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

[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.