How Elctric signals are converted into Electormagnetic signals?

[rraj.be]

Could anyone help me to understand the working of a antenna?

In antenna, we are using voltage or current as signal input and how this could be converted into em waves and vice verse in receiver side?

 

[m.s.j]

The Propagation of Electromagnetic wave from a piece of conductor which named antenna is related to frequency, lenghth of conductor and amount of pass current. For more information you can refer to Electromagnetic Riddle No.1 from http://electrical-riddes.com

 

[Corneo]

Your question is very general, but I will take a shot at it. An antenna can be viewed as just another circuit element, along with the other elements you already know about. Moreover an antenna can be viewed as a transducer - converting voltages and currents to fields. Take a simple dipole antenna for example. The antenna will have a frequency range for which it radiates, you feed your signal (of appropriate frequency) into the antenna, the antenna radiates and generates fields.

How the fields are generated and what they look like requires a strong understand of eletromagnetics.

 

[searchyan]

I think you ask a ques which is not known by all of us, including experts.
IF we want to understand it, build a antenna by our own is the only way to know how it works.
IF we don't know how it works, we will not know how to use

 

[Mike_In_Plano]

I think you've gotten in where angels fear to tread. Oh well, I'm a bit of an ox, I'll give it a go...
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The electromagnetic fields that go spreading through space are two fields that travel together. The alternating magnetic and alternating electric. they alternate together and at right angles to one another.
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If you're in a situation where one field predominates, it will give rise to the other. They are linked by the characteristics of the space they travel through. Thus the ratio of electric field to magnetic field will be different when traveling through cotton candy than it will propagating through air. But, in either case a fluctuating electric field will set up a fluctuating magnetic field and vice-versa.
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The classic antenna we all begin with is the dipole. It has two equal lengths of wire or rod that go out in opposite directions. These are the antenna's elements.
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Where the dipole's elements come together is where you attach your transmitting device. Perhaps you carry the signal from a remotely located transmitter. In that case, you'll need a transmission line to carry the signal from one point to the next. The transmission line is for all purposes a pair of wires, but the size and geometry of the wires is important to how they behave, which is really confusing for a beginner, so let's stick a pin in that concept and if you want to know more about it, I'll fill you in later.
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Anyway, we have signal that's being placed on the two elements:
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. -------------{-Signal+}-------------
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The signal is a sine wave, with one element always being driven at the opposite polarity as the other. Now, to efficiently radiate the applied signal, we want the antenna to resonate at the signal's frequency. That way, energy that isn't immediately transmitted stays in the antenna system, resonating, instead of fighting the output of the transmitter.
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For thin antenna elements, the resonance occurs when each element is about 1/4 of the transmitted signals wavelength. This kinda makes sense when you think of it this way:
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When the signal source has reached a peak on an end of an element, it's influence (assuming it can zoom along at the speed of light) won't be felt at the far end of the other element until 1/2 of a wavelength later. When that peak arrives it will have to wait another 1/2 wavelength to reach the place were the signal started. Having traveled a full wavelength, the signal arrives at the same instant that the signal has changed by 360 degrees. Thus, the antenna is a "tuned" circuit.
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Like other tuned circuits, you can change the resonance of the dipole with a little inductance and capacitance. If you put a couple of pie pans at the far ends of the dipole, that will load it with capacitance, and it will resonate at a lower frequency.
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Likewise, if you put a couple of inductors in the middle of the elements, you'll decrease the resonate frequency. If you swap those inductors for some series capacitors, you'll increase the resonate frequency.
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As you do these tricks with L's and C's, you can still get the antenna to transmit effectively, but the Q of the circuit goes up. It becomes a more narrow band device.
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Now, back to how the currents and voltages get into space. Back on the dipole, it has fairly high currents starting out where the signal drives it, but eventually they die down by the time you reach the end.
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Essentially, as you get near the end, there is less and loading on the element (unless you put the pie pans on the ends ;). On the other hand, the voltages swing more near the ends, rather like the peaks in a tube of water if you rock it near resonance.
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In reality, it is just what it seems; the impedance is increasing as you get further from the center. In fact, you can change the load impedance seen by the signal source by simply changing where you tie the connections.
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If your driving it with a 75 ohm source, than your in great shape. Tie to the starting point of the elements and that's your impedance. If you have a higher impedance signal source, than simply spread the leads. After all, as you go outwards, the current goes down and voltage goes up:
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Z = V / I
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Higher Z by spreading connections:
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... ____________ ____________
......|{-Signal+}|
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As to the magic of getting the signal into space, it's like breeding rabbits. Due to the current, you've got the magnetic field wrapping around the elements. Coinciding with that, you have an electric field that's lined up end to end. They flip together, and they're at right angles. By definition that's an EM wave, and off it goes into space...

Now, you've obviously seen cases where it doesn't work that way, take the car alarm remote as an example. There's no way you could fit our simple dipole in that tiny case. What's done instead is a current carrying loop is used - as big a loop as can be afforded. The loop has a tuning capacitor across it, which tunes it to be an LC resonator at the transmitters frequency.

Now, the LC resonator has a very high Q, and the loop only works for a very narrow frequency. But, at that frequency, high currents build up in the loop and couple into the surrounding space. Remember what I said at the beginning? Alternating magnetic fields set up alternating electric fields and vice-versa. In this fashion, the magnet field from the transmitter couples into the surrounding space and sets up a propagating EM field.
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I hope that helps.
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. Best wishes for the holidays,
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. - Mike