Understanding the physics of antennas

[harts]

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.)

 

[davenn]

Hi harts

altho I know the basic answers to your questions, I suspect you are looking for some deeper answers... that i will leave to others :)

but this question needs clarifying...

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?

2 antennas fed from the same RF source ? that is a single coax that is then split to feed 12 separate antennas ? or were you thinking of something different

in the case of 2 antennas and one source say the antennas were Yagi type and 10 dBd ( dBd = dB over a dipole) gain. If you are feeding 10Watts into just one Yagi you have 10 W x 10 gain or an ERP (Effective Radiated Power) of 100W (minus some small losses that we won't go into here)

if hnow you split the coax and feed 2 identical Yagi's with that 10 Watts, you double your power, that's an additional 3 dB ... so your total gain is now 13dBd
no, it's not 10dBd + 10 dBd = 20dBd ... many years ago, it took me a while to grasp that fact

You also have to remember that since you split that 10 Watts between the 2 antennas, each antenna is only seeing 5W from the transmitter

cheers
Dave

 

[yungman]

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. (1)

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."(2)

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?(3)

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?(4)

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.)

First of all, below is only for short dipole where current is assumed constant along the rod. It will be too complicate to assume sinusoidal. The idea is the same.

(1) As differential voltage driving into the both rods, current driving into the rod.
[tex]\tilde A=\frac {\mu_0}{4\pi}\int_{v'}\tilde J \frac{e^{-jkr}}{r}d\vec l'\;\Rightarrow\; \mu_0\tilde H=\nabla\times \tilde A, \;and\; \tilde E=\frac{1}{j\omega \epsilon_0}\nabla\times \tilde H[/tex]
So from current, you get E and H.

(2) Power radiate out:
[tex]P=\frac {1}{2}\int_{v'}\tilde E\times \tilde H^* dv'=P_{rad}+P_{img}[/tex]
At close to antenna ( near field), because the ##P_{img}##>>##P_{rad}##. Most power is imaginary, that's the reason the real radiation power is like zero. It is too long to write out the formulas. But if you use the formulas given from (1) you'll get
[tex]E_r=K_1\left(1+\frac{1}{jkr}\right), \;E_{\theta}=K_2\left[1+\frac{1}{jkr}+\frac{1}{(jkr)^2}\right][/tex]
As you can see, if ##kr##<<1 ##E_{\theta}## dominates and is complex. Same goes to H. That's the reason the book say radiating power is zero and close distance ( near field). You calculate the power and you'll see.

(3) It is not correct to say dipole field goes to zero at large distance. Look at (2) above, ##E_r\propto\frac{1}{r^2}## but ##E_{\theta}\propto\frac {1}{r}##. Therefore ##E_r## assume to be zero and only ##E_{\theta}## remains at large distance from antenna.

(4) If you refer back to (2) where I explain
[tex]E_r=K_1\left(1+\frac{1}{jkr}\right), \;E_{\theta}=K_2\left[1+\frac{1}{jkr}+\frac{1}{(jkr)^2}\right][/tex]
H is the same way. The poynting vector is too complicate to calculate using all the terms. So you make assumption for a given distance. For example if kr<<1,##\Rightarrow \frac{1}{(jkr)^2}##>>##\frac{1}{(jkr)}##>>1. So you ignore the smaller terms. So you will see the fields looks different at different distance. It is just an assumption but it is good enough.

This is a big subject. I am studying antenna right now. No body can explain in a post here. What I give you is just a starting point in trying to answer your question. Remember, the antenna theory book is thicker than the electromagnetic book!

If you are going to study antennas and EM, you better learn Latex. This and other forums all use Latex to write formulas. Without that, good luck in even asking question! I am getting to the point I can just type as if it's English!

 

[yungman]

Look at the formulas of ##\vec E## and ##\vec H## in this article. Make assumption according to the distance and calculate the Poynting vector. You'll see this a lot clearer.

http://en.wikipedia.org/wiki/Dipole_antenna

In the article, it also show you the far field assumption. Do it on the near field assumption, you can see it become imaginary and does not contribute to real radiating power.

