- Absorption wavemeter
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.htm] [http://www.tpub.com/content/neets/14188/css/14188_63.htm] [http://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 adip meter can be used.HF andVHF The most simple form of the device is a
variable capacitor with a coil wired across its terminals. Attached to one the terminals of theLC circuit is adiode , 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 agalvanometer 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 isresonant .This is given by f = 1/2π L0.5C0.5
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 andSHF =At the higher frequencies it is not possible to use
lumped components for the tuned circuit. Instead methods such asstripline are used. One design for UHF and SHF is a resonant λ/4 (quarter wave) rod which can vary in length. Another design forX-band (10 GHz) is aresonator cavity which can be changed in length.As an alternative for UHF, Lecher transmission lines can be used [http://www.intuitor.com/resonance/standingEwaveDemo.html] . It is possible to measure roughtly the frequency of a transmitter using
Lecher lines .See also
*
Grid dip oscillator
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