Absorption wavemeter
Encyclopedia
The Absorption wavemeter is a simple device for the measurement of radio frequency
energy at different frequencies.
The device can be used by the users of radio equipment to check the approximate frequency of a strong signal source, and also to check the output of a transmitter for harmonics. Many radio amateurs use one to check the purity and frequency of their transmitters http://www.darleys.pwp.blueyonder.co.uk/Radio/advanced/measure1.htmhttp://www.tpub.com/content/neets/14188/css/14188_63.htmhttp://www.brightbell.com/radio/antenna1.html#awavemeter. Also devices can be made which work in the same way that are able to detect mobile phones http://www.creative-science.org.uk/absorb.html. As an alternative a dip meter can be used.
HF
The most simple form of the device is a variable capacitor
with a coil wired across its terminals. Attached to one the terminals of the LC circuit
is 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
is wired to the terminals of the decoupling capacitor.
The device will be sensitive to strong sources of radiowaves at the frequency
at which the LC circuit is resonant.
This is given by
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
At the higher frequencies it is not possible to use lumped components for the tuned circuit. Instead methods such as stripline
are used. One design for UHF and SHF is a resonant λ/4 (quarter wave) rod which can vary in length. Another design for X-band (10 GHz) is a resonator cavity which can be changed in length.
As an alternative for UHF, Lecher transmission lines can be usedhttp://www.intuitor.com/resonance/standingEwaveDemo.html. It is possible to measure roughly the frequency of a transmitter using Lecher lines
.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
energy at different frequencies.
The device can be used by the users of radio equipment to check the approximate frequency of a strong signal source, and also to check the output of a transmitter for harmonics. Many radio amateurs use one to check the purity and frequency of their transmitters http://www.darleys.pwp.blueyonder.co.uk/Radio/advanced/measure1.htmhttp://www.tpub.com/content/neets/14188/css/14188_63.htmhttp://www.brightbell.com/radio/antenna1.html#awavemeter. Also devices can be made which work in the same way that are able to detect mobile phones http://www.creative-science.org.uk/absorb.html. As an alternative a dip meter can be used.
HFHigh frequencyHigh frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
and VHF
The most simple form of the device is a variable capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
with a coil wired across its terminals. Attached to one the terminals of the LC circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
is a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
, 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
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
is wired to the terminals of the decoupling capacitor.
The device will be sensitive to strong sources of radiowaves at the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
at which the LC circuit is resonant.
This is given by
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.
UHFUltra high frequencyUltra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
and SHFSHFSHF may refer to:* Super high frequency, radio frequencies in the range of 3 GHz and 30 GHz* Société de l'histoire de France, a society formed in 1833 to study French history* Souther-Hillman-Furay Band, a mid-1970s country-rock band....
At the higher frequencies it is not possible to use lumped components for the tuned circuit. Instead methods such as striplineStripline
Stripline is a transverse electromagnetic transmission line medium, that was invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.- Description :...
are used. One design for UHF and SHF is a resonant λ/4 (quarter wave) rod which can vary in length. Another design for X-band (10 GHz) is a resonator cavity which can be changed in length.
As an alternative for UHF, Lecher transmission lines can be usedhttp://www.intuitor.com/resonance/standingEwaveDemo.html. It is possible to measure roughly the frequency of a transmitter using Lecher lines
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 UHF and microwave frequencies. They form a short length of balanced transmission line. When attached to a source of radio frequency power such as a...
.