RF basic question (sine)

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Hello

Could someone explain me why RF waves have 'always?' sine shapes?

This is my curiosity.

Thank you

Deez

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Square wave shapes have a wide range of harmonics, and when you working on RF, the wave need to be tunned in a frequency and depending the app the harmonics must be reduced to a low value.

Regards,

Bruno Muswieck

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Define an RF wave for us.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Imagine a weight hanging from a rope. Pull it back. Let it go. It speeds up heading down, slows down heading up, stops, goes the other way. The speed is a decaying sine wave. Add just a little energy and it sustains. This is the LC resonant tank circuit. The transistor that adds the little bit of energy to the tank is one pulse of a square wave, but the voltage in the tank circuit is a sine wave.

Imagecraft compiler user

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The fundamental linear resonator behaves nearly sinusoidally like a pendulum. Crystals, LC oscillators, etc, all produce near-sine signals.

Relaxation oscillators (multivibrators, etc) produce square waves.

For RF, specifically, you want to confine the information to as narrow a band as possible so that others cane use near-by bands (frequencies) without interference. Sine signals are very narrow band but square-waves and other non-sine signals occupy very broad range of frequencies.

Also, the basic receiver techniques are designed to receive one frequency or a very narrow band of frequencies. If you transmit using a square wave, all of the energy that goes into those other frequencies is wasted because the receiver won't pick them up.

Finally, it is really quite hard to make an antenna that will work effectively over a broad range of frequencies. If you TRY to send a square-wave, the antenna will really limit you.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
Finally, it is really quite hard to make an antenna that will work effectively over a broad range of frequencies. If you TRY to send a square-wave, the antenna will really limit you.

Think like the antenna is a "band-pass filter".

Regards,

Bruno Muswieck

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Great explanations. Thank you
By the way, when we drop something in a recipient of water we see the waves and they seem like sines. When we pull up and down a rope we could see the waves to. In the air, which is the component that generates that sine waves? How it can do that when the antenna is just a piece of wire?

Is there any (simple) experience we could do at home (using simple components (OEM)) to see this working?

I read at some months ago somewhere that if we make shock a coin in a 9v battery we produce low frequency waves?! But then we must use an old radio to capture them. Is it possible to make something 'by hand' (discrete elements) to capture that waves? Do yu know other kind of simple experiences like this one?

Thanks a lot

Deez
(Just a beginner and an entusiast of RF waves)

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Quote:
How it can do that when the antenna is just a piece of wire?

The electric & magnetic field strength varies sinusoidally. Of course you can't see that, but if you measured the value at a particular point and plotted it on a graph you would see a sinusoid vs time. If you took a snapshot and measured the field strength at different distances and plotted that you would see a sinusoid vs distance. This is all assuming the RF signal is a pure un-modulated sinewave and there are no other RF signals nearby.

Quote:
that if we make shock a coin in a 9v battery we produce low frequency waves?!

Yes, whenever you create a spark, you have generated an electric & magnetic field, which will consist of many frequencies (sinewaves) (Incidentally the electric & magnetic field emanating from the spark will not be very sinusoidal.)

In any case you would hear that on an AM receiver especially in the 2-30 Mhz. range.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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The spark gap generator might be useful reading material also.

http://en.wikipedia.org/wiki/Spa...

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Ross McKenzie ValuSoft Melbourne Australia

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Do look at the animated gif in http://en.wikipedia.org/wiki/Max...'s_equations

In vacuum these are linear equations, and the sum of two solutions is also a solution. Hence any arbitrary waveform could be built up from fourier superposition of sine waves. On the other hand any sine wave could be built up from the superposition of (a complete set of) arbitrary waveforms...

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Mathematically, a square wave can be constructed by adding sine waves that are odd multiples of the square wave frequency. This means that transmitting a square wave is the same as transmitting a sine wave at that frequency, plus a sine wave at three times that frequency, and another at five times that frequency, and so on.
To give you an example of what this would do, I can use a simple example. A transmitter sending a square wave at 29MHz, the middle of the 10m amateur radio band, would also be transmitting at 87 MHz, in the middle of TV channel 6, at 145 MHZ, in the middle of the 2m amateur band, and at 203 MHz in the middle of TV channel 11. Just from this example, you can see that the potential to cause interference to many other users of the radio spectrum is there, so users of the spectrum are required to as much as possible eliminate those harmonics. This means transmitting as pure a sine wave as possible.

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Great example! Thank you

DeezPa

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all this is understandable when comparing triangular, complex, square and sine waves and ok, sine waves are simpler to create and process...

What about amplitude constant waves like thoses generated by DC power suplies. These kind of waves are even simpler right? why not use them instead of sinewaves?

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Is a flat line really a "wave" then? Sounds like a pretty philosophical question to me!

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yes, it is a flat line but it could also 'encode' and transport information, right?

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DeezPa wrote:
all this is understandable when comparing triangular, complex, square and sine waves and ok, sine waves are simpler to create and process...

