Lecher line

Early 1902 Lecher line identical to Ernst Lecher's original 1888 apparatus. Waves generated by the Hertzian spark-gap oscillator at right are coupled into the wires by the two metal plate capacitors (circles) and travel down the parallel wires. 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. The voltage goes to zero at nodes located at multiples of a half-wavelength from the end. 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 high voltage on the line makes the tube glow. When the tube reaches a node, the voltage goes to zero and the tube goes out. The measured distance between two successive nodes is equal to half the wavelength λ/2 of the radio waves. The line is shown truncated in the drawing; the length of the line was actually 6 meters (18 feet). The waves produced by the oscillator were in the VHF range, with a wavelength of several meters. The inset shows types of Geissler tube used with Lecher lines.

Lecher-line educational kit sold by Central Scientific Company in the 1930s for teaching radio theory in college. It contains everything necessary, including an absorption wavemeter for independently measuring frequency.

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. They form a short length of balanced transmission line (a resonant stub). 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. Lecher lines were used as frequency measuring devices until inexpensive frequency counters became available after World War II. 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. They are used at frequencies between HF/VHF, where lumped components are used, and UHF/SHF, where resonant cavities are more practical.