Standing Waves in a Coax Cable

Just like a mechanical wave on a string or sound waves in an open tube, one can produce standing electric waves in a coaxial cable. By using a sinusoidal function generator, we investigated this phenomena to understand the cable's resonance frequencies.

The circuit diagramed above was used to produce standing waves in the coax cable. The resistor placed at the front end of the cable was used for impedance mismatching. The scope is mismatched with the cable, due to its very high impedance; therefore, we wanted to replicate this on the front end by using a 10 ohm resistor.

A good analogy to having a standing wave in a coax cable can be seen in a typical laser. Just as light is constantly reflected off of mirrors in a laser, electrical pulses are constantly reflected off the ends of the coax cable. If the function generator is at a resonance frequency, the amplitude of the wave will become much larger in the cable.

Click for a graph of resonance frequencies.

We also focused on the fundamental resonant frequency by creating a graph of Amplitude vs. Frequency around 135.1 kHz. By fitting this graph with a Gaussian Curve we were then able to calculate the theoretical "Q Factor" of the cable.

Click for a graph of Amplitude vs. Frequency.

Introduction | Cable Info | Lab Procedure and Results | Conclusions