# Is this how a dipole antenna works?

by CraigH
Tags: antenna, dipole
 P: 198 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.
Mentor
P: 40,634
 Quote by CraigH 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.