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How does the directional coupler in an swr meter work? How are the forward and reflected waves separated out from the standing wave on the feeder?

I would like an answer in terms of sine (or cosine) waves, phasors and complex numbers, as well as a non-mathematical description. please.

clabacchio
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Harry Weston
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  • Thank you "The Photon" and "Dave Tweed". You gave partial answers to my question, enough to send me on a more informed Internet search, where I found three web sites that have brought me close to a full understanding of what goes on. when I have digested them I will write it all up and post it here. The most useful is: http://kambing.ui.ac.id/onnopurbo/orari-diklat/teknik/arrl/using-equipment/5904024.pdf http://web.cecs.pdx.edu/~greenwd/xmsnLine_notes.pdf – Harry Weston Dec 30 '12 at 16:32
  • Sorry I got in a bit of a pickle trying to give my references in a comment -- ran out of time. – Harry Weston Dec 30 '12 at 16:43

3 Answers3

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An SWR meter isn't really measuring "forward" and "reflected" power directly, but rather, it's measuring the phase relationship between voltage and current on the line. In the "forward" position, it shows the leading-phase component of the current, and in the "reverse" position, it shows the lagging-phase component. It is the relative value of these two measurements that represents the SWR.

Another way of looking at it is to think of a quadrature encoder used to detect physical motion. In a transmission line with a matched load, the current in the direction from source to load peaks 90° before the voltage does. If power is flowing in the other direction, this relationship is reversed, allowing the meter to distinguish the two, just like the quadrature encoder can detect the direction of movement.

Dave Tweed
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I don't know if it's common, but in contrast to Dave Tweed's answer, at least according to Wikipedia some SWR meters are built using directional couplers, just as you say.

Before we start, I'm not sure if you have a slight misconception about what's going on:

How are the forward and reflected waves separated out from the standing wave on the feeder?

The forward and reflected waves aren't distinct from the standing wave. The "standing wave" is just what we observe when we have a forward-travelling wave and a reverse-travelling wave superimposed on each other.

Now, for the benefit of future readers, what is the standing wave ratio or SWR? It's basically the ratio of the peak amplitude to the trough amplitude as you move a probe along a transmission line. It's given by

\$ VSWR = V_{max} / V_{min} \$

The max and min voltage amplitudes occur when the forward and reverse waves interfere with each other either constructively or destructively.

\$ V_{max} = V_f + V_r \$ and \$ V_{min} = V_f - V_r \$.

So

\$ VSWR = (V_f + V_r) / (V_f - V_r) \$,

or, to get closer to our usual formula for calculating the VSWR:

\$ VSWR = \frac{1 + V_r / V_f}{1 - V_r / V_f} \$.

So how can we extract the separate waves to compare them? The directional coupler is actually conceptually very simple. If we put two waveguides or microstrip lines (or PCB traces) carrying rf signals close enough to each other, some of the evanascent wave in the transverse direction (orthogonal to the line) will overlap with the propagating mode of the other line, and so some power will be transferred from the main signal to a parallel-travelling signal on the "coupled line".

enter image description here (CC image by SpinningSpark at Wikipedia)

Unfortunately expressing this mathematically actually requires solving the 2-d transverse electromagnetic field problem for the transmission line structure, which can't even be done analytically for many important structures (like microstrip line).

Most of the power will simply continue to propagate in the same general direction it had been, so that naturally the forward-travelling wave is coupled to just one of the output ports, and the reverse-travelling wave is coupled to the other. Of course, there are also some reflections that happen at the discontinuities in this structure, so that the directional coupler can't have perfect directivity.

Also, if you look up directional couplers on Wikipedia, you'll see there are numerous other types beyond the simplest one I've shown here, and various tricks to enable the coupled-line coupler to operate over more than a small frequency band.

The Photon
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  • But most SWR meters are *much* shorter than 1/4 wavelength, and some just use a simple current transformer instead of parallel lines. – Dave Tweed Dec 28 '12 at 22:56
  • @DaveTweed I think you are both correct, you are correct for lower frequencies, thephoton is correct for higher frequencies(which makes sense given his name). We used direction couplers in the GHz range and a transformer on the MHz range stuff. – Kortuk Dec 29 '12 at 14:01
  • @DaveTweed, I don't disagree. But your answer implied that directional couplers are never used to build SWR meters. Which may be true for HAM frequencies, but as far as I can tell isn't universally true. – The Photon Dec 30 '12 at 18:27
  • @Kortuk, to be honest, I've never used an SWR meter in my life --- I'm writing based on Wikipedia saying directional couplers are used in at least some SWR meters, and my (mostly academic) knowledge of directional couplers. – The Photon Dec 30 '12 at 18:28
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I got in a mess trying to do this in comments, so here is what I intended to say.

Thank you ThePhoton and DaveTweed. You gave partial answers to my question, enough to send me on a more informed Internet search, where I found three web sites, listed below, that have brought me close to a full understanding of what goes on. When I have digested them I will write it all up and post it here.

An inside view of directional wattmeters by Warren B. Bruene http://kambing.ui.ac.id/onnopurbo/orari-diklat/teknik/arrl/using-equipment/5904024.pdf

A Modern Directional Power/SWR Meter by Bill Kaune W7IEQ http://www.arrl.org/files/file/Product%20Notes/2012%20Handbook/KAUNE.pdf

Zo: Transmission Lines, Reflections, and Termination http://web.cecs.pdx.edu/~greenwd/xmsnLine_notes.pdf

Harry Weston
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