EM waves, longitudinal EM propagation?

In summary, the content discusses fundamental questions about electromagnetic waves and their properties. The conversation covers topics such as the relationship between electric and magnetic fields in EM waves, the amplitudes of the fields in a circuit or wire, and the behavior of EM waves in antennas. The key points include: the electric and magnetic fields of an EM wave traveling through space are in-phase, the amplitudes of the fields can vary dramatically in a circuit or wire but remain equal in the radiated wave, and antennas have multiple modes for the E and B fields depending on the distance from the source.
  • #1
artis
1,481
976
Hey, after doing some reading I stumbled across a few fundamental questions.1) Do all EM waves across the EM spectrum , if they travel through space have their E field and B field amplitudes exactly equal and in phase and shifted 90 degrees from one another?

If the answer is yes then...
2) In ordinary wires at DC or low frequency AC I can have any relationship between voltage and current, like have very low voltage and very high current or high voltage and low current, so taking the low voltage high current situation, if I made a very long antenna at low frequency, like a single phase electrical transmission wire and had low voltage but very high current run through the wire then how would the amplitudes of the radiated EM wave look like? I presume they would still have to be equal in amplitude ?
So I guess I'm asking is why do the amplitude of a traveling EM wave is equal for both E and B fields while the strengths of B and E field given in amperes and volts can vary dramatically in a circuit or wire?
Now I probably have asked enough but please one more.

3) Is it true that Em waves only travel transversely and not longitudinally?
If the answer is yes as I suppose it could be then consider this thought experiment example and please tell me the answer.

I glue a copper plate to a low frequency large amplitude excursion speaker, I charge the copper plate to some arbitrary positive or negative value.
I now have a static E field pointing out in space from the plate, I now apply a 50hz sine to the speaker and the voice coil and membrane start to resonate creating longitudinal pressure waves of sound in the air surrounding it.
What sort of EM wave will travel from the speaker plate?
 
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  • #2
The electric and magnetic fields of an EM wave traveling throgh space are in-phase, not at 90 degrees. The energy in the two fields is the same, but the amplitudes are measured in different units, and the ratio of the two is 377. This ratio is expressed in Ohms.

The voltage applied to a wire and the current flowing in it do not constitute a radiated EM wave and can have any relative value. If you use a wire carrying a large current as an antenna the radiated wave still has equal energy in its E and B fields. Energy which is not radiated by the wire is either dissipated in a resistor on the end of the wire or stored in the capacitance and inductance of the wire.

Antennas are sometimes viewed as a transition from a transmission line mode to the free space mode. This requires a very long antenna. In most cases we see a lot of stored reactive energy in an antenna, with just a small percentage being radiated.
 
  • #3
artis said:
So I guess I'm asking is why do the amplitude of a traveling EM wave is equal for both E and B fields while the strengths of B and E field given in amperes and volts can vary dramatically in a circuit or wire?
I wasn't quite sure which bit of your post to quote but the above could be a start. The 'amplitudes' and directions of B and E are only as commonly described when the wave is so far away from any source that it can be regarded as a plane wave and their phases are the same.

Start with a dipole antenna (basic structure) and look at the E and B close to the wires. There are dozens of Googleable images to help visualise this. This link shows the E field, animated and this link shows the E and B at a point in time. The relative directions are different (not orthogonal) and their phases are near quadrature when very close in; nothing like the far field that we tend to start off with.

They are 'Near quadrature' but not exactly because the supply of RF power into the dipole 'sees' a small in-phase component which represents the flow of power away from the radiator. (Radiation Resistance).

Fields inside complicated radiating systems can be very non-intuitive, which means that the Impedances (ratio of the fields) in various regions can be high, low, positive or negative reactive.
 
  • #4
Ok @sophiecentaur so since we are going into antennas let me do some questions,

1) as far as i know antennas have 3 modes for the E field and 2 for the B field, for the E field namely the 1/r3 for the static components, then 1/r2 for the near field and 1/r for the far field that you mentioned
the B field I assume has 1/r2 for the near field and also 1/r for the far , correct?So the far field E and B fields are always in phase and always same amplitude?
Then I would guess that the near field E x B components must then resemble the current and voltage with which i am driving the antenna right? Like if I have an inefficient antenna that I'm driving with lots of current and very low voltage my near field B would be very strong and E very weak, if this is so, then my question is this

2) For this high current low voltage case antenna , since the far field E x B are same amplitude and in phase , what would their amplitude correspond to, would it be the average of the sum of my near field E and B field?
In a sense that keeping the voltage constant but changing the current would change my near field B field strength but it would also change the amplitude of the far field wave?
and what about phase shift? the far field E x B are in phase but the near fields must not be in phase if the antenna current lags the antenna voltage , the near fields should follow this lag correct?

if this is what happens then how can the far field waves be in phase even though they were created from an antenna whose voltage/current are out of phase and whose near fields are also out of phase?ps. I looked at the links you gave, it seems to me that at least for the dipole antenna the waves it creates resembles "copied" versions of the dipole antenna itself, like the dipole radiates countless cloned virtual dipoles in all directions which then form the wavefront that travels outwards, is this a reasonable way of looking at it ?
 
