Polarity of an Electromagnetic Wave

[jaydnul]

I have asked a form of this question previously: https://www.physicsforums.com/threads/electromagnetic-waves-and-polarity.857347/

...but have come back to it in slight confusion.

Say I have this antenna and the voltage source is increasing with a constant acceleration of its voltage. It was concluded in the previous thread that it would just emit a singular pulse and not an electromagnetic wave since there was no defined frequency. I am curious however, a singular pulse of what? Photons? If so, what are the photons frequencies?

 

[Drakkith]

Forget everything you know about photons. Forgot you ever even heard the word "photon". It will only confuse you when trying to learn about EM waves at this stage.

Classically (as in classical E&M) an EM wave is a propagating disturbance in the electromagnetic field. This disturbance causes the electric and magnetic field vectors (the values at every point in space that determine the direction and magnitude of the force a charged particle would feel at that location) to oscillate in direction and magnitude at the frequency of the passing wave. These vectors are what is "waving" in an EM wave.

As far as I understand it, a pulse is essentially the combination of a large number of different frequencies summed together in such a way as to constructively interfere where the wave amplitude is large (which is why the amplitude is large in the first place) and destructively interfere everywhere else, causing the amplitude to approach zero. So it's not that a pulse isn't a wave, it's that it's the combination of many waves of different frequencies and amplitudes.

 

[jaydnul]

As far as I understand it, a pulse is essentially the combination of a large number of different frequencies summed together.

This is where my confusion still remains, and I think I have formulated a way of asking that will answer it.

Say the voltage source is given as $v(t) = sin(2\pi t)$ .

Now let the voltage source run for half a second (i.e. just the positive swing). This would be a pulse and generate radiation with a wide band of frequencies?

Now let the voltage source run for a full second. Now this would somehow produce only radiation at 1Hz?

 

[davenn]

Now let the voltage source run for half a second (i.e. just the positive swing). This would be a pulse and generate radiation with a wide band of frequencies?

1/2 a sec is a long time in EM, there could be 1000's of cycles depending on the frequency of your AC generator

what you have shown isn't really an antenna ... it's just part of the wires that are already connected to the ends

Now let the voltage source run for a full second. Now this would somehow produce only radiation at 1Hz?

as I stated above, it is dependent on the frequency of your generatorto answer your thread title ---- "
Polarity of an Electromagnetic Wave "

the polarity of the EM wave leaving an antenna is determined by the polarity of the antenna, be it horizontal or vertical
2 most common modes. Circular polarisation is also used , particularly for ground to space and visa versa.
The polarity is the polarity of the E field (electric field) rather than the B (magnetic field) Dave

 

[jaydnul]

1/2 a sec is a long time in EM, there could be 1000's of cycles depending on the frequency of your AC generator

what you have shown isn't really an antenna ... it's just part of the wires that are already connected to the ends

as I stated above, it is dependent on the frequency of your generator

...ok

We can have the frequency wherever, and an actual antenna/transmitting circuit. My question is this: If we only allow the positive swing of the sinusoidal input to the transmitting antenna, this emits a wide band of EM radiation? But if you then allow the fulll swing, it will create an EM wave at only that specific frequency?

 

[davenn]

to show your schematic correctly

this is the way it could have been drawn more correctly

cheers
Dave

 

[jaydnul]

to show your schematic correctly

this is the way it could have been drawn more correctly

View attachment 209640cheers
Dave

Haha you there we go.

So my question still stands with this correct antenna circuit. If we only allow the positive swing of the sinusoidal input to the transmitting antenna, this emits a wide band of EM radiation? But if you then allow the fulll swing, it will create an EM wave at only that specific frequency?

 

[f95toli]

No, you can never in reality create a wave at a single frequency; the mere fact that you at some point have to turn your source on (and at some later point off) means that every (real) signal is actually a pulse of some sort. Of course. the longer your pulse, the more "ideal" your waveform (closer to a perfect sine) will be.
A pulse that is only one wavelength long is still very short and the spectra will contain lots of other frequencies (depending on the rise-time of your pulse),

Note that this has nothing to do with physics as such; it is just a consequence of how Fourier analysis works, i.e. it is math.

