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receiver crystals. The keep-alive current is normally about 100 microamperes but falls off as the end of
the tr tube life approaches. You can also measure the keep-alive voltage between the plate of the tr tube
and ground when the voltage source is known to have the correct output. You then record this voltage for
use as a reference for future checks. However, these checks are not as reliable as recovery time testing.
Specific procedures for measuring tr leakage and recovery time can be found in the equipment
technical manuals.
Q7. A loss of receiver sensitivity has the same effect on range performance as what other loss?
Q8. You determine receiver sensitivity by measuring the power level of what signal?
Q9. When measuring receiver sensitivity, what quantities must you add to the dBm reading obtained
on the signal generator or test set?
STANDING WAVE MEASUREMENTS
(You may want to refer to NEETS, Module 10, Introduction to Wave Propagation, Transmission
Lines, and Antennas for a review of standing waves before going further.) Measurements of standing
waves can indicate the approximate operating frequency, the presence of defective transmission-line
sections, and the condition of the antenna. Standing waves present on transmission lines and waveguides
indicate an impedance mismatch between a transmitter or receiver and its antenna. When this condition
occurs, the transfer of energy between these units becomes inefficient. Reflection of energy at the load
end of a transmission line results in a wave that travels toward the generator end. This reflected wave
varies continuously in phase in much the same way that the incident wave varies in phase. At certain
points, a half wavelength apart, the two waves are exactly in phase; the resultant voltage is at maximum.
At points a quarter wavelength from the maximums, the two waves are in opposition and voltage nodes
(null points) are produced. The ratio of maximum-to-minimum voltage at such points is called the
VOLTAGE STANDING WAVE RATIO (vswr). The ratio of maximum-to-minimum current along a
transmission line is the same as the vswr. A high vswr (1.5 to 1 or higher) indicates that the characteristic
impedance of a transmission line differs greatly from the terminating impedance; a low vswr (1 to 1 is
best) indicates a good impedance match between the transmission line characteristic impedance and the
terminating impedance.
For radar applications, a low vswr is desired for the following reasons: (1) Reflections in the
transmission line cause improper transmitter operation and can result in faulty pulsing (this effect is most
pronounced when the line is long, as compared with a wavelength of the transmitted energy); (2) arc-over
may occur at the maximum voltage points; and (3) hot spots can occur in the transmission line and cause
mechanical breakdown. Since transmission lines for radar equipment are normally coaxial cables or
waveguides, slotted lines or directional couplers must be used for standing-wave measurements.
Q10. Receiver bandwidth is defined as those frequencies spread between what two points of the
receiver response curve?
Q11. The end of the usefulness of a tr tube is indicated by an increase in what quantity?
SUPPORT SYSTEMS
When you think of radar equipment with its complex electronic circuitry and other sophisticated
equipment, you may forget that the entire radar relies on other systems. These other systems are referred
to as SUPPORT SYSTEMS and are not normally thought of as part of the radar. These support systems
