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How do you test the oscillator circuit you just made that runs between MHz and MHz if all you have is a MHz oscilloscope, a few multimeters and a DC power supply? One answer is to put away the oscilloscope and use the rest along with a length of wire instead. Form the wire into a Lecher line. A Lecher line consists of two parallel wires or rods that form a balanced transmission line.
Its namesake is Ernst Lecher, a physicist from Austria who perfected the practice in 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. 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. The next one further away is a full wavelength. The oscillator is a voltage controlled oscillator. The resulting data and graph are shown below. Looking for more high frequency measuring projects here on Hackaday? This reminds me of the fun part of physics classes.
My favorite, though, was wiring our ADC circuits directly into the address bus through opto-isolators, of course of a Commodore 64 so that we could get data points every 10 ms when we wrote our apps in assembler.
Good times. But if I HAD to…. Another way would have been to use e. Why does this so drastically differ from the values in the datasheet? Given that the wires are surrounded by air, and that the relative permittivity of air is 1. There is wooden base under wires. There seems to be something to that. As done in the post: 8. How about using a metal measuring tape for one of the transmission elements?
You might have to remove paint from part of it the backside? So the distance my screwdriver was off by was 0. Looking at the chart, the error due to that is much smaller. See the red line in this photo. Does that make sense? Oh, wait. So I am still off by a lot. The loop is only around 3. Something else is still up…. Propagation velocity depends on the dielectric. Thank you for this. That setup really helps de-mystify its mechanical equivalent at sonic frequencies, the transmission line speaker, and the currently popular mathematical model for it as described by Martin J.
Really cool. I wonder if this same effect would work even if the two Lecher wires were not connected at all- I think it might because presumably inductance is conveyed by light particles.
The wikipedia article has a good description of this phenomenon. Apparently the screwdriver causes a reflection creating a standing wave of voltage and current on the line. It would be great to see a visual explanation showing using a light particle model- showing how the light particles are absorbed and create the electric potential between both sides of the diode when the screwdriver is at a node. See Velocity factor. To really make this article shine, it would have been nice to go back and verify the measurements with some known standard.
You guys should do a retrotachular on ECGs. Detecting really tiny current with a long filament, huge magnets, and some optics. How does it work — does anybody know? How does it differ from the POS? As a typical example, something like a Colpitts oscillator may be used, tuned with a varicap diode that changes its depletion capacitance strongly as the bias voltage across the diode changes — which is what allows control of the oscillator frequency from an applied bias voltage.
Also available on Arthur H. Less than half baked idea, but could a similar setup be used to detect the swinging length of a guitar string?
Or if you rather use technology from 2 centuries ago, use that I suppose. So that is what a Lecher Line is! Here I thought it was men in overcoats and galoshes at the ticket counter of an X-rated movie theatre. My Tektronix MHz scope can display waveforms up to about MHz at reduced accuracy but sufficient to see that the oscillator works.
More fun discovering Lecher Lines though. Thanks for suggesting that. I made a loop like the diode loop but without the diode and hooked it up to my Tektronix TDS MHz oscilloscope probe and probe ground and tried it. There was a 90 MHz signal of unknown origin that dominated and the induced signal from the oscillator was lower amplitude so it rode on the 90 MHz signal. I measured the frequency by spanning three of them and then multiplying by three.
This site uses Akismet to reduce spam. Learn how your comment data is processed. By using our website and services, you expressly agree to the placement of our performance, functionality and advertising cookies. Learn more. What follows is a bit about Lecher lines, how I did it, and the results.
Plain Lecher line and with test equipment in use The end with a loop A Lecher line consists of two parallel wires or rods that form a balanced transmission line. The end with the oscillator but pulled back from the Lecher line. Another angle showing the diode. NTE diode loop. Lecher line, oscillator and other equipment. Lecher line data. Graph of the data. Report comment. I wonder how easy it would be to use a fluorescent tube and measure of light intensity.
The one objection is: Why does this so drastically differ from the values in the datasheet? Because the speed of light in a transmission line is not the same as in vacuum. Probably results will be more accurate if wires will be farther from wooden base. Could be wrong, but I believe the wood right next to it will have a significant effect.
You only do that if you want correct results. Very cool stuff! Thanks for sharing! Check this out it probably has a few other neat methods in it too : Radar circuit analysis by United States. Leave a Reply Cancel reply.
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Using A Lecher Line To Measure High Frequency
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.
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. 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. Austrian physicist Ernst Lecher , improving on techniques used by Oliver Lodge  and Heinrich Hertz ,  developed this method of measuring wavelength around They were also used as components , often called " resonant stubs ", in UHF and microwave radio equipment such as transmitters , radar sets, and television sets , serving as tank circuits , filters , and impedance-matching devices.