 

[sophiecentaur]

what you have shown isn't really an antenna .

It's a Loop Antenna. Its 'magnetic' rather than 'electric'. But the far field is still an Electromagnetic wave and indistinguishable from a wave from a more familiar dipole.

it is just a consequence of how Fourier analysis works, i.e. it is math

That is a true statement as far as it goes but people should beware of thinking of it as 'just Maths'. The time domain description of a signal is no more or less valid than the frequency domain description. You must avoid saying what the signal 'really is' because it is both. Describing a signal as a "single pulse" includes the fact that there is zero for all time, apart from the duration of the pulse. To describe the pulse in the frequency domain, you may have to include all possible frequencies too. We tend to be very offhand about using the results of an FFT as 'reality'. The FFT is a Discrete Transform and assumes a repeated pulse and consists of harmonics of a fundamental repeat frequency. To get the correct answer, you need a full Fourier Transform, which has to be calculated over all time (or frequencies). But, a DFT is near enough for Jazz as long as you use a wide enough window.

 

[davenn]

It's a Loop Antenna. Its 'magnetic' rather than 'electric'. But the far field is still an Electromagnetic wave and indistinguishable from a wave from a more familiar dipole.

indeed

 

[Dale]

I am curious however, a singular pulse of what? Photons? If so, what are the photons frequencies?

There is absolutely no reason to bring in quantum concepts, like photons. This is purely classical.

To get the frequency content of your signal, simply apply the Fourier transform.

 

[jaydnul]

Ok thanks this helps alot.

In the picture below, if you perform a Fourier transform on the red signal you will get one fundamental frequency. But if you perform it on the blue signal, you will get a bunch of harmonic frequencies? If that's true, and let's say the red wave can penetrate a certain substance very well, is it possible that a lot of the harmonic frequencies of the blue wave would be blocked by the substance and what you get out on the other side would be completely distorted?

 

[sophiecentaur]

Ok thanks this helps alot.

In the picture below, if you perform a Fourier transform on the red signal you will get one fundamental frequency. But if you perform it on the blue signal, you will get a bunch of harmonic frequencies? If that's true, and let's say the red wave can penetrate a certain substance very well, is it possible that a lot of the harmonic frequencies of the blue wave would be blocked by the substance and what you get out on the other side would be completely distorted?

View attachment 209657

That is one method of Low Pass Filtering. Yes, if the low pass only let the fundamental through, you would get the same shape as the red signal (a sinusoid) but lower level.

 

[jaydnul]

I see! Thanks!

 

[jaydnul]

Sorry one more question to seal things up. If you perform Fourier analysis on a pulse that is identical to the positive swing of a sinusoid, you will get a bunch of different frequencies that add up together. But then if you take the Fourier of a full sinusoid, you will just get a single frequency, obviously.

My question is when an antenna is transmitting a signal, and this will sound stupid I'm sure, and is halfway through one period of the sinusoid (i.e. it has only completed the positive swing), how does the EM wave know that the negative swing will be completed? How does the EM wave know if it should be a combination of the fundamental frequency and a bunch of harmonic frequencies (as is the case for the positive swing pulse) or if it should just wait to be completed by the negative swing and be a pure fundamental frequency?

 

[sophiecentaur]

how does the EM wave know that the negative swing will be completed?

Good question, on the face of it. But a normal transmitter has circuits in it that are narrow band (too narrow to pass harmonics of the carrier). The transmitter just CANNOT stop the excursions of the wave. The transmitter, the matching networks and the antenna itself are all relatively narrow band so the signal will always be basically sine -like. So it's not the EM wave "knowing" anything; it's just the nature of the circuits producing it. There is a mechanical equivalent to this in an internal combustion engine, which has pistons which are hit once every two cycles with the exploding fuel, but the heavy flywheel only allows a steady rotation of the crankshaft.
It is possible to launch a wave like the one you describe if you have an appropriate transmitter (and aerial system) but, as was mentioned earlier, it would produce so many spectral products that would interfere with other transmitting bands that it couldn't be used.

