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Electromagnetic field theory. Q and A

MRC_Hans

Penultimate Amazing
MRC_Hans
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Aug 28, 2002
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This educational thread is dedicated to Roger Coghill, but others are welcome to participate.

Rules of the game:

1) This is not a debate thread. If somebody can document information given here to be at fault, they are, of course, welcome to correct it, but stick to facts.

2) This is not a thread for making assumptions or expressing opinions.

3) Only subject allowed is electromagnetics, fields, energy propagation.

4) Bioelectromagnetics is NOT the subject of this thread.

Post questions, and I (and hopefully others) will answer to the best of our ability.

OK, shoot.....

Hans
 
Why is half a wavelength a good length for a dipole antenna?

(I know. Because then the antenna's resonant frequency equals the frequency of the radiation. Or something like that. But why is that true?)

Also, an antenna's resonant frequency can be changed, without changing its length, by sticking in some extra capacitance or inductance, correct? But that isn't quite as good as if it were the right length to begin with, I think. Why not?

Thanks. Maybe I'll finally understand this . . .
 
I think I can remember this correctly. It is not just half, but some even divide or muliple. The closer to the actual wavelength, the more power you get.

An ariel is picking up a very, tiny, electromagnetic wave out of the air. This wave is of a certain length. So, you want an antennae to match that wavelength, because, as you say, it resonates.

Why is resonance important. It is resonance that gives you the biggest return on your investment of using the antenna. All antennas work to a degree, but you want one that works a lot better.

Why is resonance important? Think of pushing someone on a swing. When you push at the resonant frequency, you get to push nice and smoothly, without much effort, and they go up nice and high. If you push at the non resonant frequency, they don't go far at all. If you just push at your own rate, that is, without regard to whether they are in range for a push, you aren't going to get much of a result.
 
I'm curious to know more about shielding of EMF. I started a thread asking about this once but its still not clear to me. If I got it right its enough with a metal net where the holes are smaller than the wavelength. But efficiency? grounding? Can you trap EMF from a source by wrapping a metal net around it? If you repaper your appartment with metal nets, are you free from EMF? What if you leave holes for windows and doors? Will it protect from magnetic fields, static or varying?
 
(Good answers AUP, but I'll offer mine, too)

Q:Why is half a wavelength a good length for a dipole antenna?

A: If the antenna is not in resonance, it will act either inductive (if too long) or capacitive (if too short). This means that it will not be well matched to the transmitter/receiver and energy transfer will be less than optimal. Whole fractions of the wavelength are in resonance. Half a wavelength gives a manageable impedance (75 ohms), and a good trade-off between antenna efficiency and size.

Q: (I know. Because then the antenna's resonant frequency equals the frequency of the radiation. Or something like that. But why is that true?)

A: The wavelengt is defived from the frequency, based on the speed at which the wave propagates. In air it is very close to the speed of light. In an antenna wire it is slightly slower (by a few %), so actually, the dipole should be a little shorter that a half wavelength.

Q: Also, an antenna's resonant frequency can be changed, without changing its length, by sticking in some extra capacitance or inductance, correct? But that isn't quite as good as if it were the right length to begin with, I think. Why not?

A: By trimming the antenna into resonance, you achive impedance matching towards the receiver/transmitter, but it is still mismatched towards "the air".

Hans
 
Vitnir said:
I'm curious to know more about shielding of EMF. I started a thread asking about this once but its still not clear to me.
Click to expand...

Q: If I got it right its enough with a metal net where the holes are smaller than the wavelength.

A: They should be much smaller than the wavelength. Alternatively, the net, or grid should be at least as thick as the longest wavelenght you want to exclude then the holes can be just "smaller".

Q: But efficiency? grounding?

A :Good conductivity is important for efficiency. In principle grounding is unimportant, but you won't want a large conductive box sitting around ungrounded (it can get charged with static electricity and give a fair impersonation of lightning :eek: ).

Q: Can you trap EMF from a source by wrapping a metal net around it?

A: Yes. If you make an efficient screen, EM cannot get in or out.

Q: If you repaper your appartment with metal nets, are you free from EMF? What if you leave holes for windows and doors?

