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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 radio transmitter, the radio waves form standing waves along their length. By sliding a conductive bar that bridges the two wires along their length, the length of the waves can be physically measured. Austrian physicist Ernst Lecher, improving on techniques used by Oliver Lodge and Heinrich Hertz, developed this method of measuring wavelength around 1888.〔E. Lecher (1888) "Eine studie uber electrische Resonanzerscheinungen"(''Study of Electrical Resonance Phenomena''), ''Wiedemann Annalen'', Vol.41, p.850, cited in (Fleming, 1908 )〕 Lecher lines were used as frequency measuring devices until frequency counters became available after World War 2. They were also used as components, often called "resonant stubs", in UHF and microwave radio equipment such as transmitters, radar sets, and television sets, serving as tank circuits, filters, and impedance matching devices. They are used at frequencies between HF/VHF where lumped components are used, and UHF/SHF where resonant cavities are more practical.
==Wavelength measurement==
A Lecher line is a pair of parallel uninsulated wires or rods held a precise distance apart. The separation is not critical but should be a small fraction of the wavelength; it ranges from less than a centimeter to over 10 cm. The length of the wires depends on the wavelength involved; lines used for measurement are generally several wavelengths long. The uniform spacing of the wires makes them a transmission line, conducting radio waves at a constant speed very close to the speed of light. One end of the rods is connected to the source of RF power, such as the output of a radio transmitter. At the other end the rods are connected together with a conductive bar between them. This short circuiting termination reflects the waves. The waves reflected from the short-circuited end interfere with the outgoing waves, creating a sinusoidal standing wave of voltage and current on the line. The voltage goes to zero at nodes located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. Therefore the wavelength ''λ'' can be determined by finding the location of two successive nodes (or antinodes) and measuring the distance between them, and multiplying by two. The frequency ''f'' of the waves can be calculated from the wavelength and the speed of the waves, which is the speed of light ''c'':
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The nodes are much sharper than the antinodes, because the change of voltage with distance along the line is maximum at the nodes, so they are used.
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