how antenna radiates?


by david90
Tags: antenna, radiates
david90
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#1
Sep5-11, 05:32 PM
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From my understand, electrical current is required to generate RF. In the picture below, only one end of the antenna is connected L1. If only one end of the antenna is connected, how can electrical current travel up the antenna to generate RF?
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phyzguy
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#2
Sep5-11, 06:43 PM
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The antenna is long enough that it needs to be treated as a transmission line, and not a simple open circuit. Think of shaking a rope with the far end free (analogous to an open circuit at the far end of the antenna). Waves will travel down the rope. This is basically what happens with an antenna. EM waves travel down the antenna and the energy is radiated into space.
vk6kro
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#3
Sep5-11, 06:54 PM
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This circuit would operate at about 100 MHz and, at this frequency, one wavelength is about 3 meters. So, a quarter wave length is about 75 cm (29 inches).

If you put a 100 MHz sinewave at one end of a piece of wire that is 75 cm long, the voltage takes about 2.5 nS to reach the other end. While it is doing that, the input keeps changing, so the voltage at the end never catches up with the input voltage.

There is always a voltage difference between the ends of the antenna. The end is always 90 degrees out of phase with the input voltage

So, there is a current flowing between the two ends of the wire even though one end is open circuit. This current causes some of the input power to be radiated.

This seems alien to everything you have learned about DC circuits, but it is what happens at these frequencies.

If you are thinking of building this circuit, the part of the circuit marked with a ground symbol should be connected to some large metallic object (not necessarily actual dirt) for the antenna to work properly.

yungman
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#4
Sep5-11, 07:49 PM
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how antenna radiates?


I am currently studying antennas. This is half of the electric dipole. They assume it is on top of the ground plane or something close to it, the other half is the image. The way to calculate is to treat as if it is a dipole. The top half of the radiation pattern is exactly the same as if it is a real dipole but the bottom half is missing. Radiation power is half of the real dipole.

Regarding to where the current goes at the end, they use approximations by assuming current at the end is zero and charge is deposited at the end. This can work even if the dipole is very short( Hertzian dipole). For short dipole, it assume uniform current along the wire and actually model as electric dipole with opposite charges on either end(of cause the bottom is the image in your case). For longer dipoles, current along the wire is represented by different form depend on the driving input. Charge distribution along the antenna is very complicated and it is very difficult to get the exact expression even if it is only a wire. You have to give the exact dimension and the input current waveform before you can find the suitable approximation.

Yes, the explanation is a little out there, you can get more info in the EM book under image current and some antenna book. I am still studying this subject, so someone might have better explanation.
jim hardy
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#5
Sep6-11, 01:02 AM
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i'm real simple

What happens when you charge a parallel plate capacitor?
On one plate, electrons move closer together as more of them crowd onto that plate.
On the other plate, electrons move farther apart as some of them leave that plate.

So it appears that electrons inside a conductor can be somewhat compressed or rarified.

That opens the possibility of very brief direct current flow into one end of a conductor without any current flow out the other end.

Which i think was your question.

In a halfwave antenna electrons are alternately compressed and rarified at the ends while in the middle they just move back and forth a small distance. That's why current is highest in middle and zero at ends.

But............
there's people here who can continue the explanation better than i
it's just that i remember struggling with the same concept years ago.
and there's times when oversimplification is a necessary step to get our thinking straight.

old jim
david90
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#6
Sep7-11, 10:58 AM
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So if I take an antenna and I connect it to the positive lead (leaving the negative lead unconnected) of a function generator, there would be current in the antenna and RF radiation?
Evil Bunny
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#7
Sep7-11, 02:22 PM
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Quote Quote by jim hardy View Post
What happens when you charge a parallel plate capacitor?
On one plate, electrons move closer together as more of them crowd onto that plate.
On the other plate, electrons move farther apart as some of them leave that plate.

So it appears that electrons inside a conductor can be somewhat compressed or rarified.
Yes, but with the capacitor and a battery, for example, some electrons left one terminal of the battery and came back on the other terminal of the battery.

In other words, they had a place to go...

In this case they don't. So I'm having a hard time digesting that explanation.

And you must also be saying that if you connected a single wire to just one terminal of a battery that the electrons would rush to one end of the wire. Is this true?
Studiot
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#8
Sep7-11, 02:59 PM
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This circuit would operate at about 100 MHz and, at this frequency, one wavelength is about 3 meters. So, a quarter wave length is about 75 cm (29 inches).