What about amplitude constant waves like thoses generated by DC power suplies. These kind of waves are even simpler right? why not use them instead of sinewaves?


So many reasons...
But one really important question, if all the radio stations transmitted using a "flat wave", how could the receiver discriminate between them?

Four legs good, two legs bad, three legs stable.

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Quote:

yes, it is a flat line but it could also 'encode' and transport information, right?


Then it would stop being a flat line and become a wave (probably a square wave I guess). Surely a "wave" is defined by a period/frequency? A flat line has no period/frequency and it cannot be used to convey bits of information while it remains flat.

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When perfect sine waves left the transceiveir, they are not sinosoids anymore...

With a constant amplitude signal we can transmite morse code, for example. So we can encode other types of symbols on it too.

So que question remains, why sinosoids and not constant amplitude signals?

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No, nobody transmits morse code with a flat wave. Not in the context of RF, anyway. Over wires, maybe.

Four legs good, two legs bad, three legs stable.

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"A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off. Information is carried in the varying duration of the on and off periods of the signal, for example by Morse code in early radio. In early wireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method from damped wave transmission."

in http://en.wikipedia.org/wiki/Con...

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When perfect sine waves leave the transmitter, they DO remain sine waves. Why do you think otherwise?

Morse code is NOT transmitted by constant amplitude signal. Constant amplitude can only carry information by varying frequency. Morse code IS transmitted by varying the amplitude.

"Continuous wave" does not mean constant amplitude. In the very early days (meaning spark gap transmitters), the signal was a series of damped (ringing) impulses. On a receiver, they sound like a "buzz"; the buzz frequency was at the impulse repetition frequency. But, the RF was the ring frequency. Continuous wave refers to a signal that is steady, without the ringing impulses.

BUT "continuous wave" does not mean "constant amplitude". Morse code involves changing the amplitude in the "dot-dash" pattern. Mose code is transmitted using a kind of amplitude modulation. It is 100% modulation because the carrier is completely turned off at times (between the dots and dashes).

I really think that you are confuses about your terminology.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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DeezPa wrote:
"A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off. Information is carried in the varying duration of the on and off periods of the signal, for example by Morse code in early radio. In early wireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method from damped wave transmission."

in http://en.wikipedia.org/wiki/Con...


OK, you win.
Since DC could be interpreted as constant frequency...

I give up.

Good luck with your DC radio communications.

Four legs good, two legs bad, three legs stable.

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I am really still trying to figure out what it is that the OP wants to know!

The original question is about "radio waves". One of the critical things the OP does not seem to understand is that a propagating wave consists of two components, a magnetic component (H-field) and an electric component (E-field). These can, in theory, take on any single-valued function with time (just as an ocean wave is not sinusoidal). Radio waves, physically, DO NOT HAVE TO BE sine waves.

Until recent (last 20 years or so) efforts at wide-band communication, the technical focus has been on "single-channel" communication (AM, FM, SSB, CW, and such). For these classical communication modes, the desire is often to convey as much as possible in as little bandwidth as possible. The carrier is always sinusoidal (because we know how to make that very purely). Modulation of any kind (even Morse Code) increases the bandwidth. Low word-per-minute Morse can occupy less than 100Hz bandwidth. FM as used in broadcast has a bandwidth of around 150KHz. It is a pure sine of constant amplitude and varying frequency.

The technology of receivers and transmitters of this era favors sinusoidal signals. So, that is what has been used. A sine is easy to generate. A sine can be easily amplified at a single frequency; wide bandwidth is much harder. A sine can be modulated quite easily. A transmitter can output a sine (single frequency) easily. The antenna matching network is far simpler with a sine signal and so is the antenna. The same holds true all through the receiver. Thus, for purely practical reasons, we use sine waves.

But, recent efforts at very high data rate ultra-wide-bandwidth (UWB) actually transmits non-sinusoidal signals. The transmitters and receivers are very difficult to design and build at this point, but they will become easier as we learn HOW to actually do it. Just as it was, in the very early days, when the shift was made from spark gaps to vacuum tubes.

So, NO, pure sine waves are not the only way radio waves happen. But, now, mostly, pure sine waves ARE what we use.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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DeezPa wrote:
all this is understandable when comparing triangular, complex, square and sine waves and ok, sine waves are simpler to create and process...

What about amplitude constant waves like thoses generated by DC power suplies. These kind of waves are even simpler right? why not use them instead of sinewaves?

How do you impress information on a DC "waveform."

Ignoring that detail, what's the wavelength of a DC power supply waveform? It goes to infinity with time. That being the case, what will you use for an antenna? Any finite length antenna will have an efficiency of 0% when fed with a signal of infinite wavelength!

I'm not sure exactly what you're trying to understand, but it seems to be related to why sine waves show up so often in electronics and for that matter, pretty much all of physics. The answer is basically that the solutions to the differential equations which characterize the behavior of linear systems, are of a sinusoidal nature.

Greg

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Dear friends, thank you for your posts...

