An absorption wavemeter is a simple electronic instrument used to measure the

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...

s. It is an older method of measuring frequency, widely used from the birth of radio in the early 20th century until the 1970s, when the development of inexpensive

frequency counter A frequency counter is an electronic instrument, or component of one, that is used for measuring frequency. Frequency counters usually measure the number of cycles of oscillation, or pulses per second in a periodic electronic signal. Such an instr ...

s, which have far greater accuracy, made it largely obsolete. A wavemeter consists of an adjustable

resonant circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...

calibrated in frequency, with a meter or other means to measure the voltage or current in the circuit. When adjusted to resonance with the unknown frequency, the resonant circuit absorbs energy, which is indicated by a dip on the meter. Then the frequency can be read from the dial. Wavemeters are used for frequency measurements that do not require high accuracy, such as checking that a

radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...

is operating within its correct frequency band, or checking for

harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...

s in the output. Many radio amateurs keep them as a simple way to check their output frequency. Similar devices can be made for detection of mobile phones. As an alternative, a

dip meter :''"Dip meter" can also refer to an influential early commercial expert system called Dipmeter Advisor; or may refer to an instrument that measures the magnetic dip angle of Earth's magnetic field, the field line angle in a vertical plane.'' Grid ...

can be used. There are two categories of wavemeters: ''transmission wavemeters'', which have an input and an output port and are inserted into the signal path, or ''absorption wavemeters'', which are loosely coupled to the radio frequency source and absorb energy from it.

HF and VHF

The most simple form of the device is a variable

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

with a coil wired across its terminals. Attached to one the terminals of the

LC circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...

is a

diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...

, then between the end of the diode not wired to the LC circuit and the terminal of the LC circuit not bearing the diode is wired a ceramic decoupling capacitor. Finally a

galvanometer A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. A galvanom ...

is wired to the terminals of the decoupling capacitor. The device will be sensitive to strong sources of radiowaves at the

at which the LC circuit is

resonant Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

. This is given by

f =

When the device is exposed to an RF field which is at the resonant frequency a DC voltage will appear on the terminals on the left hand side. The coil is often outside the case of the unit so it can be brought close to the object being probed.

UHF and SHF

At the higher frequencies it is not possible to use

lumped component The lumped-element model (also called lumped-parameter model, or lumped-component model) simplifies the description of the behaviour of spatially distributed physical systems, such as electrical circuits, into a topology consisting of discrete e ...

s for the tuned circuit. Instead methods such as

stripline Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line. Description A strip ...

or resonant cavities are used. One design for ultra high frequencies (UHF) and super high frequencies (SHF) is a resonant λ/4 (quarter wave) rod which can vary in length. Another design for

X-band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxim ...

(10 GHz) is a

resonant cavity A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonato ...

which can be changed in length. As an alternative for UHF, Lecher transmission lines can be used. It is possible to measure roughly the frequency of a transmitter using

Lecher lines In electronics, a Lecher line or Lecher wires is a pair of parallel wires or rods that were used to measure the wavelength of radio waves, mainly at VHF, UHF and microwave frequencies. They form a short length of balanced transmission line (a ...

References

{{Reflist, colwidth=30em Measuring instruments Microwave technology Radio electronics

HF and VHF

UHF and SHF

See also

References