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I have an unusual request and I am new to this. I was doing an experiment where I was charting electrical values on a Lecher Line (a type of antenna that creates a standing radio wave). I'm using 148.000 MHz. I want to chart the sine wave using the details I obtain from an oscilloscope. Then I want to predict the amount of voltage change that will occur if I were "stretch" or shrink the wave. Any assistance would be greatly appreciated.

Rene Steinhauer
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  • I believe you are looking for the Q factor of the antenna, though how that relates voltage change and frequency change specifically is up to someone more experienced than I am in that field. I should note though that your oscilloscope will also display a drop in voltage with increasing frequency... – Joshua de Haan Dec 24 '19 at 00:15
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    By "stretch" and "shrink" do you mean changing the source frequency (and therefore wavelength), or changing the length of the Lecher Line? Please expound on what you mean by this. – Ben Watson Dec 24 '19 at 00:15
  • It is like a trombone to shorten the wavelength with a shorted termination with a ruler to measure 1/4 wave and 1/2 waves – Tony Stewart EE75 Dec 24 '19 at 01:17
  • Any questions René? – Tony Stewart EE75 Dec 24 '19 at 04:50
  • This experiment is designed to be similar to the Michelson-Morley experiment, except using the radio waves and electrical measurements rather than using the interferometer. As such, when I mention the "stretching" or "shrinking" of the sine wave, I describing the potential for the change of the velocity of the electromagnetic propagation. While we know that it is not possible, in this experiment it is important for me to know how much change would occur (if possible) to know if my instrument is sensitive enough to recognize the change. – Rene Steinhauer Dec 26 '19 at 19:08

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I have simulated this lab experiment for you. I've not done this since 1974 in the Microwave 401 lab. (but I have used a similar method using a return loss bridge to tune a dipole)

You can pull the slider or tick the left/right end for a fixed increment then record the data for Vpp and Wavelength mm and ns and plot it. You can then compute s11.

With the end of the transmission line shorted, you should be able to locate the lengths for the 1/4 wave inverse impedance ( open circuit = 0 current) and half-wave short circuit looking at the input voltage after the 300 Ohms.

You can change the impedance of the delay line as well as the fixed resistor. I just made them both 300 Ohms.

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Tony Stewart EE75
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  • OMG! This is really awesome! I'm not certain I can obtain the prediction for which I am searching, but it is a great start. Thanks for doing this! I will try to PM you for some additional questions if possible. – Rene Steinhauer Dec 26 '19 at 19:15
  • Did you find the 1/4, 1/2 wave points yet? – Tony Stewart EE75 Dec 26 '19 at 19:16
  • I was able to find high and null points on the letcher line. My system is different than the one you designed and I am interested in knowing more. My specialty is medicine, I am just learning some basic electronics as I am exploring this experiment. The basic concept is that if you can measure an electrical output of a letcher line in 2 different vectors there should be no change in output. If there is a change in output, it would suggest a change in velocity of propogation – Rene Steinhauer Dec 26 '19 at 19:21
  • I could not find your email online. mine is ReneSteinhauer AT Renesteinhauer.com – Rene Steinhauer Dec 26 '19 at 19:22
  • tony.sunnysky at gmail & you can change delay line impedance and source Rs – Tony Stewart EE75 Dec 26 '19 at 19:24
  • Velocity is only a function of dielectric constant . Air =1 Standing waves at 1/2 waves and all multiples transfer load to source, while 1/4 , 3/4, 5/4 invert load to source impedance. – Tony Stewart EE75 Dec 26 '19 at 19:27
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    Can you contact me via email? Rene@ReneSteinhauer.com – Rene Steinhauer Dec 26 '19 at 19:28