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Starburst The Lecher Line Starburst

The Lecher Line

The Lecher Line.
Used in high frequency electronics, generation & measurement.

Ernst Lecher of Vienna, 1856 - 1926, created the device bearing his name, probably around 1890. Originally to accurately measure the wavelength of electromagnetic signals. Also as a means of determining the speed of light.

The basic form is 2 parallel wires or rods. These form a waveguide or transmission line & can be arranged in various ways. An early variant has one end short circuited, with a signal inductively or capacitively coupled. A sliding shorting bar is moved along the line whilst a lamp is placed between them. Positioned halfway between the shorting bars, with no connections made.

If sufficient power is available the lamp will light, a small neon bulb is usually more sensitive than filament or fluorescent types. When the 2 short circuits are exactly one half wavelengths apart brightness will be maximum. A standing wave is produced, sliding the lamp along between the shortimg bars shows its distribution as a brightness variation.

The point of maximum brightness is called an antinode & is the peak of a half cycle. If the shorting bars are one wavelength apart, a minima or node will be found halfway between two antinodes. This is the zero crossing point of the waveform. It is clear that the wavelength can be readily measured, although more accurate methods are now available for this.

If the frequency is known, the speed of light can be measured. It was fairly well established by 1890 that all electromagnetic waves propogate at the same rate in a vacuum. This speed is only slightly different in air, the mechanical accuracy of the line is probably not sufficient to detect this difference.

Some literature on the subject suggests that a lamp can be 'connected' across the line. A mistake, anything across the line may cause a reflection, the wave will think it's a shorting bar.

Bead

The means of operation is straightforward, the parallel conductors form a transmission line, with a characteristic impedance. Electromagnetic waves propogate along the line at the speed of light (c).

A short circuit across the line causes an impedance mismatch, the wave is reflected. If it meets another short circuit it will be reflected again. If two short circuits are an exact number of half wavelengths apart, the waves travelling in each direction add, forming what is called a standing wave. Any deviation from this distance will cause the waves to partially or completely cancel.

Not only is the line a measuring tool, it also forms a frequency selective system with a very high Q. A response is obtained at any frequency where the half wavelength is an integral sub-multiple of the line length. It differs from a coil/capacitor tuned circuit in this respect. Line capacitance & inductance play no direct part in its frequency selectivity.

An open circuit at one end of the line is also an impedance mismatch, causing similiar reflections. If both ends of the line are open circuit its fundamental resonance is at a wavelength twice its length. It will oscillate at this wavelength in a suitable circuit.


Line
Line Wave
Line

The characteristics of this arrangement are interesting, if you like that sort of thing. Driven from one end it exhibits a high impedance at the fundamental resonance. Like a parallel tuned circuit this impedance falls rapidly at frequencies either side. Animation approximates current in an open line.

At half that frequency it forms a quarter wavelength, reflected waves exactly cancel. Exhibiting a low impedance, at frequencies either side impedance is higher. This behaviour resembles that of a series tuned circuit. Selectivity (Q) is higher than such circuits, in both cases.

Physical dimensions make it impractical for tuning except at very high frequencies (VHF), upward.

Bead

At 100 Mc/s the wavelength is 3 metres, the line will need to be half that, almost 5 foot. Half that again for quarter wave resonance, this requires a more difficult implementation at high frequencies.

In the late 1950s, whilst an apprentice, I was asked to design a transistor VHF FM receiver. It was decided to ignore the common technology of the time in many respects.

The best germainium transistors of the day had poor response at 100 Mc/s. A new device was chosen, the tunnel diode, this was being made & beta tested by a division of the company. When suitably biased it exhibits negative resistance, over part of its characteristic curve. As the Voltage is increased the current falls.

Placed in parallel with a tuned circuit it will oscillate or amplify. The standard beta device was not good for VHF. However I had access to samples that failed to meet Voltage & current specifications. Semiconductor manufacture was more hit & miss than today.

A few were found to have excellent high frequency performance, although working at unusually low Voltage & current meant they were fragile.

A Lecher Line was chosen as the RF & local oscillator selective element. Used in half wave mode its input end was simply placed directly across the diode. Results were good, an IF frequency of 125Kc/s was used with an unusual form of pulse counter. This had good linearity, giving about 0.1 percent distortion at the full deviation of 75 Kc/s.

