How does an antenna work?

[Wartex]

I always wondered, what is the fundamental principle of operation? I get the whole "AC causes electron movement which causes RF radiation", the problem is, every forum I've been to, the last, and always unanswered question is "How do you get current flowing" if antenna is *not in a circuit*, at least galvanically?

[Mechatrommer]

you need a physicist forum for this. i dont have an exact answer, just an imagination that electron movement is like ripple of water. in the antenna, its like a dead end tunnel where the ripple try to go out but then got reflected back. there will be no net electron movement, but this jiggling like motion, with proper tunnel/antenna length will create resonance and amplify the rippling hence electromagnetic force/wave. i imagine potential also is like pressure thats try to push and pull electrons in the antenna. maybe thats just my science fiction imagination.

[Tony R]

I'm not an expert in this area but I wish to point out that if you increase the voltage on an antenna, you add charge to it, the charge will want to even out thus they are moving, a moving charge is a current.

I do know that radio waves are due to accelerating electrons. acceleration means motion so they have to be moving, and the above answer is the only way i know of that you can get movement of electrons.

[Trigger]

Too much to type so...

http://youtu.be/7Sgtq-ncZAE

[johnmx]

What do you mean with: “antenna is *not in a circuit*, at least galvanically”? Between transmitter and receiver?

If you understand what causes RF radiation than you know the answer of your question.

[Mechatrommer]

http://youtu.be/7Sgtq-ncZAE

with the genius of its mathematical formulation, its still dont answer the common sense stuff, such as where the standing wave comes from? why the current moving at one direction and then another one at the opposite direction at certain points in the antenna? and it only explain dipole type of antenna, how about monopole?
edit: ok, moving electrons will create electromagnetic field (radio), thats noted.

[vk6zgo]

Wrong forum,try QRZ.com,but before you sign up & ask questions,browse through the Antenna Forum section.
You might find your answer there.
Also,Google for W8JI--Tom is a very knowledgable person,& his writings on Antennas are always worth reading.

See if you can find the ARRL handbook, the RSGB Manual,or ARRL Antenna Book at your library.
Although these books are for Amateur Radio Operators,they contain a lot of excellent information.

VK6ZGO

[the_raptor]

RF radiation is a series of alternating electrical and magnetic fields. An electrical field generates a perpendicular magnetic field and then collapses, the magnetic field generates a perpendicular electrical field and then collapses, and so on. Eventually one of those magnetic fields is generated close enough to an antenna that the new electrical field is generated ON (RF travels on the skin of a conductor) the antenna and thus can be measured to pull signal out. Making the antenna an antenna a specific portion of a wavelength (pretty much no one uses full wavelength antennas, in amateur use 1/4 wavelength is common) gives you selectivity because the fields of that frequency generated on and around the antenna won't cancel each other out as much.

RF is NOT AC in that you can not apply basic AC circuit theory and come out with an understanding of RF. Which I presume is the OP's problem. For RF you need to understand AC outside of wires and in the world of electrical and magnetic fields.

P.S. Mechatrommer. There is no such thing as a real monopole antenna. All "monopole" antennas are just dipoles which use a "ground plane" as the one half of the antenna. Which is why you can improve the send/receive ability of handheld transceivers by clipping on an extra bit of wire instead of relying on the users body being the ground plane. For non-mobile "monopole" antennas more copper is buried for the ground plane than is sticking up in the air.

[Wartex]

Hi Raptor, thanks for the explanation. I have a fairly good understanding of physics and I see the direction you are pointing me at. I will state what I understand and maybe you can clarify where I'm wrong.

So, here goes:

When there is a difference of potential between two points, i.e. variance in quantity of free electrons in the atomic structure and a conductor is applied between those two points, an electric field is propagated thru the conductor with a speed fractional to "c" (299997 Km/s), say for a straight copper conductor we will assume 0.75*c. Once the field is propagated, it interacts with electrons' own fields and repels them in the direction of propagation. The spin of moving electrons creates a magnetic field oriented perpendicular to the conductor.

