File:Lecher line educational kit 1932 labeled.png

English: An educational kit from around 1932 used to demonstrate standing waves of radio waves on a parallel transmission line, a device called a Lecher line. It consists of a vacuum tube oscillator which produces radio waves in the UHF range, with wavelength of around a meter. The radio waves are coupled through a wire loop to one end of a balanced transmission line consisting of two parallel rods, called a Lecher line. The line is shown truncated in this diagram; it was actually several meters long. The other end of the rods are shorted together with a movable shorting bar. The radio waves travel along the transmission line with a constant speed very close to the speed of light. When they hit the shorted end, they are reflected back. The outgoing and reflected waves superpose, creating a series of stationary or standing waves along the line. The voltage across the line has a sinusoidal variation that goes to zero at points where the two waves cancel each other, that are a multiple of a half wavelength (λ/2) from the end; these are called nodes. The wavelength of the radio waves can be determined by measuring the distance between two nodes and multiplying by two. The nodes can be found by sliding the neon bulb along the line until it goes out. Another alternate technique is to attach the RF voltmeter at the beginning of the line and slide the shorting bar along it until it reads minimum. A centimeter scale is included so the distance between the nodes and thus the wavelength can be read off. Once the wavelength λ is known, the frequency of the radio waves can be found from f = c/λ where c is the speed of light. Between the nodes are points called antinodes or loops, where the voltage is maximum but the current goes to zero. An alternate way is to find the antinodes by sliding the incandescent lamp attached to an inductive loop along the line until the light goes out. The wavemeter is a calibrated tuned circuit which can be used to independently check the frequency. This method of measuring the length of radio waves was devised by Heinrich Hertz, Oliver Lodge and Ernst Lecher around 1888, just after radio waves were discovered. It was widely used until frequency counters became available after WW2.

Caption:"At right is the set-up, including generator, Lecher antenna, wave meter, galvanometer, neon indicating lamps, and equipotential link."

Alterations to image: Replaced the original difficult-to-read labels with clearer labels in red.