An antenna radiates ....how can this be explained physically

I just want to know what is happening there physically in an antenna . How is it radiating ?? I tried all the basic books like Kraus, Balanis,etc .But no book even considers it .They just explain the properties , construction ,etc .But without even knowing how it works, what's the use ??

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Originally Posted by ysenthilece
I just want to know what is happening there physically in an antenna . How is it radiating ?? I tried all the basic books like Kraus, Balanis,etc .But no book even considers it .They just explain the properties , construction ,etc .But without even knowing how it works, what's the use ??
Hi!

I'm usually using the attached picture.
Imagine 2 E-field lines a-b, c-d (2 other arrows is the H field, but it's not important now).
lines are travelling within the waveguide (for example 2 wire line), with distance λ/2.

d) first line reaches the end of the transm.line and keeps boundary conditions there (E perpendicular to conductor). Next we have to accept that field line has momentum (inertia). Thus the "ends" of the first field line are "glued" to end of the guide and it's centre is travelling into free space e)

f) the second field line reached the "glued" first one and they get connected

g) the connected lines are forming a loop-spherical wave (thus rot E≠0 for HF problems!), separating from the guide and flying to the free space. Of course the separation process is some kind of "violence" to the guided wave and thus some part of energy is going back to the source (reflected wave represented by reactance X). Radiated energy is represented by R as resistor which charges the transmitter

The area where the process f) is realized, is said to be the "near field region", region where the waves are still connected to the antenna and where the field lines are going out and back, the "reactive region"

I'm sorry for antenna specialist and people who like theory, I know it was not very rigorous :)
Anyway, if one looks into FDTD simulation of say radiating waveguide, this simplified process could be seen there...

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In the book "Antenna Theory Analisi and Design" by
Stutsman and Thiele exist a section about it:
in Antenna Theory abd Desing by Shelkunoff and
Friis, John Wiley, 1952

An explanation a physicist would be likely to offer:

Whenver a charge acelerates, it radiates energy in the form of an EM wave. RF current flowing on a conductor has a non-zero first time derivative, thus the charge on a wire has a non-zero second derivative, thus it radiates.

Now, why an accelerated charge radiates, no one knows...

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5. how does an antenna radiate

Waves propagate during discontinuities in the fields of the antenna, u can imagine it exactly like dropping a stone in some water

The explanation is simple. The power comes in on a cable. If little of the power is reflecting back on that cable, and if the antenna is not melting from the heat, then the power must be radiating into free space.

The explanation is simple. The power comes in on a cable. If little of the power is reflecting back on that cable, and if the antenna is not melting from the heat, then the power must be radiating into free space.
UUUUUUHHHHHHHHHHHHH?????????

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It is pretty simple. One can measure the power reflected from the antenna. Also one can measure, with a thermocouple, the temperature in the antenna. If there is more power incident on the antenna than is reflected or turned into heat, then the antenna is radiating.

U can find very good explanation in the book Practical antenna hand book by Carr..
chapter Two ...

11. Re: An antenna radiates ....how can this be explained physic

please do help me by sending a relevant link of e-book of antennas for begginers ..please see book makes antenna theory practical........ i m looking forward for ur reply

12. Re: An antenna radiates ....how can this be explained physic

Electric dipole is a good example to think about radiation. You know that an electric dipole creates an electric field around it:
http://hyperphysics.phy-astr.gsu.edu...gele/edip2.gif
The question is: What happens to the electric field if charges move? Imagine that you swap their positions. Electric field is changed, this change starts from vicinity of charges and propagates to infinity, a propagating wave. Therefore, if you move charges you create waves, this is what is going on in an antenna radiating waves. An example: Charges swap their positions 2e9 times in a second in a dipole antenna whose radiation frequency is 1 GHz.