 

[jaydnul]

I realize it is not exactly wise to think in terms of photons, but why does the photon frequency have to match the EM wave frequency? For example, on the positive swing of the signal on the antenna, are there photon creations happening then? Or do the photons wait for the full sinusoidal cycle to be created so they know what frequency to be?

 

[sophiecentaur]

You have just demonstrated for yourself what a nonsense it is to try to think of photons at the same time as waves.
How do you actually picture the photons you refer to? Are they little bullets? What sort of volume would they occupy if the wavelength of the EM wave is 1500m? Which direction are they going in when the wave has settled down with a spherical wave front, spreading out into space. How would you identify one of them which interacts with your radio receiver and where was it going at the time? It just doesn't make sense. Quantum particles are not compatible with the classical world so you can only wear one hat at a time.

I realize it is not exactly wise to think in terms of photons

Well, try not to! It won't get you anywhere.

 

[jaydnul]

So how do you explain the attenuation of radio waves trying to pass through certain materials if you don't consider the photon interactions? (a genuine question, not trying to be uncooperative )

 

[sophiecentaur]

So how do you explain the attenuation of radio waves trying to pass through certain materials if you don't consider the photon interactions? (a genuine question, not trying to be uncooperative )

With normal classical EM theory and the bulk properties of materials.
If you really want to get involved with what happens to EM in solids you have to consider the interactions of photons with the whole of the lattice and there really isn't an arm waving explanation. I pretty much forgot what I learned in my solid state course once I had done the exams - and that was fifty blooming years ago.

 

[Dale]

So how do you explain the attenuation of radio waves trying to pass through certain materials if you don't consider the photon interactions?

There can be reflection at the surface or if the material is conductive then there can be resistive dissipation within the bulk. There is no need to consider photons for the scenario you described, and lots of reasons to avoid them

 

[jaydnul]

I double promise this is the last question lol

Good question, on the face of it. But a normal transmitter has circuits in it that are narrow band (too narrow to pass harmonics of the carrier). The transmitter just CANNOT stop the excursions of the wave. The transmitter, the matching networks and the antenna itself are all relatively narrow band so the signal will always be basically sine -like. So it's not the EM wave "knowing" anything; it's just the nature of the circuits producing it.

If the circuit cannot produce the harmonics, but you stop the voltage signal halfway (so the antenna just receives the positive swing only), what then is emitted? We already established the isolated positive swing was represented as a pulse with multiple frequencies adding up. Now those frequencies aren't being created, but you still have half of a sinusoid input signal on your antenna.

 

[Dale]

If the circuit cannot produce the harmonics, but you stop the voltage signal halfway

This is a self contradiction. If you can stop the voltage signal halfway then you can produce the harmonics. That is what the Fourier analysis shows.

 

[jaydnul]

I see, thank you

 

[sophiecentaur]

I double promise this is the last question lol
If the circuit cannot produce the harmonics, but you stop the voltage signal halfway (so the antenna just receives the positive swing only), what then is emitted? We already established the isolated positive swing was represented as a pulse with multiple frequencies adding up. Now those frequencies aren't being created, but you still have half of a sinusoid input signal on your antenna.

I could build a circuit (plus antenna) that would produce that 'half sine' waveform (so that it looked ok on an oscilloscope, at least). It would not be practical to try to Transmit that waveform at a high power level because it would involve wide band power circuitry and a load of shash all over the RF bands. Not allowed!

