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The impedance of an antenna can be lumped-represented as Rr (radiation resistance), Ro (ohmic resistance), XL (inductive reactance), and XC (capacitive reactance). The incident power from a generator via transmission line is lost as heat through Ro and is emitted into space through Rr. What happens to the power incident on the reactive portion? Is my understanding correct, that power incident upon a reactive impedance is not emitted, but reflected? If so, how do you know this? My reading of Maxwell's "Reflections III" does not seem to make this clear. TIA!

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  • \$\begingroup\$ VSWR is the key of 'emitting' wave ... \$\endgroup\$
    – Antonio51
    Aug 14, 2023 at 13:37
  • \$\begingroup\$ If a reactance were to constantly accept power -even a little- cycle after cycle, then it would accumulate (since a reactance cannot dissipate). That's impossible, so power must go somewhere else, since AC power is always in motion until it dissipates away. \$\endgroup\$
    – glen_geek
    Aug 14, 2023 at 14:33

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Where does the RF power go that is sent to a reactive portion of an antenna?

What happens to the power incident on the reactive portion?

The power does get reflected back along the transmission line, but still has to end up somewhere. There are only 2 places:

  1. It gets absorbed in the source resistance, thereby heating the transmitter.
  2. A standing wave along the transmission line, turns the transmission line into an antenna, thus radiating into the surroundings.

How do I know?

I have measured with a field strength meter along the transmission line.

I have felt the transmitter get hotter than usual.

I have measured the transmitter supply current, watching it vary with antenna match.

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What happens to the power incident on the reactive portion? Is my understanding correct, that power incident upon a reactive impedance is not emitted, but reflected?

Any power travelling along a transmission line that hits a reactive or partially reactive load produces a reflection unless, the characteristic impedance of the transmission line matches the reactive load.

If so, how do you know this?

By studying transmission line theory.

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    \$\begingroup\$ Yes, that is what Maxwell (Reflections III) is all about. Thanks! \$\endgroup\$
    – Noji
    Feb 27 at 20:55
  • \$\begingroup\$ @Noji If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. In other words, it's helpful to others if you select an answer and accept it (it doesn't have to be mine). \$\endgroup\$
    – Andy aka
    Feb 27 at 21:00
  • \$\begingroup\$ Done...thanks for the reminder! \$\endgroup\$
    – Noji
    Feb 29 at 13:21

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