13. Re: An antenna radiates ....how can this be explained physic

What hasnt really been said is ... ALL conductors carring an electric current radiate .... that is they form an electromagnetic field around themselves
think of a simple coil of wire and creating an electro-magnet.
Basic electrical theory

a mains power cable is no different you can hear the hum picked up on a radio from the cable, caused by the electromagnetic field rising and collapsing.

the difference with an antenna is that we take advantage of that radiated field and make a radiator (bit of wire/antenna) that is tuned (cut to length) to the freq of the applied AC current to make the EM radiation more efficient

power cable, speaker cable, cat45 network cable, etc ... undesirable EM radiation
which is inefficiency and power loss
a tuned antenna ... efficient by allowing as much EM radiation energy to be radiated as possible at desired freq

ok thats my 3 cents worth (2cents+ inflation)

Dave

14. Re: An antenna radiates ....how can this be explained physic

See this discussion, you will find some good radiantion diagram about a dipole and a Yagi Antenna:

http://www.uwaverf.altervista.org/fo....php?f=19&t=18

15. Re: how an antenna radiates

Originally Posted by eirp
Hi!

I'm usually using the attached picture.
Imagine 2 E-field lines a-b, c-d (2 other arrows is the H field, but it's not important now).
lines are travelling within the waveguide (for example 2 wire line), with distance λ/2.

d) first line reaches the end of the transm.line and keeps boundary conditions there (E perpendicular to conductor). Next we have to accept that field line has momentum (inertia). Thus the "ends" of the first field line are "glued" to end of the guide and it's centre is travelling into free space e)

f) the second field line reached the "glued" first one and they get connected

g) the connected lines are forming a loop-spherical wave (thus rot E≠0 for HF problems!), separating from the guide and flying to the free space. Of course the separation process is some kind of "violence" to the guided wave and thus some part of energy is going back to the source (reflected wave represented by reactance X). Radiated energy is represented by R as resistor which charges the transmitter

The area where the process f) is realized, is said to be the "near field region", region where the waves are still connected to the antenna and where the field lines are going out and back, the "reactive region"

I'm sorry for antenna specialist and people who like theory, I know it was not very rigorous :)
Anyway, if one looks into FDTD simulation of say radiating waveguide, this simplified process could be seen there...
i think this is completely correct i attach a new video to confirm this idea:

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oscillators are used to make an antenna to radiate example RC oscillator is used for low frequency oscillation and LC oscillator is used for high frequency oscillation...

Originally Posted by ysenthilece
I just want to know what is happening there physically in an antenna . How is it radiating ?? I tried all the basic books like Kraus, Balanis,etc .But no book even considers it .They just explain the properties , construction ,etc .But without even knowing how it works, what's the use ??
I have been studying this also and this is what I have discovered:

In our universe, there are phenomena that occur that we are trying to understand completely. Sometimes even though we don't completely understand we can do useful things with what we have observed. In the case of the phenomenon that make up what we know as "electricity" we know enough to predict fairly accurately what will happen.

• Phenomenon One: There is a force of nature called charge; it is a force that acts at a distance. It can attract or repel.
• Phenomenon Two: The smallest amount of charge yet to be observed is the charge associated with the fundamental atomic particle, the Electron.
• Phenomenon Three: If an electron is moving, its charge is moving and an electromagnetic (EM) field is produced, radiating into free space around the Electrons.
• Phenomenon Four: If an EM wave meets a conductor, a corresponding current (charges moving) is "induced" into the conductor.

An antenna is a conducting circuit element (like a wire) with some length, terminated by what appears to the circuit as a very very large impedance. When a current "attempts" to enter this circuit (antenna), it is met with the "brick wall" of the large impedance at the end of the antenna. Most of the energy is reflected back down the antenna back into the source.

If the current is static, that is at a fixed amplitude, nothing much happens but a simple EM wave like the coil around a nail and a battery.

But if the current is oscillating, something really cool happens; an EM wave leaves the antenna into free space. If the frequency of the oscillation is just right with respect to the length of the antenna, then a significant portion of that energy is radiated, with respect to the ground potential the earth.

Any other antenna in the path of this EM wave will have electrons excited inside, inducing a current in the circuit. If the receiving antenna is the right length with respect to the frequency of the radiated wave then a good portion of the initial radiated power is converted back into the oscillating current.

This induced oscillating current can be detected by various means.
I originally found myself stuck at the unterminated length of wire, without a “complete current path”. It is in reality, a circuit with some finite impedance, therefore power can be dissipated. It is taken up by heat and the energy transferred into the EM wave that is radiating away from the antenna. The magic is in the transference, something I aspire to understand mathematically, but need to take a course in “electrogodamnics.

David W,
Melbourne, Florida

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