how is a radio frequency produced by a wire
Reach
Posts: 107
I would gOOgle this but cant seem to use the correct words to describe my lack of understanding.
I am having trouble fully understanding how a radio signal is made from a wire or the antenna.
Lately I have been studying FM transmitters and think I know enough about how they work except the antenna part. I try to imagine a current jumping off the the wire - I know ts not how it works but can someone open my minds eye to how it does? I also think that the antenna length is critical to the transmission as in 1/2 wave, 1/4 wave etc.
I am having trouble fully understanding how a radio signal is made from a wire or the antenna.
Lately I have been studying FM transmitters and think I know enough about how they work except the antenna part. I try to imagine a current jumping off the the wire - I know ts not how it works but can someone open my minds eye to how it does? I also think that the antenna length is critical to the transmission as in 1/2 wave, 1/4 wave etc.
Comments
If that explanation seems too simple, please feel free to browse the equations on this page:
http://en.wikipedia.org/wiki/Li
You need to be happy with ideas like:
a) Moving electrons, a current in a wire, create a magnetic field.
b) A varying magnetic field can move electrons, create a current in a wire.
c) Which ultimately leads to the idea of electromagnetic radiation, radio waves, where waves of varying magnetic field create waves of varying electric field, which create waves of varying magnetic field......a traveling EM wave.
d) Learn about capacitors
e) Learn about inductors
f) Learn how an inductor and a capacitor wired in parallel can resonate at some frequency as energy moves from magnetic field in the coil to electric field in the capacitor and vice versa.
g) Which leads to the idea that the dipole antenna is like an inductor in the middle with capacitance between the ends, hence it resonates.
Quite a long road, especially if you want to get the maths behind it all.
http://en.wikipedia.org/wiki/Electromagnetism
antenna wire to oscillate. Oscillating charges always radiate, but nearby ground planes and
other conductors contain free charges that can respond by oscillating out of phase and cancelling
the oscillating field at large distances. This is called "near-field". Remember charges like to balance
out if they can and cancel.
A wire sticking out into space has no such nearby conductors to permit this so that the oscillating
field propagates efficiency outwards to distance objects - the so called "far-field".
A slightly more sophisticated way to look at things involves understanding the Poynting Vector, the
direction of energy flow in an electromagnetic field - its tends to run parallel to conductors, but at the
end of an aerial it has nowhere else to go but outwards... (Well its always at right angles to both the electric
field and the magnetic field in free-space, if you draw the fields around an antenna you'll see it flows
outwards, whereas with two parallel wires (transmission line) it just runs along parallel to the wires.
Charge up a big electrolytic capacitor and short it through a wire. Hear and see the spark.
Wind a coil, connect it to a meter and move a magnet rapidly nearby it. See the volts induced.
Charge a big, non-electrolytic, cap discharge it through a big inductor. Watch the voltage across the cap oscillate on a scope.
Mess around with transformers and signal generators.
Build an amateur band receiver and sling up a dipole antenna in the yard.
Perhaps move on to a transmitter. You might need amateur radio operators license to do that but studying for the license is a good way to learn how radio works.
http://www.ebay.com/itm/Elements-of-Radio-By-Marcus-Horton-HB-1943-Illustrated-Complete-Edition-/290990388961?pt=Antiquarian_Collectible&hash=item43c060f6e1
Excellent question!
The thing that jumps off the wire is a photon. Millions and billions and trillions of them.
I remember studying this at school and not really understanding it, and then making radio gear and getting a feel for the practicalities, and then going back more recently and studying the theory. It is fascinating to study.
Photon http://en.wikipedia.org/wiki/Photon
Take the first sentence "A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation".
I used to think of photons as being just light, but the entire electromagnetic spectrum, from low frequency radio to ultra violet and beyond are all just photons of different sizes. A light photon is tiny. The photons I find most fascinating are around 1Ghz, because they can fit in your hand. You can ask questions about something that fits in your hand.
What is a photon? I read that article, and reread it, and I think I am getting the hang of it. It is a wave, and a particle at the same time. It has a magnetic and electric component - oscillating between the two at a characteristic frequency. It travels at the speed of light. You can make them and receive them with antennas and in other ways like electrons jumping between orbitals. They carry a fixed quanta of energy. So yes, they are like little currents jumping off the wire.
But I only understand a tiny bit of it.
For instance, the clever physicist boffins tell us that photons have wave-particle duality. For tiny photons of light, they are so tiny one can sort of conceptualise them as both tiny waves and tiny billiard balls. But for a radio photon 18cm long, where is the particle? In the middle? At the end? Moving around? All over the universe it both time and space?
