antenna-theory.com

[idmond]

I've been studying the topic of the radiation mechanism of an antenna, and i am now stuck with these questions
that i hope i can find answers for them. Here's my questions:


1-are these point of views about why antenna radiates (i.e the EM fields propagate away from the charges)
when the charges is accelerated, is what really happens in an antenna, or we just don't know and we use these mental pictures to help us and other people visualize why when we have accelerated charge or antenna radiates?
- point of views:
a- the dipole and the capacitor and the E field from a loop when the capacitor is stretched and that loop is
repelled by the current E field loop in the stretched capacitor.
b- the graphical argument which was presented by J. J. Thomson in Longair’s book "High Energy Astrophysics". that the accelerated charge and the jiggling or the disturbance in the field lines that propagates away from the source .


2- what the relation between the current and the charges speed, i.e does a current that its intensity changes,its speed changes too or is it irrealevent?


3- suppose that i took a charge and moved it up and down, let's say 10 times per second, does this charge radiates considering the fact that the movement of the charge is produced by a force from my hand not from an electric or a magnetic field like what happens in an antenna?
i asked this question because i have read in an article and many other places that charges only radiates when they're accelerated by E or M field not by gravitational field or any force produced by other means. is it true?
or no one really knows for sure or they didn't prove it experimentally yet?
i'm confused here. (2 technical people, one has a PHD in EM and the other in physics) said that it'll radiate in this case.


4- what's the difference between uniformly accelerated charge and a charge under constant acceleration?


5- some people say that any piece of wire that carries a current radiates? does it mean any type of current or only currents that're changing?
because i know for sure that dc current doesn't radiate (only at the time of switching ON, the rising edge, and switching OFF, the falling edge)
because the charge isn't accelerated in that case, so no radiation happens. right?


6- does the electrons in an antenna at FM frequencies doesn't actually move to any far end of the antenna and only they travel hundreds of million of an inch and then switch direction and cut the same distance in the other direction, but doesn't reach any of the two far ends of the antenna, just sloshing back and forth and that sloshing is the only thing that's transferred to other nearby electrons in the antenna wire, just like how sound propagates through air molecules, that the molecules themselves don't rush to the other end, but only transfer movement to the nearby molecule and that molecule do the same to his nearby molecule and so on till the effect (i.e the sound) reaches the last molecule close to the far end.
is what happens with air molecules is the same same thing that happens to electrons in antenna wire?


7- is the reason that twisted pair wire that carries changing current(i,e accelerated charges) doesn't radiate EM waves, because the current in each wire flows in an opposite direction and since the wires are so close to each other, the effect of EM radiation will reach any charges in space almost at the same time, but with forces trying to move the receiving charges in opposite direction with the same magnitude , so the charges eventually don't move (because it's now subjected to two forces from the two EM waves from the two wires, that have the same magnitude but with opposite directions).
is this is the case?
and if it is the case, then that means that if i spread the wires further instead of twisting them around each other, the wires will radiate more.right?


8- Why the field line of a charge doesn't bent, when it moves at a constant speed?right?
if a charged particle were suddenly created,
its field would not instantly appear everywhere. On the contrary, the
field would first appear in the immediate region surrounding the
particle and then extend outward at the speed of light. For example,
light takes about eight minutes to travel from the sun to the Earth. If
the sun were to suddenly extinguish, we would not know until eight
minutes later. Similarly, as a particle moves, the surrounding field continually
updates to its new position, but this information can propagate
only at the speed of light. Points in the space surrounding the particle
actually experience the field corresponding to where the particle was
at a previous time. This delay is referred to as time retardation. It seems
reasonable to assume that even a charge moving at constant velocity
should cause the field lines to bend due to this time retardation.
However, Einstein’s theory of relatively states that velocity is a relative
measurement, not an absolute measurement. If the field lines are
straight in one inertial observer’s reference frame, they must be straight
in all other inertial observer’s reference frames.right?


9- a uniform transmission line is typically a poor radiator at any frequency, why?
because of its geometry. for example, the wires of a two-wire transmission line, have one wire carrying a current (and hence charge) in one direction and the other wire carrying a current (and hence charge) in the other direction.that means that at far distances, where the spacing between the two wire appear at far distances as almost zero distance, the radiation field of each wire will act on any charge at far distances,
with a force that's opposite to the other field in direction and equal in magnitude, that means that the charge won't feel any force acting on it, and hence to it, it sees no radiation field, although there's a radiation field but it has a net value of zero, because the fields tend to cancel each other at far distances. if that was true, then this means that if i want to increase the radiation of any circuit or a cable that is carrying changing current, i just spread the two wires of the cable or a circuit a little bit apart to the outside, like when you spread out a capacitor to create a dipole, so they won't cancel each other at far distances, right?


10- How did Hertz knew that what he observed in his experiment, was a radiation far field not a reactive storage near field, taking in mind that he received the wave at very close distance from the transmitter?

[bigSteve]

That's a long post idmond. I'll throw out some notes

You need accelerating charges because radiated waves at any single frequency are sinusoidal - so what gives rise to them must be changing at the same rate.

Radiation occurs when either current adds in phase (like a dipole antenna), or voltages add in phase (like a slot antenna). A circuit would not radiate because while oscillatory current flows, the current flows in two directions and therefore cancels out. A dipole antenna at a half-wavelength does not have canceling current. From maxwell's equations (www.maxwells-equations.com) nature wants to produce radiation - but things can't cancel themselves out.

Read this page if you haven't already:
http://www.antenna-theory.com/basics/whyantennasradiate.php

If you took a charge and moved it up or down, this would be current flow. It would produce a changing disturbance in the fields around your charge. This is radiation. So yes - this would absolutely radiate in proportion to the magnitude of the charge and how fast you move it.

Your 4th question doesn't make sense.

Question 5: Wires do radiate. However in a circuit you have current going in one direction and then back around in the other - the result is no real radiation because things cancel. At D.C. you do have a wrapping H-field (but no E-field that surrounds it), so you have fields that surround the wire but not really a propagating wave.

Question 6: No, it's not like sound propagation. Once the voltage is applied across an antenna, the E-field moves at the speed of light and there is a net Electric Force (E-field) along the wire. This induces the free charges in good conductors to move, but they are not bouncing into each other.

Question 7: Yes. Now the higher you go in frequency, the more likely the separation between two wires will not cancel each other out but may add together. Hence if you twist them around each other the wires are more likely to cancel out. But at high enough frequencies they will still radiate.

Question 8: See the why do antennas radiate page above.

Question 9:Same as Q7 really.

Question 10: If he did the experiment at very close distance, he couldn't have known whether he was coupling through radiated or near fields. This is a common mistake even today.