Lecher and early researchers used long thin Geissler tubes, laying the glass tube directly across the line. If the indicator has too low an impedance it will disturb the standing wave on the line, so a high impedance indicator must be used a regular incandescent bulb has too low a resistance. When the bulb reaches a node, the voltage between the wires goes to zero, so the bulb goes out. One is to use some type of voltage indicator, such as an RF voltmeter or light bulb, attached to a pair of contacts that slide up and down the wires. Two methods are employed to find the nodes. 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. The frequency f of the waves can be calculated from the wavelength and the speed of the waves, which is approximately the speed of light c:į = c λ 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 voltage goes close to zero at nodes located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. 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. This short circuiting termination reflects the waves. At the other end the rods are connected together with a conductive bar between them. One end of the rods is connected to the source of RF power, such as the output of a radio transmitter. The uniform spacing of the wires makes them a transmission line, conducting 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. 4 Characteristic impedance of Lecher lineĪ Lecher line is a pair of parallel uninsulated wires or rods held a precise distance apart.They are used at frequencies between HF/ VHF, where lumped components are used, and UHF/ SHF, where resonant cavities are more practical. They were also used as components, often called " resonant stubs", in VHF, 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 balanced transmission 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 VHF, UHF and microwave frequencies. It contains everything necessary, including an absorption wavemeter for independently measuring frequency. Lecher-line educational kit sold by Central Scientific Company in the 1930s for teaching radio theory in college. The inset shows types of Geissler tube used with Lecher lines. The waves produced by the oscillator were in the VHF 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. 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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