 

[Bart McCoy]

Not every circuit that has current (even into an antenna) will emit photons. A slow ramp of voltage would push energy into the local magnetic field where it's stored as reactive energy (1/2 * L * I^2). When you release the slow moving voltage, the energy storage will likely collapse into the local circuit, with no photons emitted. Near the circuit, you could measure the magnetic field, but the energy never escapes as a photon & become a far-field emission from the antenna. Inefficient antennas do this all the time. In terms of antenna fields, where photons are NOT released, the E and H field looks like rain droplets on tree branches- they swell up, but never quite pinches off into a separate droplet, so the energy is reabsorbed. When a photon is emitted, it looks like a rain droplet that gains enough substance to separate, so it's emitted as a bubble of energy- a photon. It's quite possible to generate all sorts of weird voltage waveforms that don't release photons, so there's no contradiction with quantum physics & photons. If you do generate the right wavelength (or combination of wavelengths) with enough energy, groups of many photons will be released, each with characteristic frequency & energy. Your voltage waveform might contain many frequency components, all the way down to DC, but the photons actually emitted might not match that at all. That's one reason voice communication data are mixed up to high frequencies in bands that the antenna will emit. You won't send out a 3 kHz voice signal from your cell phone antenna!

 

[Drakkith]

Not every circuit that has current (even into an antenna) will emit photons. A slow ramp of voltage would push energy into the local magnetic field where it's stored as reactive energy (1/2 * L * I^2). When you release the slow moving voltage, the energy storage will likely collapse into the local circuit, with no photons emitted.

You'll still emit a small amount of EM radiation, even with a slow change in voltage/current. That's how low-frequency antennas work. There is no minimum frequency you can emit at as long as it remains above zero.

Your voltage waveform might contain many frequency components, all the way down to DC, but the photons actually emitted might not match that at all. That's one reason voice communication data are mixed up to high frequencies in bands that the antenna will emit. You won't send out a 3 kHz voice signal from your cell phone antenna!

The voltage waveform sent to the antenna matches the transmit frequency, not the voice frequency that modulates it. And you certainly can send a 3 kHz signal with a cell phone antenna, it will just be very inefficient at generating the radiation since the antenna is very small compared to the wavelength of the emitted signal.

 

[sophiecentaur]

It's quite possible to generate all sorts of weird voltage waveforms that don't release photons, so there's no contradiction with quantum physics & photons.

If the generating equipment radiates any EM waves then they can be detected and that process - if you really insist - can be described as a photon interaction. But, apart from tying up loose ends of Physics-with-Electrical-Engineering, is there really any point in trying to analyse every process in terms of Photons? I've said it before and I'll say it again - we have established the general principle that EM waves can be treated (very carefully) in terms of Photons but is it really any earthly use to take a problem that's so conveniently analysed in terms of waves and insist on using a photon approach? I actually defy anyone to derive the radiation pattern of a 'simple' half wave dipole by using only photons. This is not a game of Top Trumps but, if it were, waves can beat photons on some criteria and photons beat waves on other criteria. Is it 'really' smart to solve a problem standing up in a hammock? Is the solution somehow more valid just because photons were used in the solution?

 

[jaydnul]

If the generating equipment radiates any EM waves then they can be detected and that process - if you really insist - can be described as a photon interaction. But, apart from tying up loose ends of Physics-with-Electrical-Engineering, is there really any point in trying to analyse every process in terms of Photons? I've said it before and I'll say it again - we have established the general principle that EM waves can be treated (very carefully) in terms of Photons but is it really any earthly use to take a problem that's so conveniently analysed in terms of waves and insist on using a photon approach? I actually defy anyone to derive the radiation pattern of a 'simple' half wave dipole by using only photons. This is not a game of Top Trumps but, if it were, waves can beat photons on some criteria and photons beat waves on other criteria. Is it 'really' smart to solve a problem standing up in a hammock? Is the solution somehow more valid just because photons were used in the solution?

You're right, but there is a difference between applicability and curiosity. I just find it fun to think about, that's all. Not going to use it to aid my career as an RF engineer, just for fun.

 

[Dale]

But, apart from tying up loose ends of Physics-with-Electrical-Engineering, is there really any point in trying to analyse every process in terms of Photons?

I agree. One should always use the simplest theory that applies. And distinguishing when different theories apply should be taught explicitly.