If you put a 100 MHz sinewave at one end of a piece of wire that is 75 cm long, the voltage takes about 2.5 nS to reach the other end. While it is doing that, the input keeps changing, so the voltage at the end never catches up with the input voltage.

There is always a voltage difference between the ends of the antenna. The end is always 90 degrees out of phase with the input voltage

So, there is a current flowing between the two ends of the wire even though one end is open circuit. This current causes some of the input power to be radiated.

This seems alien to everything you have learned about DC circuits, but it is what happens at these frequencies.

If you are thinking of building this circuit, the part of the circuit marked with a ground symbol should be connected to some large metallic object (not necessarily actual dirt) for the antenna to work properly.

What a good simple explanation!

Evil Bunny
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#9
Sep7-11, 05:59 PM
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So... When you touch a wire to only the negative terminal of a battery, electrons rush from the battery terminal and on to the wire?

If that's the case, then if you had a long enough coil of wire, you could drain the battery by only attaching it to the negative terminal of the battery... All the available electrons would rush out of the battery and onto the wire.

This doesn't sound right...
vk6kro
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#10
Sep7-11, 06:33 PM
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Quote Quote by Evil Bunny View Post
So... When you touch a wire to only the negative terminal of a battery, electrons rush from the battery terminal and on to the wire?

If that's the case, then if you had a long enough coil of wire, you could drain the battery by only attaching it to the negative terminal of the battery... All the available electrons would rush out of the battery and onto the wire.

This doesn't sound right...
If you had the other terminal of the battery grounded, yes.

The wire would have capacitance to ground and charging this capacitance could partially discharge a battery. The capacitance and the battery would eventually have the same voltage on them. This charge is not lost, though, as you could recover the charge from the capacitor and do useful work with it.

There would also be some power radiated as you connect the battery to the wire. This rising voltage would have harmonics in it and these would be radiated and heard as a "splatt" on nearby radios.
jim hardy
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#11
Sep7-11, 07:30 PM
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"Yes, but with the capacitor and a battery, for example, some electrons left one terminal of the battery and came back on the other terminal of the battery.

In other words, they had a place to go...

In this case they don't. So I'm having a hard time digesting that explanation.

And you must also be saying that if you connected a single wire to just one terminal of a battery that the electrons would rush to one end of the wire. Is this true?
"

Well let's think on that.
vk6kro nailed it.

Would the electrons rush to one end?
Or would they all move a little closer to the far end to make room for the ones being squeezed in by your battery?
I'd say the latter. Think of people squeezing into a subway car.
And the current would be so minute as to be almost immeasurable.
But if you ever grabbed the end of a co-axial cable that was recently carrying high DC voltage you know that a long piece of wire can hold quite a few electrons.


Now - recall what is capacitance

epsilon X A/D
where epsilon is dielectric constant, if no dielectric it's capacitance of free space,,
A is area of plate,,
D is distance to other plate.

So a piece of wire with surface area 'a' has a small amount of capacitance to the rest of the earth.
If your wire were long enough to have huge surface area, like long enough to wrap earth like a ball of yarn, well yes it could probably hold all the amp hours in a small battery. The battery being small would need its electrons replaced via connection to earth as our friend noted.

But it is hard to conceive of such an arrangement.
sorry for the confusion.

But what is significant is that electrons can squeeze into the wire.
They can also be induced to shift back and forth in the wire, alternately moving away from one end toward the other.
Like all physical phenomena that one has a natural frequency.
It's approximately twice speed of light divided by length of wire.
Speed of light, eh? Must be an electromagnetic phenomeon.


vk6kro sounds like ham radio call - if so he is well versed in antennas.
rbj
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#12
Sep8-11, 12:00 AM
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consider that we are standing some distance apart and facing one another. think of yourself holding a large electric charge and i am holding another one of opposite polarity (and about the same amount). we are restricting the motion of our charges so that they cannot move toward (or away from) one another on the line that connects them. but they are free to slide about on the plane that is perpendicular to that connecting line.

so i move my charge to my right (your left) and since your charge is attracted to mine it also moves to your left. then i move mine to your right and your charge follows. then i move mine up and your charge follows it up. that is an EM wave.

