4-5
In the case of a good or fair spectrum curve with sharply defined minimum points on both sides of
the main lobe, the distance between these two points is proportional to the duration of the transmitted
pulse.
The device most commonly used to check the frequency spectrum of a radar transmitter is the
spectrum analyzer.
Frequency-Measuring Devices
Devices used to determine the basic carrier frequency of a radar transmitter are the ELECTRONIC
FREQUENCY COUNTER, the WAVEMETER, and the ECHO BOX. One or more of these devices may
be included in a special RADAR TEST SET designed for a specific system or type of radar. Radar test
sets quite often consist of several types of test equipment. This combination of test equipments enables
both transmitter and receiver performance checks to be carried out with one test instrument. Electronic
frequency counters, frequency meters, and wavemeters are discussed in NEETS, Module 16, Introduction
to Test Equipment. The echo box is discussed in the next section. The specific equipments and procedures
required to measure the frequency of any radar system are found in the associated system technical
manuals and related PMS documents.
Q1. The spectrum of a radar transmitter describes what characteristic of the output pulse?
Q2. Where should the transmitter spectrum be located with respect to the receiver response curve?
Q3. The ideal radar spectrum has what relationship to the carrier frequency?
Q4. The display screen of a spectrum analyzer presents a graphic plot of what two signal
characteristics?
The Echo Box
The ECHO BOX is an important test instrument for indicating the overall radar system performance.
The echo-box test results reflect the combined relative effectiveness of the transmitter as a transmitter of
energy and the receiver as a receiver of energy.
The echo box, or RESONANCE CHAMBER, basically consists of a resonant cavity, as shown in
view A of figure 4-4. You adjust the resonant frequency of the cavity by varying the size of the cavity
(the larger the cavity the lower the frequency). A calibrated tuning mechanism controls the position of a
plunger and, therefore, the size of the cavity. The tuning mechanism is adjusted for maximum meter
deflection, which indicates that the echo box is tuned to the precise transmitted frequency. The tuning
mechanism also indicates on a dial (figure 4-5, view A) both the coarse transmitted frequency and a
numerical reading. This reading permits the technician to determine the transmitted frequency with
greater accuracy by referring to a calibration curve on a chart (figure 4-5, view B).