A simple RC coupled chain of ordinary germainium transistors formed the IF amplifier. Designed to progressivly clip at earlier stages as signal increased. AM rejections was better than the usual phase discriminator or ratio detector designs. Sensitivity was high, enabling clear reception of continental FM stations.

It was intended that the unwieldy Lecher line be reduced in size. This is when the first puzzle entered the picture, leading to an apparent paradox.

One idea was to wind the line into a helix, this requires a high level of precision. Any dimensional irregularity may cause reflections, interfering with the line's resonance.

It is well known that loading coils allow an aerial (antenna) to be shorter than a half wavelength and still match a receiver or transmitter. Efficiency is reduced, but is often still acceptable. This method was discounted, introducing an inductive component seemed part way to getting rid of the line.

Capacitive loading of the line's free end was introduced. This enables a shorter line for a given frequency but results did not agree with calculations. As the line was shortened its impedance dropped, to some extent this was expected, but the degree was not.

It was possible to reduce the line to just over half its free length, beyond that amplification & oscillation stopped. It was obvious that the negative resistance of the tunnel diode no longer countered the characteristic impedance. A test was tried with a valve circuit in place of the diode.

A modern (then) frame grid pentode was used. This oscillated strongly, it did not receive FM signals, but television signals, mainly a 50 cycle buzz on a radio. The oscillator frequency was in the 200 Mc/s ITV band.

Removing the loading capacitor made little difference. The standing wave was reflected from the junction, a mismatch. This did not show with the tunnel diode, because it could not operate at the higher frequency.

Another oddity with the diode circuit, frequency change was dependent on the type of capacitor used, as much as its value. Larger or higher Voltage parts allowed a greater frequency reduction. It became obvious that capacitors have a characteristic impedance, apart from their value. This seems to vary with the construction & working Voltage.

Even stranger, placing two capacitors in parallel did not lower the line's frequency any further. The main effect was to stop operation at a lesser shift. It seemed that one capacitor was acting as a poorly matched extension of the transmission line. Two acted as a further mismatch, plus probably splitting the reflective path.

Capacitor theory & specifications shed no light on this. College lecturers were unable to help in any way, suggestions that theory should be re-evaluated met with no useful response.

The design had to be finished & be commercially viable. So a standard L/C tuned circuit was used with the tunnel diode & the prototype completed. It was successful, designed in the company's Test Gear Development department, it never reached production. Company departmental politics was strange in those days & probably still is.

A lot was learnt from it & the prototype worked for many years. I decided that audio was where my interest lay, the paradox with capacitors at high frequencies was not followed up. It was only when writing the page on capacitors that it came back to mind. Then I found Ivor Catt's web site.

According to the prevalent theory, loading with the same value as the line's capacitance should lower frequency, to 0.7071 of the previous figure. The actual change varied, but was always more. 3 times the lines value should halve frequency, in fact it stopped it dead.

Theory assumed that resonance depended on line inductance & capacitance.
The usual formula is " f = 1/(2π X √(LC)), " no information to the contrary was available at the time.
In fact neither of these parameters have any effect on the frequency.

The only thing that matters is the length. It became obvious that a capacitor was merely acting as an extension to the line, a badly matched one. The 'theory' on which I based my original tests was found in a book on the subject & in a magazine article given to me. It was simply wrong, a work of fiction.

The author, in both cases, had obviously made an assumption & had never actually tried what he wrote about. How often does this happen, one wonders, probably most of the time. A Lecher Line can not be tuned in any other way than by changing its length.

Trying capacitors had begun to give an insight into their true nature. This however was not followed up, I worked for a company, in a very junior capacity. There was also the need to pass exams, which relied on learning what I knew was wrong.

The issue came up again at the start of this century. My wife & I decided to celebrate by implementing my 25 year old idea for a musical instrument. This required rapid capacitive discharge of considerable power. It worked first time, but showed up a large hole in theory.

To be continued.

Ivor Catt Ivor Catt.
Electromagnetism
& The Catt Anomaly.

A fresh look at reality.

For infomation on the Catt Anomaly & electromagnetism Ivor Catt's books on the subject are essential reading. A visit to his web site is well worth while. You will find a man, under-estimated by the establishment, who may succeed in bringing 21st century science out of its 20th century stasis.

