is usually a dielec-
based on a reflex klystron, having a horn antenna with 10°
tric plate covered with an absorbing material, placed within
beamwidth, 25 mW of power, and an error of 2 to 5% at
the waveguide. For dielectric waveguides, the attenuator is
speeds less than 50 km/h and less than 1% at higher speeds.
often a dielectric controlled plate on its surface, or a portion
There are also Gunn diode doppler autodynes that can handle
of stripline waveguide placed along the substrate of a dielec-
up to 100 mW; other electronic devices are used as well.
tric stripline waveguide. IAM
Doppler autodynes find use in miniature receive-transmit
Ref.: IEEE (1993), p. 64; Sazonov (1988), p. 153.
modules in short-range radar systems used to support the
AUTOCORRELATOR. An autocorrelator is a device that approach or prevent the collision of two objects and to deter-
produces an output signal proportional to the autocorrelation mine their relative distance and speed. IAM
function of the input signal (see CORRELATION). It con- Ref.: Khotuntsev (1982), pp. 10, 150.
tains an input filter, a signal delay circuit, a multiplier and an
AUTOMATIC FREQUENCY CONTROL (AFC) (see
integrator. The functions of an autocorrelator are similar to
FREQUENCY).
those of a frequency discriminator for signals with suffi-
ciently narrow bandwidth. Autocorrelators are used in CW AUTOMATIC GAIN CONTROL (AGC) (see GAIN).
radars in automatic frequency-tracking circuits. IAM
AVAILABILITY. Availability is the probability that a radar
Ref.: Vinitskiy (1961), p. 281; DiFranco (1968), pp. 214-219.
will be able to perform its designated operational function
An AUTODYNE is “a system of heterodyne reception
when required for use. Typically, it is the probability that
through the use of a device that is both oscillator and detec-
being in a standby mode the radar will be ready to be placed
tor.” The main functions of an autodyne are the generation of
in operational mode in any arbitrary moment and following
oscillations, signal reception, and modulation and detection
that moment will operate without failure during the entire
of oscillations resulting in extraction of the useful signal.
specified period. SAL
Autodyne theory treats the influence of the target as a
Ref.: IEEE (1993), p. 76; Aleksandrov (1976), p. 12; Jordan (1985), p. 45.2.
change in resistance of the antenna-target-antenna system,