Lecher's line. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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In electronics, Lecher lines or Lecher system are pairs of parallel wires or rods that measure the length of radio waves, mainly in the UHF and microwave bands. These wires form a short balanced transmission line. When connected to a source of high frequency energy, such as a radio transmitter, the radio waves form standing waves along the entire length of the transmission line. By moving the conductive jumper (bridge) that short-circuits both wires of the system, one can physically measure the wavelength.

The Austrian physicist Ernst Lecher, improving on the methods used by Oliver Lodge and Heinrich Hertz, developed his method of measuring wavelength around 1888. More advanced methods of measuring frequency are available today, and Lecher lines are now most often used as circuit elements when used in high frequency equipment such as televisions, Lecher lines are used as resonant circuits, in narrow band filters and in impedance matching devices. They are used at frequencies between HF/VHF where lumped components are used and on the UHF/Microwave bands where cavity resonators are used.

Wavelength measurement

A Lecher line is a pair of parallel bare wires or rods at a fixed distance from each other. The distance between the conductors is not critical, but it should be a small fraction of the wavelength. This distance can range from less than a centimeter to 10 cm or more. The length of the wires depends on the effective wavelength; The lines used for measurements are usually several times longer than the measured wavelength. The uniform spacing between the wires makes them transmission lines that transmit radio waves at a constant speed, very close to the speed of light. One end of the line is connected to an RF signal source, such as the output of a radio transmitter. The other end of the line is short-circuited through a movable conductor. This closing bridge reflects the waves. The waves reflected from the short-circuited end of the line interact with the incoming waves, creating sinusoidal standing waves of voltage and current on the line. The voltage drops to zero at nodes located at a distance that is a multiple of half a wavelength from the end of the line. The stress maxima, called antinodes, are located halfway between the nodes. Therefore, the wavelength λ can be determined by finding two consecutive nodes (or antinodes) and measuring the distance between them, which must be multiplied by two. The frequency F can be calculated if the wavelength and its speed are known, and if the speed of light C is known:

F=C/λ

For measurements, knots are usually used, since they appear sharper than antinodes, respectively, and the measurement accuracy will be higher.

Node search

Two methods are used to find nodes. One is to use voltage indicators such as an RF voltmeter or a simple incandescent light bulb attached to a pair of contacts that slide up and down the wires. When the bulb reaches the node, the voltage between the wires becomes zero, so the bulb goes out. One of the disadvantages of this method is that the indicator can affect the standing wave on the line, which leads to its reflection. To prevent this, an indicator with a high input impedance must be used; a conventional incandescent lamp is too low resistance. Leher and other researchers used long, thin Geisler tubes (Fig. 1.), whose glass flask was placed directly on the line. In old transmitters, high voltage excited a glow discharge in the gas. Nowadays, small neon lamps are often used. One of the problems with the use of glow discharge lamps is their high ignition voltage, making it difficult to accurately locate the minimum voltage. Precise wavelength meters use an RF voltmeter.

Another method used for finding nodes is to move the closing bridge along the line and measure the HF current flowing in the line using an HF ammeter included in the feeder line. The current in the Lecher line, like the voltage, forms standing waves with nodes (minimum current points) through each half wavelength. Since the line is an impedance for the source of RF energy that feeds it, and this impedance varies depending on the length of the line. When the current node is located at the beginning of the line, the current drawn from the source will be minimal, which is what the ammeter will show. If you move the closing bridge further along the line and mark two places with a minimum current, then the distance between these two minima will be equal to half the wavelength.

Rice. 1. Lecher line of the 1902 model, identical to the original design of 1888 by Ernst Lecher

The radio waves generated by the generator based on the Hertz spark gap (in the figure on the right) move along parallel wires. The wires are closed to each other (in the figure on the left side), the reflected waves run back along the wires towards the generator, creating standing voltage waves along the line. The voltage tends to zero at nodes located at a distance that is a multiple of half a wavelength from the end of the line. The nodes were found by moving a Geisler tube, a small glow discharge tube like a neon lamp, along a line (two of these lamps are shown in the figure). High voltage on the line causes the tube to glow. When the tube reaches the node, the voltage tends to zero, and the tube goes out. The measured distance between two adjacent nodes is multiplied by two to give the wavelength λ. In the figure, the line is shown shortened; at the very length of the line was 6 meters. The radio waves produced by the generator were in the VHF band and had a wavelength of several meters. The inset shows the types of Geisler tubes used with Lecher lines.

Design

The main attraction of the Lecher line is that it can be used to measure frequency without the use of complex electronics, and the line can be easily assembled from simple materials sold in a regular store. The Lecher line for measuring the wavelength is usually built on a frame on which horizontal conductors are rigidly mounted, along which the closing bridge or indicator moves, and a measuring scale, which determines the distance between the nodes. The frame is usually made of non-conductive materials such as wood, because any conductive objects near the line can disturb the standing wave regime.

In many ways, the Lecher line is an electrical version of the Kundt tube experiment that is used to measure the length of sound waves.

Measuring the speed of light

If the frequency F of the radio wave is known, then by measuring the wavelength λ using the Lecher line, you can calculate the wave speed C, which is approximately equal to the speed of light:

C=λ*F

In 1891, the French physicist Prosper-René Blondlot used this method to make the first measurements of the propagation speed of radio waves. He used 13 different frequencies between 10 and 30 MHz and got an average of 297600 km/s, which is within 1% of the true speed of light. This was an important confirmation of James Clerk Maxwell's theory that light is also an electromagnetic wave, just like radio waves.

Application in other areas

Short Lecher lines are often used as high-Q resonant circuits, which are called tuning or resonant stubs. For example, a quarter-wave (λ/4) short Lecher line acts like a parallel resonant circuit, having high resistance at its resonant frequency and low impedance at other frequencies. They are used due to the fact that at frequencies of the decimeter range (10 cm ... 1 m) in resonant circuits, small inductances and capacitances are required, which makes them difficult to manufacture and, moreover, they are very sensitive to parasitic capacitances and inductances. The only difference between closed transmission lines and conventional LC circuits is that a closed transmission line (resonant stub) such as a Lecherian line has multiple resonances at odd frequencies that are multiples of the fundamental resonant frequency, while lumped LC circuits have only one resonant frequency.

Powering High Frequency Power Amplifiers

Lecher lines can be used for resonant circuits in microwave power amplifiers.] For example, a double tetrode amplifier (QQV03-20) at a frequency of 432 MHz is described by G. R. Jessop in the manual (GR Jessop, VHF UHF manual, RSGB, Potters Bar, 1983 ) uses the Lecher line in the anode circuit as a resonant circuit.

Rice. 2. Using the Lecher line as a resonant circuit

TV tuners

Quarter-wave Lecher lines are used in resonant circuits in RF amplifiers and in local oscillators in some models of modern TVs. Tuning to various TV stations is carried out using a varicap connected to both conductors of the Lecher line.

Impedance of the Lecher line

The spacing of the Lechera conductors does not affect the position of the standing waves on the line, but it determines the characteristic impedance, which can be important to match the line to the RF power source for efficient power transfer. For two parallel cylindrical conductors with a diameter d and a distance between them D, the impedance of the line will be equal to:

For parallel wires, the formula for capacitance where L is the length, C is the capacitance per meter

Whence

Commercially available 300 and 450 ohm ribbon cables (such as a two-wire telephone noodle line) can be used as fixed length Lecher lines (resonant stub).

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