At university we did an experiment by making a directional coupler using a length of ribbon cable. (The cable acted as both the transmission line and the coupler). I think it was about 1m long, and the signal was a 10MHz sine wave. We used it to look at the source and reflected waves, and so find the reflection coefficient etc.

Anyway, for the life of me I can't figure out how the directional coupler actually works! I assume it has to do with the crosstalk from the main wire to the sampling wire, via mutual capacitance/inductance (right?). But how do you end up with only a sample of the source wave at one end, and the reflected wave at the other?

 

Simplified:

If you have transmission line with a sampling line in parallel with it (like a ribbon cable) with the sampling wire terminated at the input of the cable to ground with a resistor (to absorb reflected energy in the sample line) and at the output of the cable the sample wire has a voltage detector to measure the energy coupled to it, the phase of the forward incident wavefront coupled energy from the transmission wire at the detector will be in phase and adding because it travels the same distance to the detector as it moves from the input to the output. As a reflected incident wavefront moves from output to input it will be only in phase at the input side sampling wire resistor but out of phase at the output detector causing the voltages to cancel for reflected energy detected.

http://www.microwaves101.com/encyclopedia/vswr_visual.cfm

 

Hi, thanks for replying. I'm still trying to digest what you're saying.

The coupled line we used was terminated correctly at both ends, and we monitored each end simultaneously on an oscilloscope. I will have a think.

 

Hi, thanks for replying. I'm still trying to digest what you're saying.

The coupled line we used was terminated correctly at both ends, and we monitored each end simultaneously on an oscilloscope. I will have a think.

The detectors in my example could be properly terminated and still work correctly. but you can really only get accurate SWR readings using this method with two coupled lines and detectors at opposite ends.

 

It is a year after you posted this message, but if you are still interested in an answer, send me an email message. I just joined the group and am not sure how to actually send a message but I did check the box to allow members to do it Actually, I found this group while researching this subject.

For two years I was haunted by that question until I figured it out in terms of electron flow. Diodes, resistors and meters don't care about waves, they respond to electron flow. So if you still want to know, let me know. The explanation is too long to list here.

Note to Moderators: I would be happy to send you a copy of my unpublished paper describing the answer to this question if you are concerned about the veracity of this reply.