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Antennas.....

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georgetwo

Member
basically, an antenna/aerial is a none shielded material that helps in radiation of radio waves. Transmitting aerial is for transmission, receiving aerial is for receiving, but it is possible to use one antenna for both function. theoretically, a wave could be transmitted effectively if the length of the transmitting antenna is roughly equal to wavelength of the wave to be transmitted. google the rest.
 

HiTech

Well-Known Member
How do antennas operate????Can anyone explain me the fundamentals of antenna operations....
Have you tried entering Antenna Fundamentals or Antenna Basics into your web browser? I did, and came up with many, many links on the topic. Try it for yourself!
 

microtexan

New Member
Antenna theory

Have you tried entering Antenna Fundamentals or Antenna Basics into your web browser? I did, and came up with many, many links on the topic. Try it for yourself!

Ah, That's no fun. He wants someone else to do his homework for him.:(
 

Papabravo

Well-Known Member
basically, an antenna/aerial is a none shielded material that helps in radiation of radio waves. Transmitting aerial is for transmission, receiving aerial is for receiving, but it is possible to use one antenna for both function. theoretically, a wave could be transmitted effectively if the length of the transmitting antenna is roughly equal to wavelength of the wave to be transmitted. google the rest.
It is a half-wavelength for a horizontal dipole, and a quarter wave length for the end fed vertical. How it works is described by the solutions of the the partial differential wave equation. Hueristically waves move along conductors until they reach an impedance discontinuity. At the discontinuity a portion of the wave passes through the discontinuity and a portion is reflected back to the source. What you try to do is "tune" the antenna by changing its geometry so that at a particular frequency or a narrow band of frequencies most of the energy passes through the impedance discontinuity and only a small fraction is refelected back. This condition is referred to as resonance. A transmit antenna that is resonant on a particular frequency is also a good receive antenna for the same frequency.
 
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user_88

Member
One interesting aspect of antennas .... Electromagnetic radiation from an antenna consists of two parts: an electric field component, and also, simultaneously, a magnetic field component.

The component of the electromagnetic radiation which actually propagates to some distant point is the Electric Field, and only the Electric field ... This E field has an attenuation that is relative to 1/r .... where r is the far field distance. The Magnetic field component of the electromagnetic antenna radiation attenuates in a manner that is proportional to 1/r². Consequently, the E field can travel vast distances and still be picked up ... or received. However, the Magnetic field is only strong enough to be of consequence in a local, or short distance sense.

A more concise explanation may be found in the Lectures on Physics series by Richard Feynman.
 
Well this isnt my homework at all....its one of the papers in my course....i have tried out Jordan Balmain and Balanis....but they werent satisfactory....the concepts such as far field E and H fields,and directivity and all are explained in details....but the fundamentals arent explained properly....at least thats what they seemed to me.....i was just hoping if anyone could explain the basics of antenna radiation.....thnx to all.....
 

Papabravo

Well-Known Member
Well this isnt my homework at all....its one of the papers in my course....i have tried out Jordan Balmain and Balanis....but they werent satisfactory....the concepts such as far field E and H fields,and directivity and all are explained in details....but the fundamentals arent explained properly....at least thats what they seemed to me.....i was just hoping if anyone could explain the basics of antenna radiation.....thnx to all.....
You mean I didn't succeed in explaining it to your satisfaction. Well sorry, but what exactly is the disconnect here? What is missing?

An antenna is like any other transmission line except it is terminated in free space instead of with a matching network like a resistor. Changing the geometry (length, height, configuration) changes the impedance and the resonant frequency in such a way as to suppress reflection and enhance radiation.

The rest is just mathematics.
 

bountyhunter

Well-Known Member
Actually he wants some practical view of some one on antennas.


No, what he asked was:

How do antennas operate????Can anyone explain me the fundamentals of antenna operations....


Which is on par with asking somebody to explain how Physics works. If someone asks a vague, nebulous question they will invariably get unsatisfying answers.

I have studied antennas some years back on the first level, and even my knowledge would fill a good sized book. Antennas are complex, the theory behind how they work is VERY complex, and in some cases, not well understood at all. In fact, the joke used to be it was easy to build an antenna, the hardest part was figuring out how it worked. There is a lot of truth in that.
 

Papabravo

Well-Known Member
Seems to me the behavioral description has a lot going for it. Without getting into the mathematics you can understand from observing a water wave in a tank with a porous membrane at one end. Send a wave down the tank, when it gets to the membrane part of the wave gets through and part of it is reflected back. Waves are waves after all.
 
I think i should have explained the problem a bit more clearly....i wanted to understand the basics....like why does an accelerating electron radiate...(i know a bit now thanks to the link provided by Mr.RadioRon...)....
A couple of last things i would like to know....
1.What is it about the length of the antenna???Why does it have to be a quarter or half wavelength????
2.Why is it that only the antenna radiates significant amount of energy????Doesnt the TL which connects the source to the antenna radiate as well???Why cant we replace the antenna with just an ordinary piece of Cu wire????

A special thanks to Mr.Papabravo....your explanation about the impedance matching part was quite helpful....its just that i should presented my problem in a better manner....thnx for your efforts.....
 

