Transmission line radiation

  • #26
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OK, so you said that we can look at the transmission line as loop antenna, but in that shape it will be badly matched to free space. So I would like to re-formulate the question: What decides if the antenna will match transmission line to free space? Obviously the dipole antenna is a good match. Transmission line is not.

As I said before, is it because the EM wave from one wire in transmission line is inducing the current in other wire and being almost completely "absorbed" in the wire?
¼λ.

There are many good antenna designs that don't use quarter wavelength conductor segments, but most of the easiest to understand do.

A half λ section gives lots of umph. A quarter λ driven conductor plus the image current in the ground plane often works out to be ½λ.

A full λ section tends to reabsorb it's own photons. ½ emits and the other ½ is 180º out of phase, so it absorbs.

There are lots of exceptions to this rule of thumb. Plus there are lots of caveats like remembering the physical wavelength varies with the dielectric or remembering gaps often count as much as conductors (slot antennas, etc.).

Yet ¼λ is a good rule of thumb.

Another rule of thumb: parts less than 1/10 λ can be ignored. Again, not always true, but it usually is. Thus your gap between wires of less than 1/10 λ makes a bad antenna. If the gap were bigger (like ½λ) that wouldn't be true. Then you would need to run the field equations to see what happens.
 
  • #27
sophiecentaur
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As I said before, is it because the EM wave from one wire in transmission line is inducing the current in other wire and being almost completely "absorbed" in the wire?
Not "absorbed" but almost cancelled. Any distant radiation in a direction normal to the plane of the thin loop will be zero because the E fields due to the two wires will be in opposite directions (perfect anti phase) and the H field will be 'outwards' and not transverse. In the plane of the loop there will be a small resultant E field because the phases of the two fields from each leg are not totally in anti phase because the distances are unequal. The fields in the vicinity of the loop are more complicated and the E and H fields are nearly in quadrature. As you get into the far field, the only result is E and H fields that are in phase (the radiated Em wave).

You are talking in terms of needing to supply energy for the radiation from each wire but the only energy that's involved is due to a failure of fields to cancel.
If you study antennae and transmission lines more deeply it becomes easier to see that the transmission line is a poor antenna and that a transmission line with a wide separation or a taper starts to be a better antenna and a worse transmission line. :smile: It's quite hard stuff, I think.
 
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  • #28
OK, I think that the my confusion was due to thinking that EM radiation is produced by each wire in transmission line. Instead EM radiation is produced by total magnetic and electric field from both wires, which is near zero. But that means that there is some minimum distance only after which E and H fields become free EM wave?
 
  • #29
davenn
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But that means that there is some minimum distance only after which E and H fields become free EM wave?
yes
this is the next step in the theory where you start looking at "near field" and "far field" processes

mutually couples inductors such as transformers use the near field inductive properties

Antennas make use of the far field radiative properties

time for you to do some reading :wink:

https://en.wikipedia.org/wiki/Near_and_far_field

http://electronicdesign.com/energy/what-s-difference-between-em-near-field-and-far-field

http://www.antenna-theory.com/basics/fieldRegions.php

there's 3 references out of 100's on google :smile:


Dave
 
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  • #30
Ah, that's it. I thought EM wave is produced at conductor surface. Finally I can sleep :) And read about Near and far field tomorrow, thanks.
 
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  • #31
So after reading about near and far field I have few more questions.

From wikipedia:
Also, in the part of the near field closest to the antenna (called the "reactive near field", see below), absorption of electromagnetic power in the region by a second device has effects that feed back to the transmitter, increasing the load on the transmitter that feeds the antenna by decreasing the antenna impedance that the transmitter "sees". Thus, the transmitter can sense when power is being absorbed in the closest near-field zone (by a second antenna or some other object) and is forced to supply extra power to its antenna, and to draw extra power from its own power supply, whereas if no power is being absorbed there, the transmitter does not have to supply extra power.
In transmission line we have two wires. If I understood it correctly, in reactive near field, fields from each wire are absorbed by the other wire?
 
  • #32
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In transmission line we have two wires. If I understood it correctly, in reactive near field, fields from each wire are absorbed by the other wire?
No, any crap going on in the near field of the transmission line will affect it. This can be used to intentionally couple power or whatever, but mostly it just degrades the line. However because of the dipole effect the reactive nearfield is probably smaller in the transmission line than the average antenna. (Anything between the wires is messy though.)
 
  • #33
No, any crap going on in the near field of the transmission line will affect it. This can be used to intentionally couple power or whatever, but mostly it just degrades the line. However because of the dipole effect the reactive nearfield is probably smaller in the transmission line than the average antenna. (Anything between the wires is messy though.)
But each wire in transmission line is in reactive near field of the other wire?
 
  • #34
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But each wire in transmission line is in reactive near field of the other wire?
Yes, but they are arranged to complement one another. They work together to guide the wave.

Random stuff in the near field messes with them working together. Metal acts as a conductor. Insulators act as dielectrics. (Many of which are close enough to εr = 1 to not matter, BTW.)

As I said, if it's intentional we can achieve lots of good things with add ons, like impedance matching or coupling power. It's when random things happen that things go down hill.

Have you considered taking a course in electromagnetics? You seem interested in the field.
 
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  • #35
Yes, but they are arranged to complement one another. They work together to guide the wave.

Have you considered taking a course in electromagnetics? You seem interested in the field.
I have started studying physics in 2010, but I took a break in 2013. Hopefully I will continue this or next year. But yes, I would want to take courses in electromagnetics.

I know I can't understand it completely without looking into a math and equations, and I know that lot of weird stuff happens in near field of the antenna.
Can I at least look at it this way. There are two waves that are 180 degrees out of phase in the transmission line. Their fields cancel each other like this (b case)

image004.jpg


Not literally like in "b" case, just that part where their amplitudes cancel each other.
 
  • #36
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Yes, (b) is mostly correct, particularly (b)iii.
 
  • #37
tech99
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Just to add to the general confusion, but perhaps to deepen understanding and create interest, may I mention that if one wire is removed, the transmission line continues to function, even without the presence of ground, and radiation is still very small. The second wire can be replaced by a small "ground plane" at each end of the line. (There does not need to be capacitive coupling between these ground planes incidentally, and it will work even in space).
Radiation is small from a long line because at any point on the line, there is always another point half a wave further on which radiates in opposition. This also applies to the parallel wire line.
There are always two modes on a transmission line conductor. In one, the electric field is transverse, and crosses to the other wire. In the second, the electric field is longitudinal and connects points on the same wire at different potential (for instance, between points half a wavelength apart).
The latter mode is the one responsible for accelerating the electrons and producing the line current and also the tendency (for a short line) to radiate, because radiation arises when electrons are accelerated.
 
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  • #38
Just to add to the general confusion, but perhaps to deepen understanding and create interest, may I mention that if one wire is removed, the transmission line continues to function, even without the presence of ground, and radiation is still very small.
In that case transmission line has to be more than 1 wavelength long? In my case, the antenna is 40m long (dipole for 3.5MHz) but my transmission line is also 40m long. So in my case, if I removed one wire the other will radiate.

Since I realized that antenna doesn't radiate in near field, that makes sense. Fields cancel each other and there is almost no total field that would create EM waves that will become free in far field.
 

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