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I have see on many websites people explaining how antennas radiates energy. But I am failed to find an explanation on how a receiver (i.e. receiving antenna) generates current/voltage? Can anyone explain?

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  • \$\begingroup\$ Same way a generator does, but on a much smaller scale. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 7 '14 at 18:47
  • \$\begingroup\$ Antenna's obey the reciprocity theorem, if you know the transmit pattern you know the receive. antenna-theory.com/definitions/reciprocity.php (The wiki article is a bit thick.) \$\endgroup\$ – George Herold Oct 7 '14 at 19:42
  • \$\begingroup\$ First of all it is necessary to have a TRANSMITTER that is transmit electromagnetic energy, then receiver antenna can receive this energy then convert to electric power on a load. \$\endgroup\$ – GR Tech Oct 8 '14 at 5:08
  • \$\begingroup\$ ...simply by obeying Maxwell's equations \$\endgroup\$ – Curd Aug 24 '17 at 9:38
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For reference, check out books by Balanis. They are THE reference on antenna design/theory.

This is a very good question and is key to fundamental EE principles.

To help visualize this, let's picture a quarter wave center-fed dipole antenna, which resembles a 'T' shape. When an incident electromagnetic wave of the same frequency at which the antenna is resonant, there is a standing wave generated at either end of the "T". This is generated because the charges generated by the incident wave on the antenna are not continuous along the antenna. So in theory you could take a voltmeter with both probes touching the same end of the antenna and gradually move one probe to the other end, and you will see a difference in voltage.

If you take a look at the book Electromagnetics Explained, it will show this distribution and explain (fairly well) the linkage between Maxwell's Equations and a very basic but accurate idiom on why the charges behave the way they do.

I know this is a somewhat vague answer, but a simple forum post like this doesn't do justice to really explain the interaction of EM waves and their behavior.

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Usually the intuitive, non-math explanation of receiving antennas is avoided in introductory texts. Instead, they steer around the problem by only explaining transmitters and the emission of EM waves. Yet the absorption of EM waves by receiving antennas is quite fascinating. It's also a minor 'hole in physics' which needs some serious filling. (Phd engineering students take note!)

In order to absorb EM waves, antennas must always emit EM waves at the same time. (To receive, we must transmit.) This process involves interference patterns and wave-cancellation.

Whenever some EM plane-waves strike a dipole antenna, they induce a voltage across the two antenna elements, and they produce a current along the antenna. Yet an antenna with a current must be producing a magnetic field. And an antenna with a voltage across it must produce an e-field. These induced fields won't just sit still: our receiving antenna starts transmitting! The dipole antenna emits an EM sphere-wave which spreads in all directions. But this pattern of outgoing waves is superposed onto the incoming plane-waves, producing an interference pattern. In addition, the phase of the emitted wave is opposite that of the incoming plane-waves.

Far downstream from the receiving antenna, the plane-wave is being cancelled by the antenna's sphere-wave. Our receiving antenna casts a shadow, it "punches a hole" in the pattern of incoming plane-waves. If we could see this shadow, it would resemble a bulls-eye pattern: a dark central disk surrounded by circles of nodes and antinodes. It's an interference pattern, but it's different from typical patterns: some energy has gone missing. It does have the expected dark nodes, and high antinodes, but the average total energy is less than the energy in the two original waves. That's how EM absorption works. The receiving antenna has launched a wave which "sucks in" energy from the incoming wave. The reception of EM waves is a wave-cancellation process.

This wave-cancellation explains how light-absorption works; how atoms absorb photons, how objects cast shadows, and how antennas receive radio wattage. But it has odd features. Normally if we add two EM wave-patterns, the resulting intereference pattern has the same energy as the two waves being combined. (The extra energy found in the "peaks" has just been moved out of the "troughs," so total energy is always conserved.) Yet this conservation rule is broken when the sphere-waves emitted by a receiver are added to the plane-waves passing by. Instead, some net energy has gone missing. This energy ends up inside the receiving antenna. It's the net absorbed energy, the "received energy," and in the simplest case would heat up the load-resistor attached to the antenna wires. (Or, if no load resistor was present, it would heat up the antenna itself.)

This entire wave-absorption process has consequences. For example, the receiving antenna has thrown off waves in all direction. It absorbed some energy, but it also reflected energy backwards, and to the sides. These "scattered waves" are required by the receiving process, and are always present in both RF physics and in optics. At best, an antenna can only receive 50% of the waves with which it interacts. It must scatter another 50% away. (Notice that when an atom absorbs photons, it always must emit other photons as part of the process. This is absolutely required by the physics, but rarely mentioned in QM courses.)

Have you ever sat down and tried to grasp the operation of Yagi/Uda antennas? Or the log-periodic, or the Rhombic with it's weird little load-resistor? Or, how about the "Effective Aperture" concept, where receiving antennas can behave much larger than their physical size? Nearfield NSOM microscopes and evanescent waves for wireless power? Or, examine the tiny ferrite coil inside old-school AM radios, how can such a small device absorb significant energy, and why does it always have a variable capacitor attached? Look at crystal radios: they always stop working when the tuning circuit is opened (because the LC resonator was never just a filter that removes unwanted stations!) Terk and Select-a-Tenna sell weird products which violate basic physics?!

All these apparently-odd situations require that we understand the wave-cancellation process described above. They only seem obscure and mysterious. It's all because we usually haven't been taught the basics we need to dissect them. The majority of introductory textbooks do us a great disservice: they insist that we only learn about wave-emission. Then we're supposed to automatically understand everything about wave-absorption ...because reciprocity?

Uh, no.


PS
Be warned, because this entire topic trods some pathways far outside the normal texts. If you get too deeply into it, you may turn into another one of us mad-scientist types. For example, it shows that giant AM radio towers aren't required by Maxwell. It predicts some unexpected long-range optical forces at the nano-scale. It explains the basics of Stimulated Emission without needing QM, suggests that the gamma-ray clicks of Geiger counters aren't actually particles, implies that maybe Niki Tesla was right all along, and also suggests that the Photoelectric Effect at the heart of Quantum Mechanics has a simple non-QM Classical analog. Hmm, physics may not be finished after all, instead we may just be getting started? In other words, Einstein was mistaken about the need for photons, and that famous whiner Willis Lamb's Semiclassical Electrodynamics may actually offer a complete description, also that one guy makes a really good point about particles not existing, and also they must take away Einstein's photoelectric-effect Nobel prize and instead give it to MEEEEEE (pant pant pant.)

:)

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I believe your question (perhaps) can be reworded and answered as follows: -

An infinitely thin (very, very thin wires) dipole antenna (with apparently no physical area to "catch" radiated power coming its way) does actually capture power because, despite it's seemingly "zero" cross sectional area, it has a real aperture (measured in real square metres) and therefore the power passing by converts to a real electrical power in the antenna due to this "aperture" attribute that all antennas have.

Once you accept this (See Friis and this), then it's just a case of understanding how (for instance) a magnetic field can induce a voltage in a conductor.

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  • \$\begingroup\$ In other words, the electromagnetic shadow of an antenna wire is actually enormous. (Back in the early 1900s this was a huge issue, and the early radio experimenters were always using large metal plates and vast wire arrays, in attempts to build "solar panels" for radio wavelengths. Eventually they found that single thin wires of a dipole antenna would serve the same purpose. \$\endgroup\$ – wbeaty Aug 24 '17 at 10:58
  • \$\begingroup\$ Yes, it casts a shadow because it "removes" power. \$\endgroup\$ – Andy aka Aug 24 '17 at 11:37

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