2-15
The manufacturer’s calibration book is a very important part of the frequency meter package; in fact,
the book is so important that it bears the same serial number as the heterodyne-type frequency meter
itself. Contained in this book is a list of the dial settings and the corresponding frequencies produced by
that meter at those dial settings. Operating instructions for the meter are also included.
Absorption Wavemeter
WAVEMETERS are calibrated resonant circuits used to measure frequency. The accuracy of
wavemeters is not as high as that of heterodyne-type frequency meters; however, they have the advantage
of being comparatively simple and can be easily carried.
Q-8. What equipment uses a calibrated resonant circuit to measure frequency?
Any type of resonant circuit can be used in wavemeter applications. The exact kind of circuit used
depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and
capacitors are used for VLF through VHF wavemeters.
The simplified illustration of an absorption wavemeter, shown in figure 2-11, consists of a pickup
coil, a fixed capacitor, a lamp, a variable capacitor, and a calibrated dial. When the wavemeter’s
components are at resonance, maximum current flows in the loop, illuminating the lamp to maximum
brilliance. The calibrated dial setting is converted to a frequency by means of a chart, or graph, in the
instruction manual. If the lamp glows very brightly, the wavemeter should be coupled more loosely to the
circuit. For greatest accuracy, the wavemeter should be coupled so that its indicator lamp provides only a
faint glow when tuned to the resonant frequency.
Figure 2-11.—Absorption wavemeter circuit.
FREQUENCIES ABOVE THE AUDIO RANGE
The signal frequencies of radio and radar equipments that operate in the UHF and SHF ranges can be
measured by resonant, cavity-type wavemeters or resonant, coaxial-line-type wavemeters. When properly
calibrated, resonant-cavity and resonant-coaxial line wavemeters are more accurate and have better
stability than wavemeters used for measurements in the LF to VHF ranges. These frequency-measuring
instruments are often furnished as part of the equipment. They are also available as general-purpose test
sets.
