Lecher lines
Encyclopedia
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 wave
s, mainly at UHF
and microwave
frequencies
. They form a short length of balanced transmission line
. When attached to a source of radio frequency
power such as a radio transmitter, the radio waves form standing wave
s 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. However, better methods of frequency
measurement are available today, and Lecher lines are now more often used as components
in radio frequency equipment such as television set
s, serving as resonant circuits, narrow band 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.
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 circuit
ing 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 node
s located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. 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. 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.
or light bulb, attached to a pair of contacts that slide up and down the wires. When the bulb reaches a node, the voltage between the wires goes to zero, so the bulb goes out. One problem with this method is that the indicator can disturb the standing wave on the line, causing reflections. To prevent this a high impedance
indicator must be used; a regular incandescent bulb has too low resistance. Lecher and early researchers used long thin Geissler tube
s, 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. In precision wavemeters an RF voltmeter
is used.
The other 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. 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.
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.
:
In 1891, French physicist Prosper-René Blondlot made the first measurement of the speed of radio waves, using this method. He used 13 different frequencies between 10 and 30 MHz and obtained an average value of 297,600 km/sec, which is within 1% of the current value for the speed of light. This was an important confirmation of James Clerk Maxwell
's theory that light was an electromagnetic wave like radio waves.
resonant circuits, termed resonant stubs. For example, a quarter wavelength (λ/4) shorted Lecher line acts like a parallel resonant circuit, appearing as a high impedance at its resonant frequency and low impedance at other frequencies. They are used because at UHF frequencies the value of inductor
s and capacitor
s needed for 'lumped component' tuned circuits becomes extremely low, making them difficult to fabricate and sensitive to parasitic capacitance
and inductance. One difference between them is that transmission line stubs like Lecher lines also resonate at odd-number multiples of their fundamental resonant frequency, while lumped LC circuit
s just have one resonant frequency.
amplifier
and local oscillator
portions of modern television set
s. The tuning necessary to select different stations is done by varactor diodes across the Lecher line.
, which can be important for matching the line to the source of the radio frequency energy for efficient power transfer. For two parallel cylindrical conductors of diameter d and spacing D,
For parallel wires the formula for capacitance
is
l = length
C = capacitance per meter
Hence as
Commercially available 300 and 450 ohm
twin lead balanced ribbon feeder can be used as a fixed length Lecher line (resonant stub).
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, a Lecher line or Lecher wires is a pair of parallel wires or rods that were used to measure the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s, mainly at UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
and microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. They form a short length of balanced transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
. When attached to a source of radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
power such as a radio transmitter, the radio waves form standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
s 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
Ernst Lecher
Ernst Lecher was an Austrian physicist who, from 1909, was head of the First Institute of Physics in Vienna. He is remembered for developing an apparatus— "Lecher lines"—to measure the wavelength and frequency of electromagnetic waves...
, improving on techniques used by Oliver Lodge and Heinrich Hertz, developed this method of measuring wavelength around 1888. However, better methods of frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
measurement are available today, and Lecher lines are now more often used as components
Electronic component
An electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...
in radio frequency equipment such as television set
Television set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
s, serving as resonant circuits, narrow band filters
RF and microwave filter
Radio frequency and microwave filters represent a class of electronic filter, designed to operate on signals in the megahertz to gigahertz frequency ranges...
, and impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
devices. They are used at frequencies between HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
/VHF where lumped
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...
components are used, and UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
/SHF
SHF
SHF may refer to:* Super high frequency, radio frequencies in the range of 3 GHz and 30 GHz* Société de l'histoire de France, a society formed in 1833 to study French history* Souther-Hillman-Furay Band, a mid-1970s country-rock band....
where resonant cavities are more practical.
Wavelength measurement
A Lecher line is a pair of parallel uninsulated wires or rods held a precise distance apart. The separation is not critical but should be a small fraction of the wavelength; it ranges from less than a centimeter to over 10 cm. The length of the wires depends on the wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
involved; lines used for measurement are generally several wavelengths long. The uniform spacing of the wires makes them a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, conducting radio waves at a constant speed very close to the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. One end of the rods is connected to the source of RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
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 circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
ing termination reflects the waves. The waves reflected from the short-circuited end interfere with the outgoing waves, creating a sinusoidal standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
of voltage and current on the line. The voltage goes to zero at node
Node (physics)
A node is a point along a standing wave where the wave has minimal amplitude. For instance, in a vibrating guitar string, the ends of the string are nodes. By changing the position of the end node through frets, the guitarist changes the effective length of the vibrating string and thereby the...
s located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. 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. The frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
f of the waves can be calculated from the wavelength and the speed of the waves, which is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
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.
