Understanding Radiation Patterns in Antennas

[vig]

Ive just started studying antenna theory and after a good 2 weeks of research on the forum, understood to a certain extent about how the antenna radiates waves. But, I'm still haunted by radiation patterns. Why should the radiated power be a function of theta, and how exactly do we get the radiation patterns?..Further, the standing wave patterns, the current-voltage distributions that are shown, must also be a function of time. Why is it that we consider the distribution only along the length of the antenna?
Sorry if the question seem naive...ive just started on this subject.

 

[yungman]

I assume you study EM. Antenna is approximated by a small electric dipole or a series of small electric dipole. You can look at a simple electric dipole and the field pattern. Consider the dipole is in z-axis, the field has theta dependent. If you take the long dipole antenna, you can integrate a series of small dipoles and still get a field pattern.

On the second question. Fields radiate out, but charge stay on the antenna. So current and voltage distribution stay with the antenna.

For simple dipole antenna, Engineering Electromagnetic by Ulaby has a good description. But for studying antenna, Antenna Theory by Balanis is very good. All your questions are at the beginning of the book.

 

[vig]

Thanks a lot yungman for the reply. So radiation patterns, from waht i undestand, is basically a plot of the current distribution, taking the centre of dipole as the origin?..(centre as in the space between the two tx lines).
And what about the standing wave patterns?..all books show a plot of mod(I) against length..but doesn't talk anthing about the time factor..It'll be great if someone could help me out with this.
I do have balanis, but still am finding it difficult to start on thi subject.

 

[vk6kro]

Thanks a lot yungman for the reply. So radiation patterns, from waht i undestand, is basically a plot of the current distribution, taking the centre of dipole as the origin?..(centre as in the space between the two tx lines).
And what about the standing wave patterns?..all books show a plot of mod(I) against length..but doesn't talk anthing about the time factor..It'll be great if someone could help me out with this.
I do have balanis, but still am finding it difficult to start on thi subject.

The radiation pattern is a plot of field strength at points equidistant from the antenna but in all directions from it.

With a fixed antenna, this is measured by taking readings of field strength while moving around the antenna at a constant distance.

If the antenna can be rotated, then this is done with the measuring instrument in a fixed position. This is a lot easier than moving around the antenna.

The received signal is a sinewave but the ampltude of this varies according to the field strength.

Standing waves on the feedline are the result of reflections from a mismatched antenna feedpoint. The voltage between the feedline conductors is a sinewave at any point but the amplitude depends on the interaction between the incoming signal and the reflection.

 

[yungman]

Thanks a lot yungman for the reply. So radiation patterns, from waht i undestand, is basically a plot of the current distribution, taking the centre of dipole as the origin?..(centre as in the space between the two tx lines).
No radiation pattern is not the current distribution. It is the field caused by the charge distribution on the antenna. If yo read Balanis, it first talk about the differential dipole and the field pattern. Then integrate the differential dipole along the full length of the antenna. Remember the current and charge never leave the antenna, the field propagate out due to the consequence of the charge and current distribution.

And what about the standing wave patterns?..all books show a plot of mod(I) against length..but doesn't talk anthing about the time factor..It'll be great if someone could help me out with this.
I do have balanis, but still am finding it difficult to start on thi subject.

Review the electric and magnetic dipoles in the EM class. They talk about the field pattern there.

The standing wave pattern is phasor form which implies that time dependent component is left out. Remember the definition of phasor?

$$A\cos (\omega t -\beta z + \phi)=Re[Ae^{j\omega t}e^{-j\beta z}e^{j\phi}]=Re[\tilde A e^{j\omega t}]\;\hbox { where } \tilde A \;\hbox { is the phasor without time information.}$$

Remember what the book said, the current and charge distribution is only an approximation, the true distribution is very hard to find even for a simple straight rod. All the the book said was that the field pattern is close using the standing wave approximation. In fact, the book only use charge distribution for calculation as if the charge accumulate at both ends and form an electric dipole. I am no expert in Antenna, I am just studying it. But notice the book concentrate more on the pattern of different antenna configuration, more like addition and subtraction of fields of multiple dipole antennas that make up different antennas in the first few chapters. The book kind of avoid the physics of the charge and current distributions.

 

[vig]

Thanks vk6kro and yungman for your interest...it would really help if someone could explain how to plot the radiation pattern for a, say, a dipole of length λ...not practically, but theoretically.