With AC, we have same process plus inductance and capacitance of the wire. I noticed in some radio designs the transmitter would have a transformer at the end where the circuit is driving the primary and the secondary has one end earthed and the other going to the antenna. This to me looks like a huge LC circuit where the air between antenna and earth is a huge air capacitor, and I can imagine the electrons moving from antenna to earth and back, creating an EM field lines that start at the tip of the antenna and end at the grounding point, propagating into infinity.

The problem is that antenna would not produce any radiation if no energy is dissipated into it, so the currents must be induced in the antenna. At a minimum you need circular currents as in permanent magnets, and my understanding is that currents in the antenna are longitudinal and not circular. Ok, I'm fine with electron oscillations and zero net current, but there must be instantaneous currents - how the hell are they produced for example in solid-state transmitters where you have a 1 pin of a chip terminated with a copper trace? and I want too see a "picture" of EMF produced by a simple "stick" antenna.

[the_raptor]

Wartex, I am only just beginning my exploration of (amateur) radio so I don't know if I can answer to the depth you want.

It is my understanding that in a standard dipole even though the two arms of the antenna aren't electrically connected at their ends they are coupled by the EM field, which is what the "standing wave" in an antenna is. You don't have energy magically pumped into one element, the whole thing forms a RF circuit with part of the power that goes into the first element being returned  through the second (this is called the "Standing Wave Ratio", and the basic goal of antenna design is to get this close to 1:1).

To rephrase it slightly, the electrical current in the dipole elements and transmission line is only part of the RF circuit, the circuit is completed by the RF wave between the two elements of the antenna.

Also if the AC being feed into the dipole isn't of the correct frequency for the length of the antenna more and more current is returned down the feedline instead of "jumping off" into space as RF radiation. It is an amateur radio maxim that the antenna is more important than the transmitter. eg my main transmitter is rated at 10 watts, with the antenna I have set-up currently I doubt it puts out 1 watt of that as radiation.

The radiation pattern for a "perfect dipole" in "free space" would be an ovoid down the length of the dipole, with the greatest radiation going off the beam of the antenna and with the ends being nulls. However it quickly gets a hell of a lot more complicated with an antenna near the earth operating outside of the exact wavelength of the antenna, even ignoring parasitic elements etc.

Antennas are probably the most complicated AC subject that exists. Personally I only have a vague notion of how they work (I need the ARRL antenna theory book), and I think to have a good understanding would take several years of full time study.

[Mechatrommer]

...more and more current is returned down the feedline instead of "jumping off" into space as RF radiation...

this is even more confusing! how can a current jump off space? afaik current is opposite direction of electrons, how can an electron jumps in and out off space from an antenna?

[vk6zgo]

RF is NOT AC in that you can not apply basic AC circuit theory and come out with an understanding of RF. Which I presume is the OP's problem. For RF you need to understand AC outside of wires and in the world of electrical and magnetic fields.

A bit misleading,in that it  unintentionally perpetuates the common myth that there is something "magic" about RF frequencies.

In fact,a 1/2 wavelength dipole antenna at 50Hz will radiate just as effectively as one at 50MHz.
At such extremely low frequencies,antennas are impractically long,so this part of the spectrum is not used for this purpose.

Frequencies just above the audio region are quite commonly used,however,& some "hams" are at present transmitting
on 8kHz--inside the audio region, using a special experimental permit.

VK6ZGO

[Mechatrommer]

i dont see it answer the question vk6. we know the lower the freq, the longer the antenna, and all that audio band signal. we are talking antenna in general, no matter what frequency it is. but for simplicity, the freq should be high enough (practically), to avoid audio band and gigantic antenna size, it is also known, freq as low as 10Hz can be radiated, but surely that will be off topic.

[the_raptor]

RF is NOT AC in that you can not apply basic AC circuit theory and come out with an understanding of RF. Which I presume is the OP's problem. For RF you need to understand AC outside of wires and in the world of electrical and magnetic fields.

