Page 268 - Radar Technology Encyclopedia

loss, weighting magnetron, coaxial 258

pulse compression loss, and is included in the range equation Historically, the magnetron is the device that made
as the filter matching loss or matching factor M = L . In the microwave pulsed radar practical, and it has been in used in
m
latter case it is included as L , a component of the miscella- different types of radars for over 50 years. It was invented by
mf
neous signal-processing loss, L . A. W. Hall in 1921 for use as a diode switch, and until the
x
In a digital signal processor, the weighting function is a invention of the resonant cavity in 1939 it remained practi-
sequence of coefficients C applied to the input signal at cally a laboratory device. Now it can obtain the megawatts of
n
times t . An example is the Hamming weighting function output power with efficiency up to 80%.
n
Early magnetrons were of the unstrapped resonator type.
2pn ö
C = a – a cos æ ---------- n = 0 ¼ N – 1 To improve stability, strapped-vane forms were introduced
,
,
,
n
N ø
è
0
1
and now such magnetrons compose about 70% of these
where a = 0.53836, a = 0.46164. For this weighting func-
1
0
tion the loss is devices. The remaining 30% include primarily rising-sun
magnetrons (considered obsolete) and coaxial magnetrons.
N 1– 2
æ ö The latter developments include a strapped-vane system and
ç å C ÷
ç n ÷ high-Q resonant cavity that provides greatly enhanced fre-
è n = 0 ø quency stability. All existing magnetrons can be classified
L (or L ) = ---------------------------
m mf N – 1 from the standpoint of the operation mode (pulsed or CW),
2
N × å C n from the standpoint of frequency variation (fixed-frequency
or frequency-agile, tunable or tuned magnetrons), and from
n = 0
In the case of pulse-compression weighting, the times t n the standpoint of their structure (a variety of different types,
are spaced at intervals 1/B over the width t of the uncom- such as conventional magnetrons, coaxial magnetrons,
pressed pulse, where B is the waveform bandwidth, while in inverted coaxial magnetrons).
pulsed doppler filter weighting they are spaced at intervals t r Although magnetrons are the oldest sources of micro-
over the coherent processing interval, where t is the pulse wave energy used in radar, they are still widely used today
r
repetition frequency. because of their “innate” positive qualities: small size, rela-
The loss is independent of the number of samples, and tively light weight, reasonable operating voltages, excellent
for Hamming weighting is equal to 0.73 dB. The loss for efficiency, and long life. The main assets of magnetrons are:
some other functions is given in Table L14. (See also wide range of operating frequencies (from meter to millime-
WEIGHTING.) SAL ter waves); high attainable power combining with relatively
3
Ref.: Cook (1967), pp. 191–206, Barton (1993), pp. 98, 102.) small size and weight (from the 15 cm , 1-kW peak power
beacon magnetron to several megawatts peak power in air
Table L14 defence radars); very high efficiency inherent to crossed-field
Weighting Loss devices (up to 50 to 80%); low operating voltage (usually too
low to generate dangerous x-rays); and low cost. The main
Weighting function Loss in dB disadvantages restricting the usage of these devices in some
types of radar transmitters are the following: inability to
Uniform (rectangular) 0
ensure coherence from pulse to pulse (without using some
Cosine 1.0 special measures, such as frequency and phase-locking tech-
niques, which are complicated and not generally attractive);
Cosine-squared 1.8
insufficient range of pulse shaping (an order of few decibels);
inherent frequency drift and frequency modulation by micro-
Taylor 0.1 - 0.0041(G + 15)
s
phonics from ambient vibration; insufficient stability for gen-
Dolph-Chebyshev 0.01 - 0.05(G + 15) erating very long pulses (more than 100 ms) and very short
s
pulses (less than 50 ns); and high spurious power level that
G is the design sidelobe level in dB relative to the main-
s
lobe peak (from Barton (1993), Table 3.2, p. 98) produces considerable electromagnetic interference across the
bandwidth that is much wider than that of their signals. SAL
Ref.: Ewell (1981), pp. 22–37; Skolnik (1980), pp. 192–200, (1990), pp. 4.5–
4.9; Currie (1987), p. 448; Brookner (1988), pp. 263, 317; Sivan (1994),
Ch. 6.
M
A coaxial magnetron is one of the most common forms of
magnetron, in which the stabilizing cavity surrounds the con-
MAGIC- [HYBRID-] TEE (see BRIDGE, waveguide). ventional resonators (Fig. M1). In such a configuration the
straps are removed and the p-mode is controlled by coupling
A MAGNETRON is a crossed-field microwave tube (oscil-
alternate resonators to a cavity surrounding the anode, giving
lator) characterized by the interaction of electrons with the
an improvement in power, efficiency, frequency stability, and
electric field of circuit element in crossed steady electric and
life over the conventional magnetron. SAL
magnetic fields and converting to an RF power an output
energy extracted from a constant electric field. Ref.: Skolnik (1980), p. 193.