An improved Lecher wire experiment. To cite this article: M B Allenson et al Phys. Educ. 8 View the article online for updates and enhancements. Lecher line with short-circuited end: If the wires of the Lecher line are short- circuited at the end, then the voltage U is zero. A reflected wave arises with a phase. Lecher Lines. K6. K6. ELECTROMAGNETIC OSCILLATIONS. Two bare copper wires, 5 m long and 20 cm apart, are stretched above the lecture tables.
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In electronics, a Lecher line or Lecher wires is a pair of parallel . (PDF). Short Wave Craft. New York: Popular Book Corp. 3 (3): Retrieved March 23, In electronics, a Lecher line or Lecher wires is a pair of parallel wires or rods that were used to .. *PDF); F. C. Blake, "The Relative Intensity of the Harmonics of a Lecher System (Theoretical)". Physical Lab, Ohio State University. (PDF). It is quite some time since the article by Howes , concerning Lecher Lines, The formula for ZO for a parallel wire transmission line is given by equation. (3).
Finding the nodes[ edit ] Two methods are employed to find the nodes. 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. Lecher and early researchers used long thin Geissler tubes , laying the glass tube directly across the line. The high voltage of early transmitters excited a glow discharge in the gas. In modern times small neon bulbs are often used. One problem with using glow discharge bulbs is their high striking voltage makes it difficult to localize the exact voltage minimum.
Also in my setup the other end of the line is open, the wires are not connected together there.
A metal bar, or a screwdriver in my case, is put across the width of the two parallel wires, shorting them. As I slide the bar along the wires, it influences the waves. One of those ways is to have the bar be two terminals of a neon bulb.
The bulb goes out at the nodes, where the voltage is zero. In the photos you can see a measuring tape running the length of the wires for measuring the distance from the end of the Lecher line to the nodes.
Part of that loop is a 6. Transmission lines use specialized construction, and impedance matching, to carry electromagnetic signals with minimal reflections and power losses. What are some of the common types of transmission lines? Parallel Wire, Microstrip line and Coaxial Cables.
What is a Lecher line?
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 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. How are standing waves produced in a Lecher line and how can it be used to measure the wavelength?
A Lecher line is a pair of parallel uninsulated wires or rods held a precise distance apart. The length of the wires depends on the wavelength involved; lines used for measurement are generally several wavelengths long. The uniform spacing of the wires makes them a transmission line, conducting radio waves at a constant speed very close to the speed of light.
One end of the rods is connected to the source of RF power, such as the output of a radio transmitter. At the other end the rods are connected together with a conductive bar between them.
This short circuiting termination reflects the waves. 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.
The voltage goes to zero at nodes located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. The frequency f of the waves can be calculated from the wavelength and the speed of the waves, which is the speed of light c: 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.
How can you find out the locations of two successive nodes and hence the wavelength?
The method used to find the nodes is to slide the terminating shorting bar up and down the line, and measure the current flowing into the line with an RF ammeter in the feeder line. The current on the Lecher line, like the voltage, forms a standing wave with nodes points of minimum current every half wavelength. So the line presents an impedance to the applied power which varies with its length; when a current node is located at the entrance to the line, the current drawn from the source, measured by the ammeter, will be minimum.
In the photos you can see a measuring tape running the length of the wires for measuring the distance from the end of the Lecher line to the nodes. Part of that loop is a 6. The oscillator puts electromagnetic waves on the Lecher line using induction. In the first photo above, the oscillator is pulled back a bit to make things clearer.
As shown in the above photos, I added the diode loop to the collection of loops at the end of the Lecher line. When the screwdriver is still less than half a meter from the oscillator end, and at the node for the first half-wavelength, the voltage across the diode is above 0. When the screwdriver is between nodes, the voltage is less than 0.