For two parallel lines (diameter d, spacing D),
For parallel wires the formula for capacitance is
l = length C = capacitance per meter
Note: It is possible to purchase a 300 ohm balanced ribbon feeder which is physically the same as a Lecher line.
The Lecher wire system was developed by Ernst Lecher of Vienna circa 1890. The principles behind the Lecher wire system contributed to high-power transmission line and cavity resonator (eg., waveguide) development.
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.
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 modified Valved RF amplifiers 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 an Antenna analyzer in the following system. A source of RF is connected to the SWR meter or Antenna analyzer before the output is connected to a resistive load which is in parallel with the connections to the balanced Lecher line (A balun may be required for making the connection). A moveable short can then be used to search for voltage 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