A bit misleading,in that it  unintentionally perpetuates the common myth that there is something "magic" about RF frequencies.

I was talking about the difference between AC in the world of wires and AC involving EM fields. Basic AC circuit theory ignores the fact that current in a wire generates a field which then generates a current in other wires (unless specifically talking about transformers and the like). It is like physicists using round cows and a frictionless surface to describe momentum.

This is mainly because until the advent of high speed digital circuits you could pretty safely ignore RF because wiring was unlikely to be long enough to function as an efficient transmitter at the frequency of operation of the circuit. These days that isn't true and so the RF spectrum is being drowned out by devices that have no business transmitting.

I should have probably used "radio" instead of "RF". I am used to thinking about "RF energy", but not necessarily meaning "radio frequency energy".

[Mechatrommer]

"How do you get current flowing" if antenna is *not in a circuit*, at least galvanically?

we can google all we like about radio, but try to concentrate on the above mysterious question. i think the OP meant by saying "not in the circuit" is the antenna is not connected to the classic 2 port device, such as +ve to -ve wire or network. i think this is deeper than a ham can comprehend. this is just as mysterious to me as i pointed out the monopole. ok there is another pole at the base, but how it does it work up to the antenna tip high up there? its like a magic remote telekinesis. and if you chop some cm of the monopole, then the radio performance will be affected.

[the_raptor]

...more and more current is returned down the feedline instead of "jumping off" into space as RF radiation...

this is even more confusing! how can a current jump off space? afaik current is opposite direction of electrons, how can an electron jumps in and out off space from an antenna?

Sorry, could have been more clear. It is the energy that is "jumping off" into space. I intentionally used a fuzzy description because I am not sure myself the exact mechanism that translates a EM field around a wire into an EM wave propagating through space. As I was talking about before, just running a current into an antenna isn't enough for something to be an efficient antenna. The wave length of the current has to relate to the length of the antenna or else the energy will just flow back down the other side of the transmission line instead of being radiated (and radio people worry about Standing Wave Ratio for this reason, most amateur gear starts to cook once SWR exceeds 1:1.5).

P.S. Also AFAIK your description of current is wrong as well. Although current is often described as the movement of electrons IIRC it is more energy transmitted via the movement of electrons. The pipe analogy of electricity where you are sticking electrons in one end of a pipe and they come out the other is a "round cow" analogy.

[the_raptor]

"How do you get current flowing" if antenna is *not in a circuit*, at least galvanically?

we can google all we like about radio, but try to concentrate on the above mysterious question. i think the OP meant by saying "not in the circuit" is the antenna is not connected to the classic 2 port device, such as +ve to -ve wire or network. i think this is deeper than a ham can comprehend.

I thought that was what I answered? 

There is an EM wave/field between the two sides of a dipole that completes the circuit.

[Mechatrommer]

I thought that was what I answered? 
There is an EM wave/field between the two sides of a dipole that completes the circuit.

no worry mate, i'm also as eager as the op about this mystery. i think there's misunderstanding between "current" and "electromagnetic wave". i believe they are both different.

[Bored@Work]

I always wondered, what is the fundamental principle of operation?

The honest answer is, mankind doesn't really know how an antenna works. We use models of different granularity to describe effects we have observed in nature. Some of these models are very good, so good that one can predict nature's behavior from the model.

But the more you drill down, the more complicate and abstract it becomes and in the end still no one can tell you what it really is. Others here have argued with field theory. Maxwell's equations should come up here, too. Good stuff. And where do the fields come from, what is their fundamental principle of operation? One could now dig out more theories. And start to ask physicist about even more and more details and fundamentals. Any honest physicist will at some point tell you

"This [insert incredibly big ball of math here] is current the best theory we have. It is a very tricky, abstract and complicated model. It takes years to learn it. Not many people in the world understand the model. It can explain a lot and we have some measurements showing the model might be correct. We are missing other measurements. And there are some who claim they have conflicting measurements. And we scratch our head about some aspects. No, we don't know the real fundamental principles of how all this works. Every time we think we have it, something new comes up."

