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Is this how a dipole antenna works?

by CraigH
Tags: antenna, dipole
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CraigH
#1
Mar21-13, 04:43 PM
P: 202
I'm currently writing a report on antennas and I am really struggling to understand the theory.
Here's what I've got so far;


Basics:

.A moving charge causes a magnetic field
.If the direction of movement is reversed, so is the direction of the magnetic field
.A non moving charge has no magnetic field. (if it is not moving relative to the observer)
.Therefore a charge accelerating and decelerating causes a changing magnetic field
.A charge oscillating sinusoidally causes a sinusoidal magnetic field

.The charge also has a electric field surrounding it
.The electric field does not change everywhere at the same time as the field cannot travel faster than the speed of light
.The electric field is mirrored in the ground plate of the antenna creating a dipole
.This dipole has an opposite electric field which is in phase with the original electric field
.The resultant electric field surrounding the charge is an oscillating electric field

.The electric field is perpendicular and in phase with the magnetic field.
.This is an EM wave

Resonance:

.The current travelling through the antenna can also be described as a wave
.This wave is reflected from the end of the antenna
.Wave theory tells us that the reflected wave will be inverted
.This reflected wave will superpose itself with oncoming waves
.At certain frequencies the result of these reflections will cause a standing wave
.There will only be a standing wave at these frequencies
.The value of these "resonant frequencies" is dependent on the dimensions of the antenna, just like the resonant frequency of a string depends on the length of the string
.The standing wave has a much larger amplitude than a travelling wave, so at resonance more power can be radiated


Is this correct? If not can someone please tell me where I am going wrong.
I've been reading around this subject for about a month now, and this is the best I can come up with so if someone could finally put this subject to rest for me that would be amazing.
Thanks.
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berkeman
#2
Mar22-13, 12:03 AM
Mentor
berkeman's Avatar
P: 40,961
Quote Quote by CraigH View Post
I'm currently writing a report on antennas and I am really struggling to understand the theory.
Here's what I've got so far;


Basics:

.A moving charge causes a magnetic field
.If the direction of movement is reversed, so is the direction of the magnetic field
.A non moving charge has no magnetic field. (if it is not moving relative to the observer)
.Therefore a charge accelerating and decelerating causes a changing magnetic field
.A charge oscillating sinusoidally causes a sinusoidal magnetic field

.The charge also has a electric field surrounding it
.The electric field does not change everywhere at the same time as the field cannot travel faster than the speed of light
.The electric field is mirrored in the ground plate of the antenna creating a dipole
.This dipole has an opposite electric field which is in phase with the original electric field
.The resultant electric field surrounding the charge is an oscillating electric field

.The electric field is perpendicular and in phase with the magnetic field.
.This is an EM wave

Resonance:

.The current travelling through the antenna can also be described as a wave
.This wave is reflected from the end of the antenna
.Wave theory tells us that the reflected wave will be inverted
.This reflected wave will superpose itself with oncoming waves
.At certain frequencies the result of these reflections will cause a standing wave
.There will only be a standing wave at these frequencies
.The value of these "resonant frequencies" is dependent on the dimensions of the antenna, just like the resonant frequency of a string depends on the length of the string
.The standing wave has a much larger amplitude than a travelling wave, so at resonance more power can be radiated


Is this correct? If not can someone please tell me where I am going wrong.
I've been reading around this subject for about a month now, and this is the best I can come up with so if someone could finally put this subject to rest for me that would be amazing.
Thanks.
It is mostly correct, but with a few typos/errors. Do you have access to Stutzman & Thiele?

http://www.amazon.com/Antenna-Theory...ry+by+stremler

With respect to the resonance and radiation, you really need to include Maxwell's equations.
rbj
#3
Mar22-13, 01:53 AM
P: 2,251
imagine you're holding a sizable negative charge and i am holding a sizable positive charge. and imagine we are standing a few meters away from each other and facing each other and restricting the movement of our charges to move only up/down and left/right, perpendicular to the line that connects us.

now the charges attract each other so if i move my charge up, your charge follows it up. then i move it down, and your charge follows it down. i move it to my left (your right) and your charge follows it right, and i move it to my right (your left) and your charge follows it left.

if i move my charge up and down several times, your charge also moves up and down the same number of times. that's an electromagnetic wave. if i move my charge up and down a million times a second, you can tune that in on the AM radio. if it's 100 million times per second, you can tune it in on an FM radio. if it's 500 trillion times per second, you will see it as a blur of orange light.

my charge that i am holding and pushing around is, essentially, a transmitting antenna and your charge that you are allowing to be pushed around by the prevailing electromagnetic fields is, essentially, a receiving antenna.

in a simple 1/2 wavelength straight-line dipole, in the transmitting antenna, charge is being forced (by the transmitter electronics) to slosh around back and forth in the metal dipole element. that emits an EM wave. in the receiving antenna, that wave causes the free charge in the conductive dipole element to slosh back and forth, which is detected by the receiver electronics.

that's about as basic as i can think of regarding how a simple antenna works.

nsaspook
#4
Mar22-13, 12:14 PM
P: 639
Is this how a dipole antenna works?

Quote Quote by CraigH View Post
.The electric field is perpendicular and in phase with the magnetic field.
.This is an EM wave

Is this correct? If not can someone please tell me where I am going wrong.
I've been reading around this subject for about a month now, and this is the best I can come up with so if someone could finally put this subject to rest for me that would be amazing.
Thanks.
That's not bad for a whole month on a very complex subject. A simple wire with a few bends can't be that hard.

I think a point that should be made is that an antenna is the interface of two 'causes' of EM fields. The EM fields that are generated by the moving charges or currents (near field) in wires are localized near the charges/currents so they stay close to the antenna and store the power to be radiated. The varying field lines from the moving charges/currents around the antenna extend into space (they carry energy) and as they move they create it's own self-sustaining EM field that's not from moving charge/current but from a changing E or B field and travel into (far field) space as EM radiation. How this happens might be beyond the scope of your report but the book recommended by Berkeman will help to understand the process.


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