Lecher line

Austrian physicist Ernst Lecher, improving on techniques used by Oliver Lodge[3] and Heinrich Hertz,[4] developed this method of measuring wavelength around 1888.

A Lecher line is a pair of parallel uninsulated wires or rods held a precise distance apart.

[10] One end of the rods is connected to the source of RF power, such as the output of a radio transmitter.

[11] 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.

[11] 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.

The shorting bar is slid down the line and the position of two successive current minima is noted, the distance between them is half a wavelength.

[9][1][10] A major attraction of Lecher lines was they were a way to measure frequency without complicated electronics, and could be improvised from simple materials found in a typical shop.

Lecher line wavemeters are usually built on a frame which holds the conductors rigid and horizontal, with a track that the shorting bar or indicator rides on, and a built-in measuring scale so the distance between nodes can be read out.

[9] The frame must be made of a nonconductive material like wood, because any conducting objects near the line can disturb the standing wave pattern.

[9] The RF current is usually coupled into the line through a single turn loop of wire at one end, which can be held near a transmitter's tank coil.

The shorting bar should always be slid out, away from the link end, not in, to avoid converging on a higher order node by mistake.

In many ways Lecher lines are an electrical version of the Kundt's tube experiment which is used to measure the wavelength of sound waves.

They are used because at UHF frequencies the value of inductors and capacitors needed for 'lumped component' tuned circuits becomes extremely low, making them difficult to fabricate and sensitive to parasitic capacitance and inductance.

[16] For instance, the twin tetrode (QQV03-20) 432 MHz amplifier described by G.R Jessop[17] uses a Lecher line anode tank.

Quarter-wave Lecher lines are used for the tuned circuits in the RF amplifier and local oscillator portions of modern television sets.

[18] The separation between the Lecher bars does not affect the position of the standing waves on the line, but it does determine the characteristic impedance, which can be important for matching the line to the source of the radio frequency energy for efficient power transfer.

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.
Lecher line wavemeter, from "DIY" article in 1946 radio magazine
Experimental 300 MHz Barkhausen-Kurz oscillator in 1933, with Lecher line tank circuits. The experimenter is using a U-shaped Lecher wavemeter to measure the frequency
Lecher line as a tank circuit in an RF amplifier . Not shown in this simplified diagram are the chokes that feed the tube anodes from the HT source. Without them the two anodes are shorted together.