In practice this means that you have to take a model you can still handle, i.e. one you math is good enough for, and make the best out from applying that model. And treat the rest as "well, that's the way it is".

Simple models for antennas are rules of thumbs. E.g. describing certain antenna types with a few simple formulas, because people figured out in the past that this is good enough for many practical purposes. Often by experiment.

[Wartex]

Haha -  so far, no answer.

My point is that to generate EM field you need current flowing, be it straight or oscillating, period. It's just physics.

We know that electric field is propagated to the end of the wire when it's connected to the voltage source ON ONE END. The current starts flowing and magnetic field is created when the other end is connected, unless E field is strong enough to push electrons over the gap between the end of the wire and the other voltage source terminal.

So, there must be some magic happening when direction of *electric* field is changed. I guess the change itself created currents.

[Zero999]

Haha -  so far, no answer.

My point is that to generate EM field you need current flowing, be it straight or oscillating, period. It's just physics.

We know that electric field is propagated to the end of the wire when it's connected to the voltage source ON ONE END. The current starts flowing and magnetic field is created when the other end is connected, unless E field is strong enough to push electrons over the gap between the end of the wire and the other voltage source terminal.

So, there must be some magic happening when direction of *electric* field is changed. I guess the change itself created currents.

The question has been answered.

All "monopole" antennas are just dipoles which use a "ground plane" as the one half of the antenna. Which is why you can improve the send/receive ability of handheld transceivers by clipping on an extra bit of wire instead of relying on the users body being the ground plane. For non-mobile "monopole" antennas more copper is buried for the ground plane than is sticking up in the air.

Let's go back to the original question:

I always wondered, what is the fundamental principle of operation? I get the whole "AC causes electron movement which causes RF radiation", the problem is, every forum I've been to, the last, and always unanswered question is "How do you get current flowing" if antenna is *not in a circuit*, at least galvanically?

The answer is the antenna is a circuit galvanically, not at DC but at the frequency of operation. In a dipole the current flow in the lower half of the antenna is 180^o out of phase with the top.

In a monopole the ground plane acts like a capacitor. which sources and sinks the current because it has self-capacitance, see link to Wikipedia below.
http://en.wikipedia.org/wiki/Capacitance#Self-capacitance

This means that at high frequencies, a circuit can flow between two objects which are not electrically in contact with each other and are too far apart for them to have mutual capacitance. You can see this by holding a metal object attached to a non-conductive plastic pols near a Tesla coil and a spark will leap towards it.

I hope you can see that a loop antenna is a circuit not note that at DC its impedance is near 0 Ohms, at the desired frequency it's within the design limits, from memory typically 300 Ohm for a standard loop. A dipole antenna has a near infinite impedance at DC but is 75 Ohms at the resonant frequency.

[Mechatrommer]

so, is that means, dipole and monopole are capacitor? and loop antenna is an inductor?

[Zero999]

so, is that means, dipole and monopole are capacitor? and loop antenna is an inductor?

That's true at low frequencies.

At the resonant frequency, they're purely resistive.

Above the resonant frequency a dipole is inductive and a loop is capacitive.

The difference is a dipole responds to the E-field and a loop responds to the L-field.

[Mechatrommer]

ok, googling leds me back to maxwell/faraday equation, looking at the symbols, i better surrender rather than having my days ruined. i will just cut or add my antenna when needed thats it!

[Wartex]

Hero999, are you stating that antenna has currents in it to create EM field because each wire of dipole is capacitively coupled to the other wire?



What about early satellites? They are not grounded in any way, in near vacuum, monopole stick antennas.

Answer me this: How do you generate non-zero-net current in a single wire?

What type of field does an antenna produce?

So far all the explanations are WHY antennas work, but not how.