Or for a more practical question, take antennas that focus. A parabolic antenna makes sense - it can focus radio waves the same as light waves. But take a Yagi antenna - which can also focus. Take a Yagi that is transmitting - the photons going backwards get reflected and now go forwards - that makes sense. But what about a Yagi that is receiving - it can behave like a big parabolic antenna. So a photon that hits the Yagi head on gets collected. A photon travelling just one wavelength outside of the capture range seems to get caught by its tail (it is a wave, right?) and is bent so it is captured by the second element. This seems to bend other photons too, so they get captured by subsequent elements.
I have built Yagi antennas - they can be as simple as a piece of wood and lots of bits of wire of certain lengths and spacing. 2.4Ghz ones work well as the bits of wire are short so it is easy to add multiple elements. And they are amazing at focussing and collecting RF energy. But I really don't understand how they work!
So many unanswered questions too. What is the 'shape' of a photon? A sine wave? Several sine waves? Hmm - http://physics.aps.org/articles/v5/86 - it seems the shape can be anything you want it to be.
Keep experimenting. Build some antennas - they are great fun to play around with.
You have to be careful flipping backwards and forwards between the Maxwell equation (classical) view of electromagnetism
and the quantum electro-dynamics (QED) view - Maxwell's equations can't explain the photoelectric effect or semiconductor
bandgaps, for instance, but explains most other useful phenomena much more intuitively than QED.
I think the best approach is Maxwells for most things, then using the knowledge that QED acts like the classical theory
in the limit (outside of semiconductors, superconductors etc), talk about photons and virtual photons only when necessary
The ideas of photons comes from the idea of "quanta" of energy. That is to say when you are transferring energy from place to place via a electromagnetic radiation you find that the amount of energy you can move comes in discreet amounts, "quanta". You can't move an amount of energy from point A to point B that is a fraction of a quanta, the energy of a photon.
What is the energy of a photon? Turns out to be related to it wavelength: E = h * C / L.
Where h is a constant (Planks constant) and C is the speed of light.
For Dr_A's 1GHz radio waves we have a wave length of about 1 foot or 0.3 meters. And a resulting photon energy of 6.6261e-25 Joules.
That number is so tiny you are never going to see quanta effects with radio waves!
How big is a quanta of radio wave, or a photon of radio? One could argue, as Dr_A has said that it is somehow as big as the wave length. 1 foot at 1 GHZ. Many people would say that as well.
I disagree. I think that with a high intensity signal you could pack that 6.6261e-25 Joules of a single photon into a small space, like a few feet. But what if you are producing a very low intensity signal? Then the energy could take many cycles to accumulate to 6.6261e-25 Joules, perhaps miles. Of course at the receiving end you would not know if the quanta was spread out over miles or not, you would only see your antenna "pinged" into oscillation at some time or not. (Or something like that).
It really does not make any sense to think of the size of a photon.
That's before we even get onto the "wave-particle duality" thing.
The basic question is "How does a varying current flow in a wire cause radio waves to be launched from the wire"?
Assumptions:
1. Current flow in a wire is essentially not electron flow, it is charge flow.
2. A current flowing in a wire causes a magnetic field to surround the wire and theoretically extends to infinity.
3. This current flow stores energy in the magnetic field.
4. If the current varies the magnetic field varies proportionally in intensity.
5. If the rate of change in current is relatively slow the energy stored in the magnetic field is not lost and is totally recovered as the current goes back to zero.
Now for the hard part.
The magnetic field fills a volume around the wire.
However, the filling is not instantaneous, the filling rate is limited to the speed of light.
So if the current varies rapidly some of the energy in the field can't be recovered in time before the current changes polarity. The radiated portion is 90deg out of phase with the current flow.
This is in essence how dome of the energy stored in the magnetic causes a radio wave to be launched.
To increase the efficiency of launching the radio wave practical antennas strive to enhance the field intensity while being physically small. Other desirable attributes are things like sending the wave in one direction, tuning to exclude undesirable frequencies, and polarization of the wave.
Duane J
Thanks for the AMAZING responses. I'm going to read these over and over till it sinks in clearly then ill be back with questions again. I can digest all of this material independently but to comprehend it fully will take me a bit of time.
Thank you all so much
If you studied light, refraction, and reflection in school then you'll remember that light slows down and changes direction when it enters a different medium. This effect is the same for all electromagnetic waves. So, when electricity moves from the wire to the air it is effectively transitioning through mediums like light when light goes from the air into water.
If the impedance is not matched between the two different medium then the electromagnetic wave is reflected. The amount of reflection will vary depending on how badly the impedance is mismatched.