Papabravo

Well-Known Member
I think i should have explained the problem a bit more clearly....i wanted to understand the basics....like why does an accelerating electron radiate...(i know a bit now thanks to the link provided by Mr.RadioRon...)....
A couple of last things i would like to know....
1.What is it about the length of the antenna???Why does it have to be a quarter or half wavelength????
2.Why is it that only the antenna radiates significant amount of energy????Doesnt the TL which connects the source to the antenna radiate as well???Why cant we replace the antenna with just an ordinary piece of Cu wire????

A special thanks to Mr.Papabravo....your explanation about the impedance matching part was quite helpful....its just that i should presented my problem in a better manner....thnx for your efforts.....
1. In the solution to the partial differential equation, the quarter wavelength and half-wave length boundry conditions enhance the ability of the antenna to radiate at that frequency(wavelength). At the same time other frequencies are not radiated so efficiently.
2. The transmission line does radiate in some cases, especially when the RF currents use the shield as a return path because an unbalanced coax is coupled to a balanced antenna. We can replace an antenna with a long piece of wire. It has different characteristics and is often used with an antenna tuner with surprisingly good results.

If you are familiar with the concept of return loss, or have ever seen it plotted as a function of frequency, the picture of what is happening will become clearer. Terrific antennas have about 25 dB return loss. That means that the reflected signal is 25 dB down from the forward signal. A superlative antenna may go as high as 40dB return loss. Return loss is just 20*log(rho), where rho is the magnitude of gamma, the reflection coefficient.

IMHO you understand a great deal more than was evident from the original post. Thanks for the clarification.
 
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RadioRon

Well-Known Member
1.What is it about the length of the antenna???Why does it have to be a quarter or half wavelength????
2.Why is it that only the antenna radiates significant amount of energy????Doesnt the TL which connects the source to the antenna radiate as well???Why cant we replace the antenna with just an ordinary piece of Cu wire????
.

An antenna does not have to be a quarter or half wavelength, it can be any length of wire, or indeed any shape of conductor or semiconductor. But the length or size is directly related to the antenna's driving point impedance (and radiation resistance) and this is the crux of this matter. In the early days of RF engineering, the industry agreed to standardize on a few transmission line characteristic impedances and nowadays the most common ones are 50 ohms and 75 ohms. It just so happens that the half wave antenna presents a very good impedance match to this range of transmission line impedances, and while doing so it radiates with very good efficiency. Of course you may choose any length, but the input impedance will require some additional efforts at a matchng circuit. Also, if you choose a very short length, the radiation impedance becomes so small that the radiation efficiency drops quite low. The quarter wavelength antenna is a variation on a half wavelength antenna, so the same reasons apply. These antennas just happen to be some of the most simple and best performing types, that's why you see them so often in the literature.

In fact, in many examples of real antennas, the transmission line does indeed radiate a fair bit. This is unintended, but real. However, a good antenna is designed to have some isolation from its transmission line. Sometimes this is done by insuring that the antenna is a balanced circuit and feeding it through a balun device from a coaxial line. This balun looks after making sure that the RF currents flowing on the antenna do not inadvertantely go back onto the outside of the coax. It is also worth mentioning that coaxial transmission line would not provide any isolation or shielding at all if we didn't have the skin effect, so make sure you understand that concept too. Perhaps a general study of transmission line principles would be helpful too.
 

bountyhunter

Well-Known Member
Seems to me the behavioral description has a lot going for it. Without getting into the mathematics you can understand from observing a water wave in a tank with a porous membrane at one end. Send a wave down the tank, when it gets to the membrane part of the wave gets through and part of it is reflected back. Waves are waves after all.

Maybe, but electromagnetic fields are hardly that easy to model and the problem is they behave differently for no reason anybody can understand. If I recall correctly, Einstein went to his grave unsuccessfully tring to come up with a theory that could unify the behavior of fields.
 

Papabravo

Well-Known Member
Maybe, but electromagnetic fields are hardly that easy to model and the problem is they behave differently for no reason anybody can understand. If I recall correctly, Einstein went to his grave unsuccessfully tring to come up with a theory that could unify the behavior of fields.
I don't agree with that statement at all, but why should I argue with you. We don't need the simplified explanations. We've jumped the shark.
 

bountyhunter

Well-Known Member
Quote:
Originally Posted by bountyhunter
Maybe, but electromagnetic fields are hardly that easy to model and the problem is they behave differently for no reason anybody can understand. If I recall correctly, Einstein went to his grave unsuccessfully tring to come up with a theory that could unify the behavior of fields.


I don't agree with that statement at all, but why should I argue with you. We don't need the simplified explanations. We've jumped the shark.

I don't really care if you agree, since the statement I made is true, that's irrelevant.

In physics, a unified field theory is a type of field theory that allows all of the fundamental forces between elementary particles to be written in terms of a single field. There is no accepted unified field theory yet, and this remains an open line of research. The term was coined by Albert Einstein who attempted to unify the general theory of relativity with electromagnetism.

Unified field theory - Wikipedia, the free encyclopedia

Odd that Einstein stonewalled, what with "waves are waves after all".
 
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