Finding the nodes
Two methods are employed to find the nodes. One is to use some type of voltage indicator, such as an RF voltmeterVoltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
or light bulb, attached to a pair of contacts that slide up and down the wires. When the bulb reaches a node, the voltage between the wires goes to zero, so the bulb goes out. One problem with this method is that the indicator can disturb the standing wave on the line, causing reflections. To prevent this a high impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
indicator must be used; a regular incandescent bulb has too low resistance. Lecher and early researchers used long thin Geissler tube
Geissler tube
A Geissler tube is an early gas discharge tube used to demonstrate the principles of electrical glow discharge. The tube was invented by the German physicist and glassblower Heinrich Geissler in 1857...
s, 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. In precision wavemeters an RF voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
is used.
The other 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
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
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. 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.
Construction
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. The frame is usually made of a nonconductive material like wood, because any conducting objects near the line can disturb the standing wave pattern.In many ways Lecher lines are an electrical version of the Kundt's tube
Kundt's tube
Kundt's tube is an experimental acoustical apparatus invented in 1866 by German physicist August Kundt for the measurement of the speed of sound in a gas or a solid rod...
experiment which is used to measure the wavelength of sound waves.
Measuring the speed of light
If the frequency f of the radio waves is independently known, the wavelength λ measured on a Lecher line can be used to calculate the speed of the waves, c, which is approximately equal to the speed of lightSpeed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
:
In 1891, French physicist Prosper-René Blondlot made the first measurement of the speed of radio waves, using this method. He used 13 different frequencies between 10 and 30 MHz and obtained an average value of 297,600 km/sec, which is within 1% of the current value for the speed of light. This was an important confirmation of James Clerk Maxwell
James Clerk Maxwell
James Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...
's theory that light was an electromagnetic wave like radio waves.
Other applications
Short lengths of Lecher line are often used as high QQ factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
resonant circuits, termed resonant stubs. For example, a quarter wavelength (λ/4) shorted Lecher line acts like a parallel resonant circuit, appearing as a high impedance at its resonant frequency and low impedance at other frequencies. They are used because at UHF frequencies the value of inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s needed for 'lumped component' tuned circuits becomes extremely low, making them difficult to fabricate and sensitive to parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
and inductance. One difference between them is that transmission line stubs like Lecher lines also resonate at odd-number multiples of their fundamental resonant frequency, while lumped LC circuit
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
s just have one resonant frequency.
Power amplifier tank circuits
Lecher line circuits can be used for the tank circuits of UHF power amplifiers. For instance, the twin tetrode (QQV03-20) 432 MHz amplifier described by G.R Jessop uses a Lecher line anode tank.Television tuners
Quarter-wave Lecher lines are used for the tuned circuits in the RFRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
and local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
portions of modern television set
Television set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
s. The tuning necessary to select different stations is done by varactor diodes across the Lecher line.
Characteristic impedance of Lecher line
The separation between the Lecher bars does not affect the position of the standing waves on the line, but it does determine the characteristic impedanceCharacteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
, which can be important for matching the line to the source of the radio frequency energy for efficient power transfer. For two parallel cylindrical conductors of diameter d and spacing D,
For parallel wires the formula for capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
is
l = length
C = capacitance per meter
Hence as
Commercially available 300 and 450 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
twin lead balanced ribbon feeder can be used as a fixed length Lecher line (resonant stub).
External links
- "Index to Physics Demonstrations; Lecher wires". Physics Demonstrations, The University of Minnesota. 1997-06-16.
- "E-82. Electromagnetic Radiation; Demonstration Short-wave apparatus". Electricity/Magnetism, Lecture Demonstrations. Purdue University.
- M B Allenson, A R Piercy and K N R Taylor "An improved Lecher wire experiment". 1973 Phys. Educ. 8 47-49. doi 10.1088/0031-9120/8/1/002.
- F. C. Blake and B. H. Jackson, "The Relative Intensity of the Harmonics of a Lecher System (Experimental)". The Ohio Journal of Science. *PDF)
- F. C. Blake, "The Relative Intensity of the Harmonics of a Lecher System (Theoretical)". Physical Lab, Ohio State University. (PDF)