The reason why the antenna needs to be 1/4 the wavelength of the signal is so that destructive interference from the impedance mismatch between air and metal is minimized. If it isn't 1/4 the wavelength then the reflections that occur when the signal tries to travel from metal to air will destructively interfere killing the signal power.
Think about different sine waves with different phases adding together, if the reflections produced have a certain phase they don't destructively interfere with the signal. This works as long as the impedance coupler is a certain length (the antenna).
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If you need more information about this Google "anti-reflective coatings" for cameras and you'll find some articles about which talk about the problem in terms of light. However, its the same for all electromagnetic waves.
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One more, thing, all electricity traveling through metal produces electromagnetic waves, if you toggle a microcontroller pin at 1 KHz then that pin will produce a 1 KHz electromagnetic wave (along with many other frequencies due to the fact that the pin does not generate a pure sine wave). To transmit the signal effectively, all you need to do is make sure that you have an amplifier (to boost the signal power) and a good antenna.
Of course, then you have to receive the signal too which brings up a bunch of other issues. RF work is all about how to receive the signal is the sea of noise of the real world.
To make construction more compact, 1/4 wave length antennas are often used.
Ham Radio had the appeal of having short wave frequencies, and band were defined at 10 meter bands, 20 meter bands, and so on.
SO you see there is a physical correlaton of wave length to frequency. If a transmitting antenna is not properly constructed to the transmission frequency, a huge amount of energy is wasted. The antenna is creating a standing wave that propagates into the atmosphere. It is not as though all the energy just jumps off the tip of the antenna... it is the whole length of the antenna and its physical configuration that propagates the transmission into the earth's magneto-sphere.
Of course that all deals with only the Carrier frequency... we add information by modulating the carrier frequency .... such as AM or FM. ... and there are newer more complex schemes.
Reception antenna are not subject to the same precision at transmission antenna. Essentially, they have a filtering system that will tune into a particulary carrier frequency and ignore the rest of the background.
http://en.wikipedia.org/wiki/Dipole_antenna
That is nice in terms of analyzing "lumped circuits" where we assume the inductance and capacitance are located in the coil and capacitor components.
In a dipole antenna we can start to think that the length of the wire is an inductor, after all it is part of a circuit just like a coil. and we can see there is capacitance between the ends of the antenna. Both C and L are distributed over the whole antenna.
When you get to frequencies where wavelengths are approaching the physical size of your circuit you find the "lumped" analysis falls down. The whole thing becomes more like a wobbly jelly where every part affects every other part by electric and/or magnetic coupling.
Then:
http://www.youtube.com/watch?v=hUJfjRoxCbk
, or
Here's a great look at impedance: (perhaps watch this one first...)
http://www.youtube.com/watch?v=DovunOxlY1k
Go deeper conceptually with more Feynman:
http://www.youtube.com/watch?v=xdZMXWmlp9g
Another thing that is fascinating - electromagnetic waves have electrical and magnetic components, and there are antennas that work on the electrical part, and ones that work on the magnetic part. (eg the ferrite loop antenna in an AM radio).
addit - heater said
Which is why it is intriguing to see the clever physicist boffins doing just that - eg http://phys.org/news/2011-02-breakthrough-photons-microwave-frequency-range.html with beam splitters where it has to make a decision which way to go and it can't go both ways. I bet all the quantum weirdness still applies.
Yes... yes... the antenna creates electro-magnetic radiation.
And the carrier frequency is driven by an oscillator of some sort that is ideally a sine wave (as that ideally has no harmonic content), modulation is inserted, and then it is passed into a power amplification stage that outputs some genuine power ( a 5 watt, a 100 watt, or a kilowatt transmitter).
If the antenna is not constructed to handle the power of the standing wave, the power stage is likely to self-destruct due to negative feedback.
It is not merely electrical radition (I guess that might be sparks and not tuned to a particular frequency.).
Current jumping off a wire would be sparks and just create a very wide spectrum of noise. It is not that.. it is subtle... like a precise tuning fork that adds a hum to the background noise in a room.
Reach asked a simple question in the title of this thread, "how is a radio frequency produced by a wire ".
I paraphrased that question more precisely in message #11.
The basic question is "How does a varying current flow in a wire cause radio waves to be launched from the wire"?
Most seems to be fixated on the practical methods of how to make antennas work, however this doesn't answer Reach's question.
I attempted, in message #11, to give the underlying physics of how a radio wave actually gets launched.
Did I not give a good enough explanation?
Was it understandable?
Duane J
Duane,
Your explanation sounded good to me, but do you think you could elaborate some on the following? ....
Charges, though, do move near the speed of light. An example is charge flow in RG-58 coax cable of about 0.6c.
