Why Doesn't a Transmission Line Radiate All Its Power Away?

[GhostLoveScore]

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?

 

[Jeff Rosenbury]

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.)

 

[GhostLoveScore]

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?

 

[Jeff Rosenbury]

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 ε_r = 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.

 

[GhostLoveScore]

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)

Not literally like in "b" case, just that part where their amplitudes cancel each other.

 

[Jeff Rosenbury]

Yes, (b) is mostly correct, particularly (b)iii.

 

[tech99]

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.

 

[GhostLoveScore]

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.

 

[Craigmail]

There is a fair amount of current research on the topic of transmission and reception of radio waves from relatively open transmission lines. Below is a link to one of them. It seems that although two wire transmission lines radiate energy, for most cases of interest, the radiated power is a rather small (about 1%) percent of the power flowing on the line.

This is why "twin leads" are good feedlines for antennas and, with some exceptions (Beverage, Rhombic), not commonly used as transmitting or receiving antennas. I hope this is helpful.

https://arxiv.org/pdf/1701.04878.pdf

 

[sophiecentaur]

The photons are very roughly equal to the wavelength in size, so much bigger than the distance between the wires. Most of them never had a chance to break free.

Years later, I have read this statement. Unfortunately, it's nonsense. Photons have no defined 'extent' and are not needed for an 'explanation' of a wave phenomenon like an antenna and feeder.
Stick with JC Maxwell for this.