Electronic – Can we measure reflected power in a transmission line

powertransmission line

This is a question related to radio transmission, I hope this is the proper forum to ask it. It relates to transmission lines connecting a radio transmitter to its antenna.

In his book "Reflections", at the start of Chapter 8, M. Walter Maxwell says "Energy reflected by a mismatched line termination can be entirely separated from the forward-travelling wave, and then be dissipated in a temperature calibrated resistor, and accurately measured as I^2R heat."

He later says "I have performed these measurements many times"

Now I find this book long-winded and too assertive, so I haven't read it all. Is he right? How can we do this measurement? What does it really measure?

I believe that forward and reflected power are mathematical artifacts used to make the calculations possible, and do not exist as separate physical entities. Am I right?

Best Answer

Energy reflected by a mismatched line termination can be entirely separated from the forward-travelling wave, and then be dissipated in a temperature calibrated resistor, and accurately measured as I^2R heat.

This is more or less correct, with a couple of caveats.

First, it is possible to mostly, but not entirely separate the reflected wave. This is done a directional coupler. Practical directional couplers have isolation error, which causes a small portion of the input signal to appear at the measurement port, in addition to the reflected signal that is intended to be measured.

Second, the measurement is not typically done by heating a resistive element. This can be done and it is called a bolometric power sensor. However it's more common in my experience to use an rf detector based on a diode. The nonlinear response of the diode converts some of the rf energy to a dc voltage, which is read with a voltmeter.

Bolometric sensors might be used for very high power conditions, or when calibration to a non-electrical standard is required (e.g. a thermometer).

Edit Replying to your comment, "the generator supplies only the actual power that is transmitted to the load."

This depends a lot on the details of the generator. You refer to a white paper that suggests the following scenario:

Suppose a lossless line is terminated by a pure open circuit, and suppose the the line is exactly one wavelength long at the operating frequency. In this case the current at the generator will be zero, and so the current in its internal impedance will be zero, so there is no power dissipated in it.

This is correct if the generator is actually a perfect voltage source with a 50-ohm series resistance. But an actual benchtop generator might contain other circuits like a levelling circuit or power monitor between the actual generator and the front panel port. Also you rarely know the actual line length to the load --- maybe there is some internal transmission line between the actual source and its front-panel port. If you don't know you have perfectly tuned the transmission line length, then the reflected power is the power you should be prepared to absorb at the generator, even if you don't have to absorb that much in every case.

Also, the case of an open circuit termination and half-wavelength line means that the generator sees an effective open-circuit load (that's why the current is 0). But not every type of generator is designed to work correctly with an open circuit load. A practical circuit could end up demanding more power from other elements within it, or generating more harmonic content when incorrectly terminated. This could still damage the generator even if the ideal components view of the circuit says there's no power transferred in the standing wave.

Finally, if you did insert a directional coupler into this scenario, you would transfer power through the coupled port and into whatever terminates that port (assuming it's not a perfect open or short). This means you would have "separated the forward and reverse waves" as suggested by the author you quoted, even though you did it in a system that was not transferring power before you inserted the directional coupler.

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