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. The length of the wires depends on the wavelength involved lines used for measurement are generally several wavelengths long. 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. They are used at frequencies between HF/VHF, where lumped components are used, and UHF/SHF, where resonant cavities are more practical.Ī Lecher line is a pair of parallel uninsulated wires or rods held a precise distance apart. 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. Lecher lines were used as frequency measuring devices until frequency counters became available after World War 2. Austrian physicist Ernst Lecher, improving on techniques used by Oliver Lodge and Heinrich Hertz, developed this method of measuring wavelength around 1888. By sliding a conductive bar that bridges the two wires along their length, the length of the waves can be physically measured. When attached to a source of radio-frequency power such as a radio transmitter, the radio waves form standing waves along their length. They form a short length of balancedtransmission line (a resonant stub). 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 microwavefrequencies. The inset shows types of Geissler tube used with Lecher lines. The radio waves produced by the oscillator were in the UHF range, with a wavelength of several meters. The line is shown truncated in the drawing the length of the line was actually 6 meters (18 feet). The measured distance between two successive nodes is equal to half the wavelength λ/2 of the radio waves. When the tube reaches a node, the voltage goes to zero and the tube goes out. The high voltage on the line makes the tube glow. The nodes were found by sliding a Geissler tube, a small glow discharge tube like a neon light, up and down the line (two are shown on the line). The voltage goes to zero at nodes located at multiples of a half-wavelength from the end. The wires are short-circuited together at the left end, reflecting the waves back up the wires toward the oscillator, creating a standing wave of voltage along the line. Radio waves generated by the Hertzian spark-gap oscillator at right travel down the parallel wires. Early 1902 Lecher line identical to Ernst Lecher's original 1888 apparatus.
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