# EM wave question

Discussion in 'General Electronics Discussion' started by Ledwardz, Nov 28, 2012.

1. ### Ledwardz

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Dec 21, 2010
Okay so i am wondering how it is that i can transmit a EM wave in the microwave region many miles yet when i transmit energy via a coil the EM wave produced only acts over a small distance.

Is this because of the frequency it is transmitted at?

wavelength x frequency = c

Eph = hc / wavelength

i assume the wavelength in the coil is minute and so the energy and frequency is large. The small wavelength has trouble penetrating things due to some interaction with the atoms in the substance it penetrates?

and the opposite for a microwave - small frequency large wavelength and so more penetrating allowing it to reach a further distance. Does the electric and magnetic field created by a single charge at the frequency of a microwave stretch many miles? are there minute waves going through me now to my phone? does this cause electrons on my phone antenna to move?

i Suppose what i am asking is how can i determine the distance an electric / magnetic field stretches given a certain frequency energy and wavelength.

any thoughts?

Lee.

2. ### Ledwardz

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Dec 21, 2010
One more question.

y does a moving electron emit a photon. I get the whole electron moving to a valence band and so it tries to get rid of its energy producing a photon but why does a moving electron emit the EM wave?

3. ### Ledwardz

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Dec 21, 2010
Having thought about this, waves become more penetrating as the frequency increases and so the wave length must become smaller. So perhaps it is the opposite way around and the AC current in the coil is not at a high enough frequency.

This would explain the whole energy of a photon equation

Eph = hf

substituting the 2 equations i stated before.

so the higher the frequency the more energy it has and i assume more penetrating power.

4. ### davennModerator

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Sep 5, 2009
no not really

a brick wall will effectively stop a microwave signal aimed at it where lower freqs will get through

Ground penetrating radar usually uses relatively low freqs, less than 30MHz

Dave

5. ### davennModerator

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2,015
Sep 5, 2009
hi lee

you have given no info on this coil transmitter so its impossible to comment on your observed results. but keep in mind 1W of 30MHz will go world wide, 1W at 5GHz will go line of sight for a much shorter distance or till it hits some object in the path

Dave

6. ### poor mystic

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Apr 8, 2011
Hi Ledwardz
In the matter of your second question:

I had, and still have, lots of questions about electrons and photons myself, some of which I have found answers for, while others remain a conundrum.
Of course the theories of quantum electrodynamics (QED) are questions of physics, so I expected to find that a physicist would be the best person to help me understand the theory. Not so!
The people who know about electron energy levels and atomic electrical quantum states are mostly chemists to whom such questions are of central importance; electrical engineers tend to develop an interest as have you although I think QED is of no practical use in daily electrical engineering.. Physicists aren't very interested in QED on a daily level any more as far as I can see.
So, I suggest you look at an undergraduate course in chemistry, or make friends with a competent chemist.
Mark

7. ### Ledwardz

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Dec 21, 2010
Perhaps i should have explained a little bit about what i am doing. I am analysing the feasability of transmitting power wirelessly. As such, i have started with the rather generic resonant inductive coupling. I went on and analysed the magnetic field at a point in space due to a moving point charge, due to current in a wire and a coil where the magnetic field is focused in the centre of the coil. Having then plotted the graph for various values of capacitance and inductance i can calculate the frequency and plot the graph for the distance that the magnetic field extends from the coil. The results were dissapointing even when a resonant frequency was used.

I used calculations to experiment with the frequency. I found that making the capacitor or inductor smaller would create a bigger frequency which would in turn give the photons more energy although reducing the capacitor reduces the amount of available charge and reducing the inductance will reduce the magnetic field and hence the induced voltage in the other coil. However if the photon has more energy surely it should be able to penetrate further through the medium of air. It appears not. I should also add that the higher the frequency the less current pulled if that makes sense? I differentiated the amount of charge in the capacitor to get -current = (omega * initial charge) which should give me current i believe. perhaps this is the problem with using low frequency in the coil - heat and resistance.

Anyways..... I am actually beginning to think that i should be studying the medium, air. Perhaps the frequency that the coil operates at is a frequency that air can easily absorb? same with the brick wall maybe that explains why a higher frequency can't penetrate but a lower frequency can? like electrons and forbidden energy gaps maybe the air has something similar for waves? I dunno just talking crap probably.

anyways i would imagine that the coil would probably operate at what around 1Mhz assuming say a 200nH and 200nF ??? (I aint done the maths just guessing) this is some where in the middle of the radio frequency range.... why the hell does the coil only transmit 5 cm?? if this was an antenna i could pick it up with a radio miles away..... i know the voltage in the radio would have to be amplified etc but surely a coil the right size would pick up something? maybe the size of the coil needs to be the size of the wavelength and at lower frequencies this aint possible it would have to be huge.

This has got me puzzled and thanks for the reply.

Lee.

Last edited: Nov 28, 2012
8. ### Ledwardz

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Dec 21, 2010
Thanks for the reply. Electrons are fiesty little things that i don't think anyone understands even slightly. I don't think i will ever need to know any of this stuff again as an electrical engineer but i need to understand it to understand wireless transmission. For some reason nature decided that a moving electron would produce an EM wave which is able to transmit energy, so the electron seems pretty damn essential for what i am doing.