At the beginning, I was searching the reason why RF communication is always based on sinewaves and not triangular or squares, etc. I got it! Sine waves are simpler to build because, at the limit, they are just one frequency and a triangular wave is make of lots of frequencies with different amplitudes. I got it.

But sudently, I remember the DC current which produces a signal of type: y=const.

I think this kind of signal (y=const) can carry info by modulating it in i)amplitude or ii)turn on and turn off the singal for different time intervals (modulation in time? I dont know the name.).

Now, my doubt is: having a sinewave and a y=const signal to choose, why the sinewave is always used and not y=const signal? y=const is also easy to build, i think.

(When I said that when a signal left the emitter it is not a pure sinewave anymore because the pure sinewave was modulated before transmition. I also suspect that non human made device can produces a 100% pure sine wave, but very close to it)

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Have you considered taking up knitting?

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Or Cardiology?

Four legs good, two legs bad, three legs stable.

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You clearly need to understand about electromagnetic wave propagation. And, me thinks, a crash course in modulation would also help.

Lacking that, why not try Rocket Science or Geology?

To partially answer your question. a constant voltage cannot be "modulated" because it is no longer constant. You have a contradiction in terms. Try thinking a little, please!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Brain surgery might be a little easier...
http://www.youtube.com/watch?v=r...

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"There are no wrong questions, just wrong answers..."
I listen to it very often...

I said to you that I have no skills on electronics but i would like to learn more about it. So, it is normal, i think, so say things that could not make sense.

When I refered constant signals (y=const) I was thinking in using DC current to produce a way to transmite info. Now I guess that the right name for that is transmit using static (e.g. spark plug).

But nowadays, radios use continuous sine waves to transmit info...

By the way, finally I found the complete answer why sine waves are used.

Thank you anyway.

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Well, then, lets refer to some basic principles.

First, lets assume that we are talking about waves rather than something confined to a "wire". That assumption seems to fit best your original question.

The thing that you need to think about is: just how do "signals" get from one place to another? We now know that this happens due to electromagnetic waves that propagate outward from the transmitting point.

What is it that propagates? Once established, a "DC" field does not propagate, it just sits there. We have methods for detecting DC fields and measuring them. But, if YOU devised a way to establish a DC field, your measuring apparatus could not distinguish it from the normal background static (ie, DC) field that forms in the atmosphere. That varies from a few V/m to a few 10s of V/M though, inside a thunder cloud, it can be much higher.

What we CAN detect and distinguish from the background field are changes in the field. Changes in the field propagate (at approximately the speed of light). What is the easiest way to change it? Sinusoidally(!) At a specific frequency, it is possible to filter out all of the other possible fields that propagate hither and yon (a highly technical term).

Lets try an analogy. Suppose that we have a really big flat pan that is partly full of water. The height of the water represents a DC field. Lets the pan settle for a while and protect it from the breeze and it will be flat (DC). How are you going to transmit information from one side of the pan to the other? You can add water (make the field stronger). the observer senses only one event, when the water rose, but nothing after that, because the water depth becomes constant again.

Now, suppose that you have a stick and you stand at one corner an put the stick in the water, then move the stick up and down. Waves form. The observer can easily detect these waves. And what sort of waves are the? If they are very low waves, they are sinusoidal? Could you make square waves in the water, or triangle waves? I doubt it. But, you can very easily make sine waves.

Next, you can vary how fast the stick is moved up and down. That is frequency modulation and the observer can easily detect this by measuring the distance between wave crests at the other side. Or, you can some times move the stick just a little and other times, move it much. The observer can detect this by measuring the height of the waves (AM). Or, you can wiggle the stick, then stop, then start again, and so forth, to send Morse Code (another kind of AM).

Perhaps this will help your understanding. Please, however, recognize that the analogy, above, is only approximate. You should not consider it a precise model of how real E or M fields behave.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thank you Jim, your analogy is great and shows the big picture!

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To expand this analogy just a little, you previously referred to undamped waves vs damped waves. The damped waves were generated in the VERY early days of radio using a spark gap and a tuned circuit.

In the water analogy, consider what happens if you drop a small stone (pebble, rock) into the water. It is like the spark., You generate ripples that travel outward. The ripples represent the frequency of the signal. If you watch at any point out in the water, there will be an initial "wave" followed by additional waves with decreasing amplitude (damped). You get one set of these waves for every stone.

To the listener with a modern radio receiver tuned to the wave (damped carrier) frequency, you hear a sound for each spark. If the sparks happen often enough, there is a "buzz" sound. If you turn the power to the spark gap on and off, the buzz goes on and off, making it possible to send Morse Code.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Another way of looking at it, you could do DC communication by turning on and off the current in a solenoid, and detecting the change in magnetic field at faraway points. This is essentially amplitude modulation at 0 Hz. But everyone else transmitting at the same time will cause massive interference, there is nothing to separate your signal from the others.

One solution is to use separate channels by modulating at 10KHz, 20KHz, ... up to many GHz. As long as you don't "over modulate" your base frequency, receivers tuned to adjacent channels will not pick up your signal, when averaged over many RF cycles.