Sine Wave

Sine Voltage/Current

Sine Voltage/Current Lead

Triangle Voltage/Current

Triangle Voltage/Current
A capacitor has always been something of a conundrum to thinking people. As distinct from those who just accept classical teaching at face value. Andre Ampere's brilliant work formed the basis of much of our knowledge of electricity. Anomalies became clear as soon as changing or alternating currents were considered.

In basic form a capacitor is 2 conductive plates spaced apart.. Effectively a break in a circuit, yet an alternating or changing Voltage causes current to apparently 'flow'. Early on it was realised that no current actually flows across the gap. Yet the connecting leads do carry current.

With DC applied, an electrical charge is said to accumulate on one plate, fed by current in the connecting wire, An equal opposite charge builds up on the other plate. This continues until equilibrium is reached, when the potential difference across the plates equals the source P.D. At this point the capacitor is 'charged'.

Changing or reversing the Voltage causes further current to flow. Until a new equilibrium is acheived. A sinusoidal alternating Voltage will thus cause a matching current to flow.

The sticking point was the magnetic field caused by or causing any current flow. This field 'rotates' around the current. Since no current actually flows across the gap it was thought that the capacitor could not have a magnetic field. In the steady DC case this was obviously so. When AC was considered, Ampere's law no longer produced correct results.

Bead Home Bead

Maxwell's Equations

At the time of the American civil war James Maxwell solved a great many problems, including this one, by inspired theoretical thinking. He produced a set of equations still in use today. These were later rationalised by Oliver Heaviside, making them simpler & more useful.

To correct errors he introduced the concept of 'Displacement Current'. This explained why introducing insulating (dielectric) material between the plates increases capacitance. Adding terms to take account of this current gives correct results with Andre Ampere's & Michael Faraday's equations in the AC case.

Many University & college sites etc. go into the mathematics in depth. There seems little point in duplicating such work here.

Displacement current is is explained as the minute movement of charge in the molecules of insulators, when subjected to an electric field. In a capacitor, negative charge moves towards the positive plate. This leaves the side of molecules facing towards the negative plate with a net positive charge.

Including this displacement current solves problems in calculating the performance of capacitors. The concept is obviously right, the work done by this current even precisely explaining capacitive dielectric loss & heating.

Even as he was working on this theory Mr. Maxwell was the first to realsise it had an obvious large hole. One not yet plugged, even today. A capacitor still works with a vacuum between the plates.

The displacement current is still required to balance the equations, but in what is it now flowing? James Maxwell's solution was 'the aether' sometimes spelt without the 'A'. This indetectable substance was said to pervade all of space & fill any vacuum. Electromagnetic waves, which he prophesied must exist, propogate through it.

The atoms of this ethereal matter supply the displacement current in this case. We all now know that the aether does not exist, yet its postulation still holds the theory together. There is worse to come, for example: with Transverse ElectroMagnetic (TEM) waves. **

Many years ago, when I was a teenage apprentice, I was given the task of designing a transistor VHF receiver. I used a new device, the tunnel diode, as a local oscillator. Transistors of the time were usually not up to the job. Initially an arrangement called a Lecher Line was used as the resonant circuit. Basically an unterminated transmission line, with length chosen to suit the frequency required.

Anomalies with accuracy & operation of this gave cause for some pondering. Because the job had to be done I finished up using a coil & capacitor like everyone else. I did not pursue the reason for deviations from the theoretical model. If I had, avenues to a new understanding may have opened up much sooner.

Bead Home Bead

The nature of capacitors.

** There is a surprising concorde between my thinking on this subject & that of Ivor Catt. My viewpoint originated from a study of VHF sine waves along a Lecher Line, essentially a transmission line. Due to this similarity I have adapted text from his book for the following example. It is not verbatim, but gives the general idea.

Ivor Catt Ivor Catt.
Electromagnetism
& The Catt Anomaly.

A fresh look at reality.

For infomation on the Catt Anomaly & electromagnetism Ivor Catt's books on the subject are essential reading. A visit to his web site is well worth while. You will find a man, under-estimated by the establishment, who may yet succeed in bringing our discipline out of the dark ages.

This subject may not, initially, seem to be associated with capacitors, but patience. A capacitor is, usually, 2 parallel plates or strips.

< Start with 2 parallel wires of length L in a vacuum, a transmission line. One end connected, via a switch, to a source of Voltage, the other to a resistor.

Shut the switch, a TEM wave, with its front perpendicular to the wires, will travel at the speed of light, (c). After a time L/c it will arrive at the resistor & current will start to flow. Where does this current come from? The first thought is usually that the electrons flowing through the wire are the current.