 

[yungman]

Thanks vk6kro and yungman for your interest...it would really help if someone could explain how to plot the radiation pattern for a, say, a dipole of length λ...not practically, but theoretically.

I look at the Balanis again, it seems to assume that you have basic knowledge of small dipole and linear antenna. I studied three different books on this basic part. The explanation in Balanis is not as good. Try go on line and read this book:

https://www.amazon.com/Field-Wave-Electromagnetics-David-Cheng/dp/0201128195/ref=sr_1_1?ie=UTF8&qid=1327862523&sr=8-1

Look at p614 where he talked about Linear Dipole antenna and the patterns with different length.

Don't you have an EM book? A lot of them get into the linear dipole antenna. Balanis might assume you studied these already. Don't just go out and buy this book as it is only one part that is helpful. In fact the chapter gone down hill from this on as he tried to explain arrays and others in simplified manner and it really get bad.

 

[vig]

I did go through the analysis of dipoles...which finally led to plotting of field patterns...I was just thinking if there was a completely analytical way to explain the same. Since we know the current and voltage distributions, i thought that the plotting of the field intensities could be analytically done too...

 

[yungman]

Problem is you don't know the exact current distribution. It is only an approximation. The book said it very clearly. That's the reason you don't get the exact current distribution. Everyone is using an approximation like standing wave. I have a few books, they all pretty much the same.

 

[vig]

So there is no method to plot an approximate radiation pattern analytically?

 

[sophiecentaur]

What do me mean by "analytically"?
I think you may mean 'numerically' as the formulae tell you the radiation pattern from the assumed current distribution (That's analytical).

 

[yungman]

So there is no method to plot an approximate radiation pattern analytically?

The book do give you formulas to get the field at every point in space using the approximation just like in simple dipole case. You can just use the formula to plot if you want to. I thought you said you want to exact current distribution to plot the field.
But why do you want to do that as books always give you the plots anyway. Are you trying to verify the book?

 

[vig]

I'll rephrase my question...All I wanted to know was whether it was possible to explain and/or predict the field pattern just by LOOKING at the voltage and current distributions (even if its approximate). I do understand the mathematical approach.

 

[yungman]

I'll rephrase my question...All I wanted to know was whether it was possible to explain and/or predict the field pattern just by LOOKING at the voltage and current distributions (even if its approximate). I do understand the mathematical approach.

Yes, and the books show how to integrate the field of a small dipole to become the field pattern of the linear dipole. In the Cheng's book, it even show the field pattern of different length relative to the λ. Go online to see whether you can download the book free and look at p614 where it give different field patterns of the linear dipole ( that's what you are referring to of length between λ/4 to λ).

Balanis seems to skip the detail of the basic dipole. Look at Cheng.

 

[sophiecentaur]

I'll rephrase my question...All I wanted to know was whether it was possible to explain and/or predict the field pattern just by LOOKING at the voltage and current distributions (even if its approximate). I do understand the mathematical approach.

I would say No, actually. The radiation pattern is a diffraction / interference pattern, which you can calculate, of course, but it's asking a bit much to decide by just looking.
It may or may not help if I say the radiation pattern (angle) is the Fourier transform of the current distribution (spatial) of the radiating system. So, with experience, you can make a stab at it.

 

[sophiecentaur]

I could go further, actually. The radiation patterns of a short or long(ish) dipole are much the same (the ring donut shape). Longer dipoles and monopoles tend to have a 'tighter' vertical pattern and are still omnidirectional around their 'equator'.
Arrays of antennae have a much bigger range of radiation patterns and there are some simple rules of thumb to establish the spacing of nulls and maxima. Could that be what you are after?

 

[vig]

Thanks a lot to everyone here..So from what i understand..radiation patterns are best done by the mathematical approach..that is, find current distribution, then the magnetic vector potential, followed by B and finally E..

 

[sophiecentaur]

Yes. You have swung from one extreme to the other
'By looking at it' really isn't a possibility but even 'calculate the currents' only works by approximating and making assumptions.

From an engineering point of view, the detailed pattern of a single radiator is often not too relevant because any antenna operates in the vicinity of other structures which also have an effect. Also, the impedance of the antenna can be more relevant (which is why the half wave element is so popular). There is far more interest in the behaviour of arrays of elements, which can be used, cleverly, to produce all sorts of useful, very directive /selective patterns.

 

[yungman]

Well said sophiecentaur. It is much more important to know the characteristics of the radiation pattern than to know exactly the vector field at any point as there are so many other external factors.

 

[vig]

thanks!