# Antenna induction, oscillating circuit generating EM Waves

• B
• Steels
Steels
TL;DR Summary
Antena induction, oscillating circuit, EMW
Hello!
Now i need a bit explanation, so "oscillating circuit" capacitator + coil in series. Let's look at this as infinite, without any loss. We get changing magnetic field and electric field. Capacitator discharges, current goes to coil increasing its magnetic field, then starts magnetic field change, and then starts coil induction (Lentz law), when capacitator is empty, coil starts to lose energy and magnetic field energy turns back to electric energy and capacitator recharges, but current direction has changed.

When we straighten capacitator and coil and we make antenna, we add AC voltage, current starts to travel from one side of antenna to the other side. And then we get Electromagnetic waves. AND we get EMW in antenna because there is no longer induction in coil (because simply there is no coil anymore?) AND current is now traveling from one side to another side without negating each other?

(by negating each other i mean, when you look at coil magnetic field and current directions are different, check picture, in A picture, we see different current directions I, and magnetic field they produces negates each other and look at picture B in coil current direction produces magnetic field, but they negates each other because...) My question is, did I said everything correctly? Can I say, that in closed oscillating circuit is just too weak to produce EMW? OR if its poweful enough, but it only produces EMW inside of that cuicruit? Can I say that straighten antenna with AC makes larger and stronger waves (stronger, more powerful frequency)?

Any help would be super apreciated.
Thank you and have a nice day!

Last edited by a moderator:
Too many question marks, and too much confusion.

If a current flows, there will be a magnetic field generated, and there will be an EM wave radiated.

Steels said:
Can I say that straighten antenna with AC makes larger and stronger waves (stronger, more powerful frequency)?
Power and frequency are independent variables.

An antenna will be designed to radiate a stronger signal than a compact oscillator.
The antenna does not have to be straight, it could be a loop.

Every wire has inductance, so the opened out conductors have L and C and still have resonances. The only requirement for an electric circuit to be a good antenna is for it to be comparable in size with the wavelength. Very small circuits do not radiate very much. There is no requirement for resonance for radiation, it is just the current and dimensions which matter. However, in the resonant condition, the generator can see a resistive load, which is desirable.

Steels said:
TL;DR Summary: Antena induction, oscillating circuit, EMW

Hello!
Now i need a bit explanation, so "oscillating circuit" capacitator + coil in series. Let's look at this as infinite, without any loss. We get changing magnetic field and electric field. Capacitator discharges, current goes to coil increasing its magnetic field, then starts magnetic field change, and then starts coil induction (Lentz law), when capacitator is empty, coil starts to lose energy and magnetic field energy turns back to electric energy and capacitator recharges, but current direction has changed.

When we straighten capacitator and coil and we make antenna, we add AC voltage, current starts to travel from one side of antenna to the other side. And then we get Electromagnetic waves. AND we get EMW in antenna because there is no longer induction in coil (because simply there is no coil anymore?) AND current is now traveling from one side to another side without negating each other?

(by negating each other i mean, when you look at coil magnetic field and current directions are different, check picture, in A picture, we see different current directions I, and magnetic field they produces negates each other and look at picture B in coil current direction produces magnetic field, but they negates each other because...) My question is, did I said everything correctly? Can I say, that in closed oscillating circuit is just too weak to produce EMW? OR if its poweful enough, but it only produces EMW inside of that cuicruit? Can I say that straighten antenna with AC makes larger and stronger waves (stronger, more powerful frequency)?

Any help would be super apreciated.
Thank you and have a nice day!

View attachment 343836

View attachment 343837
The last image you included in your post is interesting, could you post the source of that image or the text that went with the image. A similar graphic appears in, https://global.canon/en/technology/s_labo/light/001/11.html

Thanks.

The link provided says the following: "electromagnetic waves-in which an electric field and magnetic field alternately appear-are created in the space between the two electrodes and travel into their surroundings."
However, the magnetic and electric waves are in-phase, which is in conflict with this description.

tech99 said:
The link provided says the following: "electromagnetic waves-in which an electric field and magnetic field alternately appear-are created in the space between the two electrodes and travel into their surroundings."
However, the magnetic and electric waves are in-phase, which is in conflict with this description.
I don't think I read the text. The picture of the capacitor morphing into an antenna forces you to also imagine how the displacement current also changes. For some poor reason I think that I thought that only between the charging capacitor plates is where you found the displacement current, not true, right?

If I know the electric field far from a dipole antenna the displacement current field looks the same as the electric field that has been shifted in time a quarter cycle?

In particular, if the divergence of the electric field is zero so is the corresponding displacement current field? Where the electric field is zero (edit, and changing) the displacement current is maximum?

Thanks.

Last edited:
The mechanism of radiation cannot be explained using the properties of the fields alone. A capacitor which has a vacuum as its dielectric will not radiate, only the wires leading to it will do that. To understand radiation we need to consider charges, such as electrons, which are subject to acceleration.
By the way, in the original question you mention currents flowing in opposite directions in an LC circuit. This is correct, so to obtain efficient radiation the loop must have significant size and the current must be large. As this is a "closed" circuit it is actually easy to obtain large current.

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