How does antenna transmits electromagnetic waves?

In summary, the antenna is a conductor that allows charges to move freely when a voltage is applied, creating an electromagnetic wave. The motion of charges in the transmitting antenna affects the charges in the receiving antenna, causing a small movement of charge that is amplified by receivers and RF amplifiers. When encountering an interface, electromagnetic waves are partially reflected and partially transmitted due to a change in characteristic impedance of the medium. The material of the interface must be different in order for this reflection and transmission to occur.
  • #1
Physicsissuef
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0
How does antenna transmits electromagnetic waves? For example radio antenna. How does the antenna produces moving electromagnetic waves?
 
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  • #2
The antenna is a conductor, so charges are free to move inside the antenna. You apply a voltage, charges move, you apply an opposite voltage, charges move the other way.
 
  • #3
DaleSpam said:
The antenna is a conductor, so charges are free to move inside the antenna. You apply a voltage, charges move, you apply an opposite voltage, charges move the other way.

But what makes them travel?
 
  • #4
Physicsissuef said:
But what makes them travel?

The simple fact that charges in (accelerated) motion *do* radiate electromagnetic waves. Oscilating motion between two ends of a copper wire is such motion.

This is a feature not easily quantified and many textbooks have whole chapters dedicated to describing this effect alone. However I was able to find this wiki-article on a http://en.wikipedia.org/wiki/Dipole_radiation#Dipole_radiation" what mostly cover the important conclusions and features a nice animation of the electric field
 
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  • #5
Troels said:
The simple fact that charges in (accelerated) motion *do* radiate electromagnetic waves. Oscilating motion between two ends of a copper wire is such motion.

This is a feature not easily quantified and many textbooks have whole chapters dedicated to describing this effect alone. However I was able to find this wiki-article on a http://en.wikipedia.org/wiki/Dipole_radiation#Dipole_radiation" what mostly cover the important conclusions and features a nice animation of the electric field
If I move faster the dipole, will I create bigger frequency? (which is logical, I think so)
 
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  • #6
the antenna is connected to a transmitter which is designed to output a voltage as a function of time. this voltage is an e.m.f. that forces free charge in the conductive element of the transmitting antenna to slosh back and forth along the element. in the receiving antenna, there is free charge in the conductive element that are affected (because like-signed charges repel and unlike-signed charges attrack) by the movement of charge that is happening in the transmitting antenna. because of the usual large distance between the transmitting antenna and the receiving antenna, that movement of charge in the receiving antenna is minute, much smaller than the quantity of charge and movement in the transmitting antenna. but that is what receivers and RF amplifiers are for; making that small movement of charge control a much larger movement of charge (that eventually finds its way to your radio loudspeakers).

so you have the motion of charge at one location affecting the movement of charge at another location. since the reaction of charge in the receiving antenna is not an instantaneous reaction (from the POV of an observer that is equal distant between the two antennae), what is it that is in between the two antennae that, after a finite period of time, forces the charge in the receiving antenna to move?
 
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  • #7
A pretty simplified way to look at it is this:
When you close a circuit you have an electrical field E,E create a current in the antena.
Now as you know a wire that have a current flowing in it creates a magnetic field B.

P = E X B , and that's the radiation you recieve.
 
  • #8
It is similar somewhat to the fact that you tap you hand on the water surface periodically and you make waves.
 
  • #9
Ok, I have another question connected with antennas. I found a text from wikipedia, and also found this quote:
wikipedia said:
When encountering an interface, the waves are partially reflected and partially transmitted through.
Why the electromagnetic waves are partially reflected? When they encounter an interface, I think that there is process called absorption, so the electrons release the excess of energy in form of radiant energy, shouldn't that all energy be reflected?
 
  • #10
Physicsissuef said:
Why the electromagnetic waves are partially reflected? When they encounter an interface, I think that there is process called absorption, so the electrons release the excess of energy in form of radiant energy, shouldn't that all energy be reflected?


if you have a wave traveling along a "string" and somewhere in the middle of that taut string, it changes to 1/4 inch nylon rope (the two are spliced together, then is pulled tight). when the wave is incident upon the splice, some of it will transmit through and some will be reflected.

now replace the 1/4 inch nylon rope with a massive brick wall. here, very little is transmitted and all of it is reflected.

it's because of a change of characteristic impedance of medium.
 
  • #11
rbj said:
if you have a wave traveling along a "string" and somewhere in the middle of that taut string, it changes to 1/4 inch nylon rope (the two are spliced together, then is pulled tight). when the wave is incident upon the splice, some of it will transmit through and some will be reflected.

now replace the 1/4 inch nylon rope with a massive brick wall. here, very little is transmitted and all of it is reflected.

it's because of a change of characteristic impedance of medium.
In the sentence it says like, touching no matter what interface, it will partially reflect and partially transmit the waves. So it matter, what is the material of the interface, right?
 
  • #12
Physicsissuef said:
In the sentence it says like, touching no matter what interface, it will partially reflect and partially transmit the waves. So it matter, what is the material of the interface, right?


the stuff on the other side of the interface has to be different. a different density and/or a different stiffness or compressibility. if you have an interface of some kinda Jello on one side and it's the same Jello on the other side, all of the incident wave will be transmitted.
 
  • #13
rbj said:
the stuff on the other side of the interface has to be different. a different density and/or a different stiffness or compressibility. if you have an interface of some kinda Jello on one side and it's the same Jello on the other side, all of the incident wave will be transmitted.
I understand, thank you very much.
 
