|Insertion loss of three pieces of wire, indicating their resonant frequencies, measured with a spectrum analyzer and a directional coupler.|
This sounds cool, so I bought one at ebay (24 Euros). So, let's see if it works and what can be done with it!
Easiest case first: The signal goes into the IN port, and we look at the signal coming out of the OUT port.
|A signal going from IN to OUT can pass through the coupler (almost) without anything happening to it.|
The y axis (vertical) denotes how much of the power from the TG was left after the signal passed whatever is between the output and the input port of the analyzer (in this case, because we're using the TG, but the signal could also have a different source). If you just connect output and input port with a cable, you get a straight line at "0", meaning "no power was lost". The scale is logarithmic, so "-10dB" means "90% of the power was lost", and "-20dB" means "99% of the power was lost". Positive values usually don't occur for passive devices.
So, to summarize this again: Each x-y-point on the curve tells you how much power was lost on the signal's way though the network (y), at one very specific signal frequency (x).
Now, back to topic! In the above diagram, it's easy to see what happens: close to nothing. The signal just passes from IN to OUT through the coupler, as it should be.
You get about the same result if you make the signal go from OUT to IN, 1% going to the COUPLED port isn't really that much.
Next, let's send the signal into the IN port and measure what we get at COUPLED:
|Signal input at the IN port, OUT open, measuring the power at COUPLED.|
If we send the signal into the OUT port and measure at COUPLED, one could intuitively expect that nothing reaches the COUPLED port, right? But this is not the case:
|Signal input at the OUT port, IN open, measuring the power at COUPLED.|
We can now connect a 50 Ohm terminator to the IN port. Then we get what you maybe had expected previously:
|Signal input at the OUT port, IN terminated (badly) by a 50 Ohm load, measuring the power at COUPLED.|
This measurement technique now makes it possible to check how well a device is matched to 50 Ohms, something which is very important when building RF stuff. For example, you can check the matching of a dipole antenna ("piece of wire"):
|Signal input at the OUT port, IN attached to a piece of wire, measuring the power at COUPLED.|