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. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

radio wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...

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 electronics, electronic measuring instrument, instrument, or Electronic component, component of one, that is used for measuring frequency. Frequency counters usually measure the number of cycles of oscillation or pulses p ...

s, which have far greater accuracy, made it largely obsolete.

Overview

A wavemeter consists of an adjustable resonant circuit 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 (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna with the purpose of signal transmissio ...

is operating within its correct frequency band, or checking for

harmonic In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st har ...

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 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 In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

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

diode A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...

, 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. Galvanomet ...

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. 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 components for the tuned circuit. Instead methods such as

stripline In electronics, 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. De ...

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 frequency, 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 set at approximately 7.0� ...

(10 GHz) is a resonant cavity 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.

References

{{Reflist Measuring instruments Microwave technology Radio electronics

Overview

HF and VHF

UHF and SHF

See also

References