  • #14
Can you tell me how dipole antenna works?
Here is http://www.colorado.edu/physics/2000/waves_particles/images/fig14.jpg" [Broken]
What the dipole antenna is it for?
 
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  • #15
I'd like to ask a related question. I understand that the transmitter outputs a voltage as a function of time (which creates an electric field) thus causing a flow of charge or current (creating the magnetic field) and these two fields travel down the antenna, but why don't they stop once they reached the end of the antenna. How do they get blasted into space?

Any insight would be greatly appreciated!
 
  • #16
unnamedplayer said:
I'd like to ask a related question. I understand that the transmitter outputs a voltage as a function of time (which creates an electric field) thus causing a flow of charge or current (creating the magnetic field) and these two fields travel down the antenna, but why don't they stop once they reached the end of the antenna. How do they get blasted into space?

Any insight would be greatly appreciated!

"Blasted"? Maybe if you think of it in classical terms (waves) rather than quantistically (photons) it will not require you to see anything material actually exiting the antenna, but just the energy...

Think of a large canvas, such as a bed blanket, covering your bed with edges unbound. With a hand, you pick up the middle point and you start pulling it up/down. You'll see waves flowing outwards the blanket, but there is nothing material really blasted out, you don't shoot around pieces of fabric ;)

The effect is similar with the antenna. By vibrating the dipole, you change the EM field around the antenna, but the variation is not instantaneous everywhere, instead it propagates with a finite speed (c, the speed of light). It is the constant attempt at changing the whole EM field coupled with the finite speed that will make the field "look" like it's moving outwards.

It's more than a look of course, since you can say that there is an energy associated to that "spinning" of the EM field.
 
  • #17
Physicsissuef said:
How does antenna transmits electromagnetic waves? For example radio antenna. How does the antenna produces moving electromagnetic waves?

Actually antenna propagation is based on the fact of insetia of electric and magenetic field. As we know that the moving charges produce magetic field and static charge produce elctric field. So when an antenna is excisted by time varying voltage the elctrons moves forth and back. And the field is generated. For +ve cycle of voltage the electric field direction is opposite as compared to the -ve cycle. When the rate of change of this is very high the filed will take some time to get changed. And the inertia of this will come in picture. The field which is generated by the +ve cycle will reflect the -ve cycle generated field. And there is not transmission of charges in the free space only the filelds is getting transmitted in the antenna process. Thats is why the low frequency signal cannot be transmitted.
Let me know if anybody can explain any other idea.
Thanks and Regards
Komal Kumar Dhote
 
  • #18
unnamedplayer said:
I'd like to ask a related question. I understand that the transmitter outputs a voltage as a function of time (which creates an electric field) thus causing a flow of charge or current (creating the magnetic field) and these two fields travel down the antenna, but why don't they stop once they reached the end of the antenna. How do they get blasted into space?

Any insight would be greatly appreciated!

The transmitter voltage does not create the electric field. The transmitter outputs current and voltage simultaneously, as well as an E and H field simulataneously. E, H, I, & V are all in unison for a resistive t-line (Z0 = real). There is a finite impedance value for the transmission line, Z0. The current and voltage are simultaneously present and Ohm's law is always upheld. For a 300 ohm Z0, the transmitter connected at the input outputs a V and an I, say 600 mV and 2.0 mA. The I and V waveforms travel along the t-line and when the end is reached the E and H fields radiate power into space.

The reason is that an antenna only works for high enough frequencies. The current here is displacement current. No closed path is reuired for such. The t-line ends abruptly in mid-air, yet current exists. The E and H fields are present in between the conductors of the t-line. Then at the end where it ends, the fields continue to propogate into space.

Have I helped or made matters worse?

Claude
 

1. How does an antenna actually transmit electromagnetic waves?

An antenna is made up of conductive materials, such as metal, that are designed to resonate at specific frequencies. When an alternating current is applied to the antenna, it causes the electrons in the conductive material to move back and forth, creating an electromagnetic field around the antenna. This electromagnetic field then radiates outwards as electromagnetic waves.

2. What is the role of frequency in antenna transmission?

The frequency at which an antenna resonates is directly related to the wavelength of the electromagnetic waves it transmits. The length and shape of the antenna determine its resonant frequency, which in turn determines the frequency of the electromagnetic waves it emits. Higher frequencies result in shorter wavelengths and vice versa.

3. How does the size of an antenna affect its ability to transmit electromagnetic waves?

Generally, the size of an antenna is directly proportional to the wavelength of the electromagnetic waves it can transmit. This means that larger antennas are capable of transmitting longer wavelengths and therefore lower frequencies, while smaller antennas are better suited for higher frequencies.

4. Can an antenna transmit multiple frequencies at the same time?

Yes, an antenna can transmit multiple frequencies simultaneously. This is achieved by using a technique called frequency multiplexing, where different frequencies are combined and transmitted together. This allows for more efficient use of the electromagnetic spectrum.

5. Are there any factors that can affect the efficiency of antenna transmission?

Yes, there are several factors that can affect the efficiency of antenna transmission. These include the design and size of the antenna, the material it is made of, the surrounding environment (such as buildings or other obstructions), and interference from other electronic devices. Proper placement and tuning of the antenna can also greatly impact its efficiency.

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