Radar System Engineering

SEC. 1615] THE STABLE LOCAL OSCILLATOR 659
when amplitude modulation is avoided. Information on this point is nut
available for the other types.
Magnetron frequency pulling with antenna scanning can be objectionable
with any type of tube. It may be caused either by imperfect rotary
joints or by obstacles in front of the antenna. If the pulling is slow compared
with 10 Mc/sec2 (see Table 16.1), ground clutter will remain canceled
at all azimuths. However, when the absolute detuning becomes
excessive, moving targets will be lost owing to their high video spectrum,
as is explained in Sec. 16.5. This type of detuning is particularly
insidious.
Objectionable modulation of the transmitted pulse by the use of
60-cps power on the heater has been observed with the type 4J47. This is
probably caused by the magnetic field .of the heater current. No such
difficulty has been found with the 2J32, owing apparently to the design
and construction of the heater. Three methods of avoiding this modulation
are available. At high PRF’s the heater current can be turned off
after starting the system, since the cathode temperature will be maintained
by bombardment. Alternatively the PRF can be made an exact
submultiple of the power-line frequency. Since neither of these methods
can be generally applied, it will sometimes be necessary to operate the
magnetron heater on direct current. Heater hum modulation should
eventually be made a matter of tube-manufacturing specification.
The use of a supersonic delay line necessitates a constancy of PRF to
& of a pulse width. This is the most severe new requirement placed by
MTI upon the modulator. It has been successfully met by hard-tube
and by hydrogen-thyratron designs. In their current state, triggeredgap
modulators were not quite adequate for MTI systems in 1946. Since
little effort had been put upon the jitter problem, it seemed reasonable to
expect that triggered gaps can be considerably improtied. The rotary
gap can be used for MTI only if a storage tube serves as delay element.
16.15. The Stable Local Oscillator.-Two types of 10-cm local
oscillator tubes have been investigated: the type 417 and type 2K28
reflex klystrons. The 417 has a built-in cavity and is therefore more
microphonics than the 2K28, which has a remountable cavity. However,
the external cavity gives the 2K28 a lower Q, which causes larger noise
fluctuations and greater sensitivity to external 60-cps magnetic fields.
Both tubes show frequency modulation when the heaters are run on a-c
power.
The 2K28 was chosen for incorporation into the local oscillator unit
shown in Fig. 16.26. The outer metal case of this component is lined with
sound-proofing material. The parts within are mounted on a metal
base-plate which is in turn supported by four circular shear rubber shock
mounts of conventional design, not visible in the photograph.