Electrons however, have mass & cannot travel at the speed of light. If they did, their mass would become infinite, requiring infinite energy to move them. A second possibility is that the wave provides the current. There are however several problems with this.

The wave is travelling along the space between the wires at speed c. If displacement current between the wires is a component of this wave it will set up a rotating magnetic field. In the forward direction it will be moving in excess of c, a theoretical impossibility. >

If the resistor matches the characteristic impedance of the line there will be no reflection. It will be absorbed and dissipated as heat. Current through the resistor will then cease, until current from the wires catches up. There is no practical evidence of this two step start of current, in any case the wave is transverse to the resistor and cannot cause current in it.

If the wave causes no current then flow will not start until electrons arrive. In this case why is the wave dissipated? If it causes no current, how does it know the resistor is there? Let alone that it is the right value.

A further problem is that current flows from the souce at a speed below c. Ahead of it no current flows, In the return wire current flows to the source at a similiar speed. Behind it no current flows. Where is it coming from? We have already ruled out displacement current crossing the gap.

Add these anomalies together, with others to be found later. Then realise that two parallel wires are no different, in principle, to two parallel strips, an unwound capacitor.

When I was a teenager these anomalies led me to believe that the Physics & Electrical Engineering courses I was taking were wrong. I knew no-one I could discuss this with, lecturers said, in essence, 'this is what the course says, these people have a century of experience behind them'.

I was effectively told that disagreeing with those who knew more was foolish. Certainly using the theory as it has always stood has rarely caused problems with my work. On the other hand, occasionally doing something that 'cannot work' has made me a minor mark in history.

The fact that Very High Frequencies caused so many unexplained oddities probably helped my decision to stick with frequencies I can hear. This has worked for me, audio & music was always a passion. I have always had this feeling however, that we know so much less than we think.

A huge number of questions remain unanswered. For example:

it is obvious that electrons are busy, active dynamic little things. Whirling in constant motion, with associated waveform & frequencies. Also a clear mutual repulsion, yet frequently working together. Rather like the Chipmunks in their cage in our workshop.

How can they sit still, in a pile at the end of a wire, waiting patiently, for a chance to jump the gap to another empty wire? Energy & matter are interchangable, one can become the other. How can particles of matter come to a halt without ceasing to exist? Unlike our Chipmunks they surely do not hibernate?

A thought experiment to check for particles: Take a 1 Farad capacitor & charge it to 1 Volt. This requires 1 Coulomb, 6.24 X 1018 electrons, a mass of 5.685 nanograms. Dismantle the capacitor & weigh its plates. Assuming they had the same mass uncharged, the negative plate should now weigh 11.37 nanograms more than the positive plate. Has this been tried?

Now I find, via the Web, that I am not the only one. Another fledgling branch of science is around, studiously ignored by the mainstream, but increasingly answering questions that established theory skates round. If I can make formal contact, I will keep everyone informed of progress.

It will be good to help formulate, to a reasonable level of certainty, what I have always instinctively known. It is time for younger people to escape from the trap built by physicists, electricians & experimenters of yesteryear. They did their best, without the facilities available today, but much of their theory is inspired supposition, adapted to fit measured phenomena.

If we are ever to leave the confines of our past & our planet we must learn something of the universe's true nature. Everything is not built out of blocks, like the bricks in a child's play box.

A Form of Words

On various pages of this site, including this one, we refer to current flow, capacitors being charged etc. This is merely a convenient form of words, based on habits spanning 4 centuries. It does not mean we accede to the dualist or particle theories. Nor do we accept quantum mechanics, the uncertainty principle & all their ramifications.

Until we get our house in order, a new form of language, in keeping with reality, will not fully evolve. So, for the present, we are stuck with the arcane way of describing things.

The Origin of Capacitors

In 1746 physicist Pieter van Musschenbroek, of Leiden University in the Nederlands, tried pumping electricity from an electrostatic generator into a bottle of water. There's no accounting for some people. Nothing seemed to happen but, when he picked up the jar to disconnect it, he got an almighty wallop. The Leyden Jar was born, called a condenser, because it held 'condensed' electricity!

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Lecher Line Ron Lebar, Author. Updated: 14-5-2005. Loaded:

We Wish ALL the World Peace, Justice, Equality, Prosperity & an End to Fanaticism.