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Q & A: energy and out-of-phase antennas

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Most recent answer: 10/02/2017
Q:
Suppose you have two radiofrequency transmitters, whose antennas are located in the same point in space (for example, the antenna is shared between them).If they transmit with the same power and frequency, but with a 180 degrees phase shift, the resulting destructive interference will result in a wave with zero amplitude (and energy) everywhere.Where does the energy supplied to the two transmitters ends up?
- Marco (age 38)
Italy
A:

Great question!

In general, the answer is back into whatever was driving the antennas. Let's think of a simple case, in which each antenna is driven by a resonant LC circuit with some stored energy. Each antenna on its own would gradually drain the energy out of the LC, so that the radiated power would decrease, approximately exponentially.Bringing the two antennas together out of phase would suppress that process, so the energy would stay in the circuit. You could think of it as the energy emitted by each antenna being absorbed by the other one.

Of course in real circuits the energy will run down for other reasons, mainly resistance in the circuit, but this story conveys the key idea.

Mike W.


(published on 10/02/2017)

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