if i move it left to right and back a million times per second, your charge follows. my charge is essentially a transmitting antenna and your charge is a receiving antenna. and we are both tuned to the middle of the AM band.

anyway, that is all that an antenna is; at the simplest level, it is a conductor where charge is allowed to slosh back and forth in it. for a transmitting antenna, that charge is forced to slosh back and forth by the transmitter electronics. for a receiving antenna, the charge is influenced by the moving charge in the transmitter antenna which causes that charge to slosh around (which sensitive electronics in the receiver detects).
Evil Bunny
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#13
Sep8-11, 06:44 AM
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I always thought the sloshing around was in a complete circuit... Up into the antenna from the source and back to the source on the shield of the coax.

In fact, this is how we assemble transmission lines. If your shield isn't connected properly to the connector at the end of the cable, your antenna does not radiate.
fleem
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#14
Sep8-11, 07:17 AM
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I've always felt it was a mistake to imply to newcomers to electronics that a complete DC circuit is necessary for there to be current (before they've been taught about AC). Rather, I'd prefer they be given at least a hand-waving understanding of capacitance by use of plumbing analogies. The OP shows a common question that arises because it is not taught this way.

Electrons move away from other negative charge and toward positive charge. If there are extra electrons on an isolated chunk of metal, then the extra electrons flow to the extremities of that chunk of metal because they are repelling each other. If another chunk of metal that does not have an excess of electrons comes in contact with that first chunk, then some of the extra electrons on that first chunk will flow to the second to get away from the extra electrons on the first chunk. So here we certainly have electron flow without a complete circuit. The electrons will also slosh a bit (move to and fro) for a short time immediately after those two chunks come in contact, depending on how big those chunks of metal are and what their electrical resistance is. The sloshing can be acounted for by the inertia of the electrons (but with electrons, that kinetic energy is in the form of a magnetic field). An antenna is just a relatively big chunk of metal designed such that electrons can slosh to and fro along its various elements such that it has a real (not imaginary) impedance to the transmit circuit--which means the energy given to it never returns to the circuit, so it must go somewhere (the receive antenna, and everything else in the universe). (On a side note, if there were nothing else in the universe to couple to the transmitter antenna, then there would be no radiation from it.) We've analyzed the process of radio waves leaving an antenna pretty well. We have a classical understanding (the waves leave one antenna and eventually encounter another receiver antenna) and a quantum physics understanding (there is a sort of non-local coupling between the electrons in the transmit and the receive antennas). Also classically, we see that one can describe the magnetic field as simply a relativistic electric field and vice versa. However, we still really don't know what the electromagnetic field is or whether our current picture of it is best for gaining a deeper understanding (not that its possible to ever know what anything really "is").
Evil Bunny
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#15
Sep8-11, 08:20 AM
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So... are there two different types of antennas out there? ( I mean, I know there are all kinds of different styles and sizes and shapes, etc. but that's not what I'm talking about here).

One type is just an open circuit with electrons "sloshing" (I like that description, by the way) around back and forth on a wire...

And the other type is a completed circuit with electrons traveling in loops, leaving the source, travelling up the antenna and returning to the source on the sheild of the transmission line?

It's not that I don't understand these explanations, they have been helpful, but it seems that we usually talk about "static electricity" as being something different than "current electricity". We've all seen how electrons attract and repel each other in physics lab with the glass and the fur, etc... so it's easy to envision your examples of these charges moving back and forth on a wire with these external influences.

But it starts to get a little blurry when you start to marry these two concepts together... When you study circuits, it is wise to always remember that whatever leaves the source MUST return to the source.

Capacitive coupling always rears its head in these discussions, because the answer always turns out to be that the source must be connected to ground and that is how it's returning. But in most cases it seems that we're not talking about having the source connected to ground, so it all seems a little... unsatisying, I guess.

EDIT: Here is an interesting article about Tesla and single line transmission.
rbj
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#16
Sep8-11, 12:43 PM
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Quote Quote by Evil Bunny View Post
I always thought the sloshing around was in a complete circuit... Up into the antenna from the source and back to the source on the shield of the coax.

In fact, this is how we assemble transmission lines. If your shield isn't connected properly to the connector at the end of the cable, your antenna does not radiate.
so, for a simple center-fed 1/2 wavelength horizontal dipole antenna, what is happening at the center where the inner conductor is soldered to the left half and the outer conductor (shield) is soldered to the right half? nothing else is connected to either halves anywhere along the element. does that count as a "complete circuit"?