A: There must be no holes, not even slits. Any hole that is comparable to a wavelenght in ANY DIMENSION (which means it can be a narrow slit) will allow radiation of that wavelenght and lower to pass. So doors and windows must be screened, doors and opening windows must be fitted with contact systems that make contact all along the opening edges. but it does not stop there; all wires passing into the room must be filtered.

Q: Will it protect from magnetic fields, static or varying?

A: No.

Hans
(Edited for some of the typos)
 
Here's a little "stump the expert" question:

When you have electric and magnetic fields perpendicular to each other, the fields contain momentum. You can construct static situations where the field integrated over all of space has a net linear momentum. For example, imagine a sphere with uniform electric polarization along x, and uniform magnetization along y. There will be a net momentum in the fields, pointed along z (you can work this out if you really care, but you can take it as a given, that's not the tricky part). But this is static, so there shouldn't BE any net momentum in the system as a whole. Where is the hidden momentum?
 
Ziggurat, I should perhaps have stated that this is intended to be on the primary level (it is too late to edit the opening post). Please put questions like yours in the puzzles section.

Hans
 
Q: If I got it right its enough with a metal net where the holes are smaller than the wavelength.
A: They should be much smaller than the wavelength. Alternatively, the net, or grid should be at least as thick as the longest wavelenght you want to exclude then the holes can be just "smaller".
Click to expand...

So the metal nets in hardware stores is no good even though the wires are thicker than 1mm. (If you want to protect yourself from the 0.15mm EMF from cell phones).

Q: Will it protect from magnetic fields, static or varying?
A: No.
Click to expand...
This supprised me since varying/alternating magnetic fields can induce electrical currents, right?
 
Vitnir:
Q: So the metal nets in hardware stores is no good even though the wires are thicker than 1mm. (If you want to protect yourself from the 0.15mm EMF from cell phones).

A: The wavelenght from cell-phones is much longer (about 16 cm), so many types of nets are (reasonably) effective here. I'd say up to chicken fence gauge.

Q: This supprised me since varying/alternating magnetic fields can induce electrical currents, right?

A: The crunch is, how fast does it vary ;)? When the wavelenght of the varying magnet field becomes comparable with the size of the cage, shielding begins to occur. When I simply answered "no", I assumed ELF or VLF range.

Hans
 
Right I forgot that the speed of light was 300 000 km/s not 300 000 m/s :hb:
And yes these "sensitive" dudes are scared of 50/60 Hz
 
What happens to any living or non living substance kept in electric and magnetic fields for some time?
 
Kumar said:
What happens to any living or non living substance kept in electric and magnetic fields for some time?
Click to expand...
Depends on the wavelengths involved, Kumar. Humans in the sun, for instance, produce Vitamin D and a pretty good tan. :D

Sorry, Hans, couldn't resist.
 
BillHoyt said:
Depends on the wavelengths involved, Kumar. Humans in the sun, for instance, produce Vitamin D and a pretty good tan. :D

Sorry, Hans, couldn't resist.
Click to expand...
I couldn't understand it.

I just want to know how elecric & magnetic fields can effect any living or non living thing?
 
I have a vague recollection of a seminar I was on a year ago where some interesting results were presented. Cells cultures were found to respond to magnetic fields with a frequency of around 9 Hz. I don't remember what type of cells or how the response manifested though. The only effect of EMF fields is heating if we are talking about microwaves.
 
Kumar said:
What happens to any living or non living substance kept in electric and magnetic fields for some time?
Click to expand...
That is within the scope of bioelectromagnetics, and will not be discussed in this thread.

Hans
 
Kumar said:
I couldn't understand it.

I just want to know how elecric & magnetic fields can effect any living or non living thing?
Click to expand...
Well, open another thread for it.

Hans
 
69dodge said:
Why is half a wavelength a good length for a dipole antenna?

(I know. Because then the antenna's resonant frequency equals the frequency of the radiation. Or something like that. But why is that true?)

Also, an antenna's resonant frequency can be changed, without changing its length, by sticking in some extra capacitance or inductance, correct? But that isn't quite as good as if it were the right length to begin with, I think. Why not?

Thanks. Maybe I'll finally understand this . . .
Click to expand...

To this I add another question: What length of dipole antenna best matches the dipole impedence to that of free space? :D
 
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