How do RF antennas work?

Question

I understand how an antenna absorbs RF radiation at a certain frequency. What I need to know is, how does the electricity from an antenna get into a circuit? If it is just a piece of wire attached at one end, how does electricity go through it? I thought that a circuit had to be joined at both ends for electrons to go through it. Please help me understand how I could for example, charge a battery from radio waves. Thanks a bunch!

Is the voltage difference between the ground and the antenna?

so is the energy transfered by inductance?

Answer 1

With antennas the earth acts as the second conductor so the energy in the conductor you see (metal of the antenna) and the one that is not as obvious (earth) complete the circuit.

The amount of energy in a transmitted wave is not going to be enough for you to charge a battery with. Even those whose houses are in the near field of a TV transmitting tower do not get much energy to do anything with.

Answer 2

1) The energy available in radio wave used for communication are far to small to charge a battery.
2) The theory one needs to understand an antenna is not part of DC circuit theory. When AC current is traveling down a pair of conductors and the conductors have a length significant compared to a wavelength at that frequency the oscillating electric and magnetic fields become very significant.
3) These electric and magnetic fields surround a wire. One thing one must understand about magnetic fields is that a changing magnetic field generates an electric field and a changing electric field generates a changing magnetic fields.
4) A radio wave is nothing more than oscillating magnetic and electric fields. Since one cannot exist without inducing the other it is possible for oscillating electromagnetic fields to propagate in free space where no wire exits. See http://en.wikipedia.org/wiki/Electromagnetic_wave_equation
5) When a electromagnetic wave hits an antenna with the correct polarization the oscillating fields induce an oscillating current in the antenna.
http://en.wikipedia.org/wiki/Dipole_antenna#Half-wave_dipole
6) The current is zero at the free ends of a dipole antenna but it can be non-zero along the rest of the length and a maximum near the middle. To understand this it would be helpful to understand the concept of a standing wave. Standing waves can exist on a transmission line just a standing transverse wave can exist on a string fixed at both ends.
See
Standing waves on an antenna
http://www.tpub.com/content/neets/14182/css/14182_173.htm
Standing transverse waves on a string (similar concept)
http://www.kettering.edu/~drussell/Demos/string/Fixed.html
http://mail.sci.ccny.cuny.edu/~physdept/AltLabMan/string.pdf
7) once the current is flowing in the antenna it needs to be amplified in the receiver. There are other elements of a receiver but I think thats another question.

Answer 3

You are right about antenna absorbing at frequencies.This is call resonance.All antennas have a frequency at which they are resonant.Think of a musical instument the xylophone for a minute.The shorter the keys the higher in frequency and the longer keys the lower in frequency.The same goes for antennas.You should always make an antenna as close the lenght of resonate frequency to get maximum transfer of power to and from the antenna.
A few formulas to remember for a half wave dipole antenna.
A simple antenna design

984
lenght in feet =__________ or
Freq in mhz.

300
lenght in meters =___________
Freq in mhz.

Now for the rest of what you asked

When radio frequency current passes through a conductor of an antenna it creates both electrical and magnetic fields around the conductor.In much the same way that voltage and current through a wire.These fields are at 90 degrees out of phase with each other.The electrical field determines the polarity of the antenna.So a vertical antenna has vertical electrical field around the antenna.The magnetic field is horizonal and vice versa.The electrical field ionizes the air's atoms and eventually a receiving antenna will receive it and convert it back to something we can hear or see.Radio and tv
That is a bit of simple answer to a bit of a complex question.I hope that answers it for you.

If not email me at [e-mail]

Answer 4

There is an old story I heard once about a farmer who had a long fence running under a power line. Enough power coupled into his fence that he got a shock from it. Next he hooked up a light bulb and it worked. The power company got upset but it was a legal gray area.

Not sure if this is true or not.

Answer 5

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Answer 6

as a practical matter, the field strength of radio waves is measured in micro volts. the only way to get significant power is to locate yourself right under a broadcast tower but be careful as the radiation at this point could be strong enough to cause physical harm.