Quote Quote by Evil Bunny View Post
So... are there two different types of antennas out there? ( I mean, I know there are all kinds of different styles and sizes and shapes, etc. but that's not what I'm talking about here).

One type is just an open circuit with electrons "sloshing" (I like that description, by the way) around back and forth on a wire...

And the other type is a completed circuit with electrons traveling in loops, leaving the source, travelling up the antenna and returning to the source on the sheild of the transmission line?

It's not that I don't understand these explanations, they have been helpful, but it seems that we usually talk about "static electricity" as being something different than "current electricity". We've all seen how electrons attract and repel each other in physics lab with the glass and the fur, etc... so it's easy to envision your examples of these charges moving back and forth on a wire with these external influences.

But it starts to get a little blurry when you start to marry these two concepts together... When you study circuits, it is wise to always remember that whatever leaves the source MUST return to the source.
it's the same set of physics. Kirchoff's current law is not the most fundamental physics. it is the result of an assumption that no charge can build up at any node.
sophiecentaur
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#17
Sep8-11, 12:49 PM
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There are, indeed two ways of designing an antenna. Some are based on dipoles (or monopoles with an earth) and the others are based on loops or even slots cut in large sheets (many mobile phone antennae are the slot variety).

In all cases there are currents flowing and voltages appearing at various points. So, for instance, the current along a dipole gradually tapers to zero at the ends ('cos it can't go anywhere beyond that). The essential thing is that they will radiate Energy (as em waves). To radiate any significant amount of energy, the antenna has to be a 'reasonable' size compared with the wavelength. Round about 1/2 wavelength is good but you can get a reasonable amount of RF power from an antenna only 0.1λ if you 'match it' right.
What has been written above is fair enough but it doesn't really describe how Power is actually radiated from an antenna. A transmission line and a Capacitor don't radiate any appreciable amount of power because the currents and volts are in quadrature everywhere (90 degrees of phase) and so can dissipate no power. In a larger structure, the phases are, actually, not exactly in quadrature - there is a Real component of VI and that is due to actual radiated power. 'Something' happens to the fields around the antenna because it takes time for the changes in the fields to propagate over the space it occupies. The E and B fields add together so that wave is generated that carries energy away. When you look into the feedpoint of an antenna, you actually measure a resistive component, called the Radiation Resistance so space actually acts like a resistive load- taking power away.
jim hardy
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#18
Sep8-11, 12:52 PM
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""So... are there two different types of antennas out there? ( I mean, I know there are all kinds of different styles and sizes and shapes, etc. but that's not what I'm talking about here).

One type is just an open circuit with electrons "sloshing" (I like that description, by the way) around back and forth on a wire...

And the other type is a completed circuit with electrons traveling in loops, leaving the source, travelling up the antenna and returning to the source on the sheild of the transmission line?.....""

EB nice summary!

Indeed that's so.
If you look in the back of an old high quality floor model radio you'll see a big (often square) loop of wire arranged around the cabinet. Usually it has several turns. That's your second type, 'completed circuit', antenna. It's called a "loop antenna".
It is the AM antenna, and is used because at frequencies of AM the length of wire required to achieve 'sloshing' at its natural frequency (that's "resonance") would be unmanageably long. So instead they use a tank circuit inside the radio to achieve resonance.

If you look at a classic TV antenna you'll see multiple horizontal metal tubes. They are your first type 'sloshing' antenna. Each is horizontal tube resonant somewhere in the TV broadcast band where frequencies are much higher, hence length is manageable.


Resonance in an antenna greatly improves its effectiveness.

That a simple piece of wire has an electrical resonant frequency is surprising when first encountered. My high school electronics teacher explained it to us boys using an analogy of kids in a hallway running back and forth, if you like i could try to recall it. It painted a simple mental picture that let me swallow resonance.

A more scientific explanation might start along the lines: a straight wire has self inductance, and its ability to comprress or rarify charge at its ends is not unlike capacitance. It is therefore capable of electrical resonance.

BUt there are much better theoreticians here than me so i'll not pretend to be an expert on antennas. Honestly i never quite grasped Maxwell's equations.

old jim


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