 

[PJC01]

First of all, it's great that you are taking the time to study and prepare for your challenge exam. Antennas can be a complex topic, but with some additional clarification, you should have a better understanding of how they work.

To start, let's clarify what an antenna does. An antenna is a device that converts electrical energy into electromagnetic energy and vice versa. This means that when an electrical current flows through the antenna, it creates an electromagnetic field, which can then be received by another antenna and converted back into electrical energy.

Now, let's focus on the half-wave or dipole antenna. This type of antenna consists of two metal rods, as you mentioned, with an alternating voltage source in the middle. This alternating voltage source is what creates the changing current in the rods, which in turn creates the electromagnetic field. The process of flipping the charges on the two rods is called oscillation, and it is achieved through the use of an LC oscillator. This oscillator is a circuit that uses an inductor (L) and a capacitor (C) to create a resonant frequency, which allows the antenna to efficiently emit and receive electromagnetic waves.

The phase difference between the electric field (\vec{E}) and magnetic field (\vec{B}) is due to the fact that they are perpendicular to each other and also perpendicular to the direction of propagation of the electromagnetic wave. This is known as transverse wave propagation. The Poynting vector, which represents the direction and magnitude of energy flow in the electromagnetic field, is also perpendicular to both the electric and magnetic fields. So, while the energy flow may be zero at points near the dipole, this does not mean that no energy is being radiated. In fact, the energy is being radiated in all directions, but the net energy flow is zero.

As for the distance at which the dipole fields become negligible, this is due to the inverse square law, which states that the strength of an electromagnetic wave decreases with distance from the source. So, while the fields may be negligible at great distances, they are still present and can be received by another antenna.

Now, let's address the concept of radiation at great distances. As the book mentioned, at these distances, the radiation is caused by the continuous induction of electric and magnetic fields. This is known as electromagnetic radiation or radio waves. These fields are in phase because they are being continuously induced by each other, creating a self-sustaining wave. This

 

[PJC01]

First of all, it's great that you are actively seeking to understand the physics of antennas. They can be complex devices, but once you have a solid understanding of the underlying principles, it becomes much easier to grasp the different types and how they work.

To answer your question about how antennas flip the charges on the two rods, it's important to understand that an antenna is essentially a transducer - it converts electrical energy into electromagnetic energy. In the case of a dipole antenna, the alternating voltage source in the middle creates an alternating electric current in the rods. This current then creates a changing electric field around the antenna, which in turn creates a changing magnetic field. This process continues back and forth, creating an electromagnetic wave that radiates outwards from the antenna.

Now, let's talk about the LC oscillator. This is a type of circuit that uses an inductor (L) and a capacitor (C) to create an oscillating current. In the case of an antenna, the LC oscillator is used to create the alternating voltage source that drives the current in the rods. There are other types of oscillators as well, but the LC oscillator is commonly used in antenna design.

Regarding the phase difference between the electric and magnetic fields near the antenna, this is due to the fact that the two fields are generated by the same source (the alternating current in the rods) but at different times. As you mentioned, integrating the Poynting vector over a full current/charge cycle would indeed result in a net energy flow of zero. This is because the energy is constantly being transferred back and forth between the electric and magnetic fields.

As for the distance at which the dipole fields become negligible, this is simply a result of the inverse square law - the farther you are from the source, the weaker the fields will be. Antennas can still work when they are close to the receiver, but the strength of the signal will be much stronger at a greater distance.

Now, onto the "strangest part" of the book. What it's trying to say is that at greater distances from the antenna, the changing electric and magnetic fields create a new type of wave called an electromagnetic wave. This wave is different from the near-field dipole fields, which are primarily electric or magnetic in nature. The properties of this wave, such as its frequency and power, are determined by the source (the antenna) and can be calculated using equations such as the Poynting