  • #5
Unfortunately there is a lot of stored reactive energy around a resonant antenna. These reactive fields are nothing to do with those in the far field of the antenna. The radiated part of the energy is created right at the conductor when the electrons accelerate. It is perhaps easier to consider radiation from a very long conductor supporting a traveling wave, when all the energy is radiated and none stored.
 
  • #6
so if what you say @tech99 is true then it would mean that the EM far field which consists of ExB fields being in phase is radiated from the conductor at it's surface and "passes" through the near field which is also set up around the conductor surface only doesn't radiate very far?

Although I can't imagine how can those two fields not interact with one another since they both at some point exist in the same space.
Although I guess at the near field territory of the antenna I could not detect the far field but just the near field and the same just vice versa would be true for the near field at further away distances?
 
  • #7
If you measure a field with a field strength meter, of course it does not know which it is measuring. What you need is a meter which has a phase reference so it can decide what it is looking at. For the case of a radiation field, it seems that the phase is delayed as you go away from the antenna, but for an induction field, where the energy flows both away from and towards the antenna, I suggest that you will measure constant phase.
The reactive fields are in quadrature to one another, but of course they cannot still both be in quadrature to a radiated field. So what happens is that the B induction field is initially in-phase with the B radiation field. The E induction field is initially in quadrature with the E radiation field.
Regarding the total (radiation + induction) field strengths we might measure, I have found that when we approach a dipole along its equatorial plane, the B field continues to grow with approx 1/D as we approach right up to the conductor. This is just the same as for any long conductor carrying a current. On the other hand, the E field remains approx constant at distances less then about lambda/2 pi and is mainly radiation. For a dipole, the electric energy is mainly stored near the ends so we don't see it close up in the equatorial plane. In a practical case when we approach very close to the feedpoint, we see an increase due to the driving voltage.
This is what I have observed in experiment.
 
  • #8
But both the far field and the near field is essentially created by the same current as there is only one current entering the antenna?
So the field created by the antenna (all antennas I assume) is just a single field that has different properties depending on the distance from the antenna that it is observed?
ps. At my OP I also asked about longitudinal waves, they are said to not exist within the EM fields and are normally applied to sound waves.
But I mentioned the example of a vibrating charged plate that moves back and forth , to an observer some distance away from the plate , I wonder what the field pattern would look like?
 
  • #9
These guys do an excellent job Visualizing Electromagnetic Waves with simple equipment.
 
  • #10
artis said:
But both the far field and the near field is essentially created by the same current as there is only one current entering the antenna?
So the field created by the antenna (all antennas I assume) is just a single field that has different properties depending on the distance from the antenna that it is observed?
ps. At my OP I also asked about longitudinal waves, they are said to not exist within the EM fields and are normally applied to sound waves.
But I mentioned the example of a vibrating charged plate that moves back and forth , to an observer some distance away from the plate , I wonder what the field pattern would look like?
This example is just a longitudinal electric field of the sort that any object with an alternating potential will have. It is not radiation but just an induction filed where energy is stored.
It is possible to have longitudinal EM waves where there is a waveguide. Here we see a component acting along the direction of travel. An example is the propagation of a wave on a single wire, where the E field is longitudinal, linking between the + and - peaks of the traveling wave.
 
  • #11
@tech99 you think like in a Gobbau line which is a single conductor, where a potential forms along a length of wire representing the wavelength of the signal and then this length propagates along the wire much like a train on tracks at the speed of light in the given medium?

With a static E field I guess you cannot make a longitudinal wave because a sound pressure wave travels physically from the source outwards while the E field either increases or decreases it's strength but either way the increase or decrease happens everywhere around the source.ps. thanks for the video @nsaspook , although I feel I haven't got an answer for what I asked about whether we could say that the field from the antenna both the far and near is one phenomenon just with different properties at different distances? If this is true then what law or property accounts for the fact that a field can change it's (phase in this case) with distance even though both the current that created it was not in phase with voltage and the near field was also not , yet the far field is?
 
  • #12
artis said:
@tech99 ps. thanks for the video @nsaspook , although I feel I haven't got an answer for what I asked about whether we could say that the field from the antenna both the far and near is one phenomenon just with different properties at different distances? If this is true then what law or property accounts for the fact that a field can change it's (phase in this case) with distance even though both the current that created it was not in phase with voltage and the near field was also not , yet the far field is?
 