Another way to think about it is like a wave on water. The wave propagates quite fast yet the actual water remains in the same location it was before the wave past.
Another way to think about it is there is an exceedingly large number of electrons in the cross section of a wire. If ALL the electrons move this would be millions of amps in even small wires.
Anyway, I wanted the reader to think of charge motion instead of electron flow.
BTW, my explanation of the launching of radio waves was based on current flow in conductors.
The complement to this is changing charge voltage. A voltage also presents a voltage field that fills a volume on to infinity. As with the magnetic field, that doesn't move instantaneously, the voltage field movement is limited to the speed of light.
A rapidly changing field can launch a radio wave the same way because some of the voltage field's stored energy can't be recovered when the voltage changes polarity.
Duane J
So would it be safe to say that it's kinda like if you have a 10-foot pipe full of marbles, and when you cram a marble into one end of the pipe, another marble pops out the other end? The marble you crammed into your end of the pipe did not move the entire 10 feet, but you still get the "effect" of a single marble moving that length?
I'd clarify that by saying electron "drift velocity", but can be a fair bit higher - something like 1 mm/s is the normal
speed for currents in energy transmission where the cost of the copper has to be considered. The tiny interconnect
wires in ICs probably carry far larger current densities and thus drift velocities.
Here are four speeds to compare:
drift velocity, responsible for currents at the macro level, order 10^-3 m/s http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html
speed of electromagentic waves _inside_ the conductor, controlled by conductivity and permeability of the metal and frequency, perhaps 10^4 m/s or so at 1GHz for copper - though this is normally thought of as a large complex refractive index value, or as the skin effect. http://www.renardson-audio.com/sd.html
electron fermi speed, determined by conduction energy bands and quantum mechanics, order of 10^6 m/s for copper
speed of electromagnetic waves on the surface, controlled by the insulating medium above (such as air, plastic) order of 10^8 m/s
The fermi speed coupled with the mean-free-path for electrons scattering in the metal determines the ohmic resistance, as electrons
accelerated in an electric field give up (on average) their acquired energy and drift at each scattering, the more frequently electrons
scatter, the higher the resistivity, the more free electrons the lower the resistivity. Superconductors have bound electron pairs that
don't scatter at all (because two fermions bound tegether act as a boson), these carry current with no resistance. In metals lower
temperatures mean less scattering. In intrinsic semiconductors lower temperatures mean vastly fewer free electrons (and holes),
which dominates the scattering. Doped semiconductors behave like metals until high temperatures when the thermally generated
electrons and holes overtakes the doped ones. Diamond only starts to generate electron-hole pairs significantly at temperatures
well above ambient, and is hard to dope, but people are starting to make diamond semiconductor devices and operation at 500C
is feasible.
How does anybody know what speed actual electrons travel down a wire?
All electrons are the same. You cannot tell what you put in one end of a wire or what comes out the other end.
So how can you know the "drift velocity" of a single electron?
Your explanation is excellent and the bit quoted is a really interesting way of looking at it. So (as per your explanation and that link I posted earlier) you might pump 1W into an antenna and over a number of waves, you build up energy stored in antenna to 100W, which oscillating back and forth at the resonant frequency of the antenna which is matched to the frequency you are feeding in. And each cycle, 99% of the energy goes back and forth, but 1% can't be recovered. It is not lost through resistance. But it has to go somewhere and is thus lost as radiated energy.
Am I on the right track?
@heater,
Solve that and you can win a Nobel prize. But sorry, the prize was already claimed in 1923.
i) Find out the unit charge of an electron http://en.wikipedia.org/wiki/Oil_drop_experiment
ii) Do some maths - I think it is something like this iii) And in a very practical demonstration, set up a circuit where part of the current goes through ions in a liquid that change colour, and watch the colour move http://www.schoolphysics.co.uk/age16-19/Electricity%20and%20magnetism/Current%20electricity/text/Free_electron_motion/index.html
And this goes back to Reach's question - the above is for DC current in a wire. For AC current going to an antenna, the electrons move back and forth but there is no net electron flow. So no electrons ever get 'lost' from the antenna. It is something else (photons) that are radiated.
Thanks. You reminded me that I actually measured the charge on electrons by Milikan's oil drop experiment back in university. It was a bit mind bending to realize that the movement of those oil drops I was watching through microscope was due to the presence (or absence) of single electrons.
Now it looks like I need to find some potassium permanganate...
This sites gives a good explanation. http://www.intuitor.com/resonance/radioTVres.html
Be careful.. I believe that potassium permanganate is a highly concentrated laxitive.
@pliers
Yes indeed. antenna design and construction is high art and a very serious engineering topic in its own right. Some engineers spend their whole lives just working on antennas.