Cheers,
Lee.

9. ### davennModerator

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Sep 5, 2009
very basically, when additional energy is imparted into an atom, electrons on the outer layers, the valence electrons, can move to a higher energy level. When they fall back to their "normal" energy level, they can release energy, a photon. Now depending on the input energy levels involved, that photon may be anything from a low freq RF photon to a mid range visible light photon or maybe really high energy X-ray or gamma ray photons

I dont think a single photon constitutes an E-M wave, so you cant really say "but why does a moving electron emit the EM wave?" ( that may be open to confirmation)
rather "packets" of photons make up an E-M wave

Dave

PS
Hey PM --- how ya doing Mark, trust all is well in FNQ ?

10. ### davennModerator

13,983
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Sep 5, 2009
yup and I suspect you will be severely disappointed in wireless power transmission efficiency over any distance greater than a few cm.
it works well over close distances, up to a couple of cm for stove tops induction heating elements, battery charging on my electric toothbrush and things like that
Once you get any significant distance between the transmitter and the receiver, the inverse square law will beat you every time and leave you powerless

a TV transmitting station antenna putting out 100kW of power, yet at 10 - 20 km
distance the power level at the receiving antenna is only a few microWatts

cheers
Dave

11. ### Laplace

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Apr 4, 2010
The only active consideration of wireless power transmission that I am aware of is for space-based solar power where a gigantic solar power satellite in geosynchronous orbit collects energy for transmission back to earth in a focused microwave beam to a huge antenna field where a rectanna recovers DC power for conversion to the distribution grid.

For instance, see www.ipcbee.com/vol6/no2/51-f20017.pdf WIRELESS POWER TRANSMISSION OF SPACE BASED SOLAR POWER from the 2011 2nd International Conference on Environmental Science and Technology,

or, http://www.utdallas.edu/~pxm017500/gap4s/Doc/Ref/Recten/00141357.pdf Beamed Microwave Power Transmission and its Application to Space.

12. ### Ledwardz

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Dec 21, 2010
So what you are actually saying is that it is the retardation of the electron that emits a photon. The Eph is also a function of wavelength of the particle so surely even a single photon has a de broglie wave characteristic. Unless like you say the each separate photon only makes part of the wave?

13. ### Ledwardz

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Dec 21, 2010
This bring me another question. I said before that the lower frequency, the higher the current needed to be. For this i assumed that the voltage over the inductor and capacitor would remain constant so say for example 20 v. Does this relate to the power needed by the system. the whole P = VI thing and if so how would i create this power? I mean a battery only tell you voltage or A Hours right would i have to work out the power needed by the system and find a power source capable of delivering this power. Where would i look for this? battery specifications??

14. ### Ledwardz

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Dec 21, 2010

Having read all of the top report i can quite confidently say it is useless, the author clearly has no idea what they are talking about. The second report on the other hand is quite good.

it throws an equation that relates efficiency between the receiver and transmitter antenna to the distance between the antennas and wave length. It has also gave me a few ideas i never considered like friis equation.

Thanks for this,

Lee.

15. ### poor mystic

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Apr 8, 2011

Well Lee, here's another 5 cents worth.
In your sentence beginning "However if the photon has more energy..." it sounds to me as though you expect to be able to characterise electrical waves as though they were photons, whose energy is proportional to frequency. This is the wrong approach to take with macro-scale electrical behaviour.
In electrical power systems, please visualise a load, which is supplied at some voltage and whose power may be calculated by Ohms Law. Please imagine that the load is connected to a hydroelectric generation system, and that a monitoring circuit at the hydroelectric power station keeps the voltage stable by reducing or increasing the flow of water through the turbines.
The power consumed by the load comes from the energy of falling water and is transmitted by electrical means between the generator and the load. Occam's Razor says that quantum physics cannot improve this answer. (Occam's Razor is a useful principle for theoreticians, which more-or-less says that the best answer to a question is the simplest, observably correct answer.)
Yet even in the case of small signals, detected as tiny fluctuations in the ambient electromagnetic field, the findings of Planck (f=hv) do not apply. I'm don't feel sure enough of my footing on this ground to give you the real reasons for this, though, and I hope some suitably knowledgable person soon steps in.

Mark

16. ### Ledwardz

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Dec 21, 2010
Hmmm..... so planks constant doesn't hold up in all cases? which means the relationship between my energy and frequency is actually bollex? I don't really get what your second paragraph is getting at? Are you saying that wireless transmission is pointless in a nice way?

Thanks,

Lee.

17. ### poor mystic

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Apr 8, 2011
I don't think that Planck's ideas are wrong, actually, but the single piece of wire might behave as a single entity whose properties are different to single atoms.

However a 1MHz wave at 1 volt will deliver the same power as a 1 volt, 100MHz wave into a resistive load.

My second paragraph attempts to point out a simpler approach to the calculation of electrical power, which is a macro-scale effect. I believe that the individual atoms' properties are not important within the context of such a huge entity as a piece of wire conductor.

I am aware of some electrical power transfer systems that operate over meters rather than millimeters - I haven't bothered to really look into the theory since I no longer really do electronics.