  • #13
Yes I agree with you here. The induction fields are a quite separate phenomena to the radiation fields. They can be altered by making physical changes to the antenna; for instance, by going from a dipole to a slot, or by using a loading reactance in the antenna. It still radiates the same.
Regarding the Goubau Line, any wire even without an insulated coating, will support this mode if it is in isolation. The mode is also found in all coaxial cables to a certain degree depending on dimensions. A similar mode exists as the ordinary radio "surface wave", which is used for AM broadcasting.
 
  • #14
artis said:
@tech99 you think like in a Gobbau line which is a single conductor, where a potential forms along a length of wire representing the wavelength of the signal and then this length propagates along the wire much like a train on tracks at the speed of light in the given medium?

With a static E field I guess you cannot make a longitudinal wave because a sound pressure wave travels physically from the source outwards while the E field either increases or decreases it's strength but either way the increase or decrease happens everywhere around the source.ps. thanks for the video @nsaspook , although I feel I haven't got an answer for what I asked about whether we could say that the field from the antenna both the far and near is one phenomenon just with different properties at different distances? If this is true then what law or property accounts for the fact that a field can change it's (phase in this case) with distance even though both the current that created it was not in phase with voltage and the near field was also not , yet the far field is?

I think you should think of the field at X point in space as a local effect. Due to the finite speed of propagation phase relationships are governed by the local far field free-space resistive electromagnetic properties, not those of the near field reactive signal origin at the antenna.

Take a look at this book in the antenna section.
https://archive.org/details/in.ernet.dli.2015.6364/page/n89/mode/2up

Section 35 here:
https://archive.org/details/in.ernet.dli.2015.6364/page/n193/mode/2up
 
  • #15
Ok I think I see what goes on here. Just as the current to voltage phase shift depends on the type of "medium" or circuit the current if slowing through so does the EM field's phase of ExB depend on the medium it travels in.

So for a purely resistive circuit voltage is in phase with amperage as an example and free space is resistive in nature not inductive or capacitive so the EM field travels in phase.

The EM field close to the antenna or any wire resembles that of the wire instead of free space because it's ExB relationship is dictated by the electrons in the wire yet as this phase shifted near field expands further away the phase shift disappears because now the field is not impacted by the original electrons in the wire anymore but instead the field sustains itself from the energy it originally got.If this is close to truth, then I guess no matter what kind of phase shift one puts in an antenna the far field will always be ExB in phase.
Now I get the phase modulation technique where they simply transmit a sine with either a full period or a half period depending on the number or place in the signal it needs to represent.

Oh one more thing , by this same logic one also cannot radiate a square wave? The shape of the far field wave can only be a in phase sine wave or not ? I think the near field could be squared shape if an antenna was fed such a waveform?
 
  • #16
Beyond my level of expertise but I assume there are analytical solutions of Maxwell's equations modeling the wave propagation for arbitrary waveform functions as a set of sinusoidal harmonics because modulation creates them. In perfect vacuum, in isolation you would expect to see the waveform retain its original shape but in other media the phase relationships and shape could change due to different rates of propagation at different frequencies.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.610.4374&rep=rep1&type=pdf
 

1. What are EM waves?

EM waves, short for electromagnetic waves, are a type of energy that travels through space in the form of oscillating electric and magnetic fields. They are produced by the acceleration of electrically charged particles and can travel through a vacuum or a medium.

2. How do EM waves propagate longitudinally?

EM waves propagate longitudinally by oscillating parallel to the direction of travel. This means that the electric and magnetic fields are both perpendicular to the direction of propagation, creating a wave that moves forward in a straight line.

3. What are the properties of longitudinal EM waves?

Longitudinal EM waves have several properties, including wavelength, frequency, amplitude, and velocity. They also exhibit characteristics of both electric and magnetic fields, such as polarization and the ability to be refracted and diffracted.

4. How are longitudinal EM waves different from transverse EM waves?

The main difference between longitudinal and transverse EM waves is the direction of oscillation. Longitudinal waves oscillate parallel to the direction of propagation, while transverse waves oscillate perpendicular to the direction of propagation. Additionally, longitudinal waves can travel through a vacuum, while transverse waves require a medium.

5. What are some practical applications of longitudinal EM waves?

Longitudinal EM waves have a variety of practical applications, including communication (radio waves), medical imaging (X-rays), and heating (microwaves). They are also used in technologies such as radar, satellite communication, and wireless internet. Additionally, they play a crucial role in understanding the behavior of light and other forms of electromagnetic radiation.

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