I'm currently preparing for a challenge exam, and I'm having trouble understanding antennas. My physics book is good, but it is very brief in its section on antennas. I know there are more complicated types of antennas, but let's just stick with the "simple" half-wave antenna (or dipole antenna, or whatever you want to call it).
As I understand it now, this type of antenna is essentially 2 metal rods with an alternating voltage source in the middle. My book tells me that because the current in this rod is constantly changing, it will emit electromagnetic energy.
How do common antennas flip the charges on the two rods? The book mentions an LC oscillator - how do those work? Are there other kinds of oscillators?
I think where I'm having the most trouble is in understanding how exactly energy is radiated by the antenna.
Here's what my book says: "because [itex]\vec{}E[/itex] and [itex]\vec{}B[/itex] are 90° out of phase at points near the dipole, the net energy flow is zero. From this fact, you might conclude (incorrectly) that no energy is radiated by the dipole."
Is this phase difference because of the oscillator in the middle? Is it trying to tell me that if I integrate the Poynting vector over a full current/charge cycle I would get 0?
It goes on to tell me that at great distances from the antenna, the dipole fields become negligible (since they are proportional to 1/R3). Does that mean that antennas don't work when they are close to the receiver?
Here's the strangest part of the book for me: "At these great distances, something else causes a type of radiation different from that close to the antenna. The source of radiation is the continuous induction of an electric field by the time-varying magnetic field and the induction of a magnetic field by the time-varying electric field". According to the book, these fields ARE in phase. So, where did they come from, and where were they at the closer distances? Were they overruled by the earlier dipole fields? How are the properties of this wave determined? How could I find the power and frequency of said wave?
What if I had two antennas with the same power and frequency? Would it make the strength of my signal in the transmission area better or worse?
Thanks (EDIT: I don't really know how to do vectors properly on physics forums. Sorry about that.)
As I understand it now, this type of antenna is essentially 2 metal rods with an alternating voltage source in the middle. My book tells me that because the current in this rod is constantly changing, it will emit electromagnetic energy.
How do common antennas flip the charges on the two rods? The book mentions an LC oscillator - how do those work? Are there other kinds of oscillators?
I think where I'm having the most trouble is in understanding how exactly energy is radiated by the antenna.
Here's what my book says: "because [itex]\vec{}E[/itex] and [itex]\vec{}B[/itex] are 90° out of phase at points near the dipole, the net energy flow is zero. From this fact, you might conclude (incorrectly) that no energy is radiated by the dipole."
Is this phase difference because of the oscillator in the middle? Is it trying to tell me that if I integrate the Poynting vector over a full current/charge cycle I would get 0?
It goes on to tell me that at great distances from the antenna, the dipole fields become negligible (since they are proportional to 1/R3). Does that mean that antennas don't work when they are close to the receiver?
Here's the strangest part of the book for me: "At these great distances, something else causes a type of radiation different from that close to the antenna. The source of radiation is the continuous induction of an electric field by the time-varying magnetic field and the induction of a magnetic field by the time-varying electric field". According to the book, these fields ARE in phase. So, where did they come from, and where were they at the closer distances? Were they overruled by the earlier dipole fields? How are the properties of this wave determined? How could I find the power and frequency of said wave?
What if I had two antennas with the same power and frequency? Would it make the strength of my signal in the transmission area better or worse?
Thanks (EDIT: I don't really know how to do vectors properly on physics forums. Sorry about that.)