- Lecher lines
Lecher lines are a form of parallel conductor that can be used at UHF for creating resonant circuits. They are used at frequencies between HF/
VHF where lumped components are used, and UHF/SHF where resonant cavities are more practical.Ernst Lecher of Vienna, 1856 - 1926, created this device bearing his name, probably around 1890.Description
For two parallel lines (diameter d, spacing D),
Z_0 = 276 log_{10} left(D/d + sqrt{(D/d)^2-1} ight) = (120/{sqrt{epsilon_r) cosh^{-1} (D/d)
For parallel wires the formula for
capacitance isl = lengthC = capacitance per meter
:C = pi epsilon_0 epsilon_r /ln {( 2D / d )},
Hence as
:Z_0^2 = L/C
:c = frac 1 {sqrt{(L/C) cdot C^2:frac 1 {Z_0cdotleft(piepsilon_0epsilon_r ight)cdot left [ln left({2D/d} ight) ight] }
: = 1/{Zo . (π εo εr) / [ln ( 2D / d)] }
Note: It is possible to purchase a 300 ohm balanced ribbon feeder which is physically the same as a Lecher line.
Lecher wire system
The Lecher wire system can be used to measure
frequency ofstanding wave s. This amounts to a yardstick approach in analyzing thewavelength ofradiant energy , being anelectronic filter with a high Q. When the measured distance of the weak points (which have narrow locations) in atransmission line (eg., several feet of twowire s in a parallel line, at or below a foot apart) an understanding of thestanding wave ratio could be obtained.The Lecher wire system was developed by
Ernst Lecher ofVienna circa 1890. The principles behind the Lecher wire system contributed to high-power transmission line andcavity resonator (eg.,waveguide ) development.Applications
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 Jessopref|jessop uses a Lecher line anode tank.
Frequency measurement
Lecher lines can be used for a crude measurement of frequency. In many ways this is an electrical version of the
Kundt's tube experiment which is used to measure thespeed of sound in a gas [http://hyperphysics.phy-astr.gsu.edu/hbase/class/phscilab/kundt2.html] .The technique relies on observing when a Lecher line transmission line with an infinite SWR is a
resonant stub . Similar methods can be used to create stub filters using lengths of coaxial cable.The 'stub filter' method could be used in the following way. If a pair of parallel conductors is used as the
anode tank circuit for a modifiedValved RF amplifier s stage, then, if the moveable short was to be moved up and down the line, a dip in the anode current would be observed every half wavelength along the line as the amplifier's tank circuit comes to resonance."Warning"' High voltage electronics. This circuit is not for anyone who is not entirely sure of what they are doing. If you are in any doubt as to the hazards of high voltage electronics, then do not attempt to build such a device"
A second method requiring less electronic equipment to be made is to use a
SWR meter or anAntenna analyzer in the following system. A source of RF is connected to theSWR meter orAntenna analyzer before the output is connected to a resistive load which is in parallel with the connections to the balanced Lecher line (Abalun may be required for making the connection). A moveable short can then be used to search forvoltage nodes on the lecher lines. When the short is at a voltage node the line will be an open circuit at this frequency, it will then be possible to measure the distance between voltage nodes.When using either method the distance between the voltage nodes (λ/2) has been measured then as the speed of waves along the line can be calculated the frequency can then be calculated of the waves.f = v/λ
Notes
# G.R. Jessop, "VHF UHF manual", RSGB,
Potters Bar , 1983, ISBN 0900612924See also
*
Transmission line External links
* Ron Lebar, " [http://www.ronlebar.com/lecher.htm The Lecher Line] ; Used in high frequency electronics, generation & measurement". 14-5-2005.
* "Index to Physics Demonstrations; [http://groups.physics.umn.edu/demo/old_page/demo_gifs/5N10_50.gifLecher wires] ". Physics Demonstrations, The University of Minnesota. 1997-06-16.
* "E-82. Electromagnetic Radiation; [http://www.physics.purdue.edu/reports/demo/elect/El-82.html Demonstration Short-wave apparatus] ". Electricity/Magnetism, Lecture Demonstrations. Purdue University.
* M B Allenson, A R Piercy and K N R Taylor " [http://www.iop.org/EJ/abstract/0031-9120/8/1/002 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, " [https://kb.osu.edu/dspace/bitstream/1811/1986/1/V18N05_145.pdf The Relative Intensity of the Harmonics of a Lecher System (Experimental)] ". The Ohio Journal of Science. *PDF )
* F. C. Blake, " [https://kb.osu.edu/dspace/bitstream/1811/1987/1/V18N05_163.pdf The Relative Intensity of the Harmonics of a Lecher System (Theoretical)] ". Physical Lab, Ohio State University. (PDF )
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