SilentHunter7 t1_j8kphnf wrote on February 15, 2023 at 12:35 AM

I'm an EE grad student, who's specializing in microwaves and antenna theory.

So a good way to think about this, is that changes in the Electric and Magnetic fields don't propagate instantly; they travel at the speed of light. So if I turn on an electromagnet, it will take some time for any metal near it to feel that force. Even though that time will probably be measured in nanoseconds, it still takes a nonzero time between the magnet being energized, and for the metal to feel the magnetic force.

Now imagine an antenna as a tiny electromagnet that's being flipped from positive to negative billions of times a second in a sinusoidal pattern (this is an EXTREME oversimplification of what an antenna is, but for the purposes of this discussion, it's enough). The changes in the field will only propagate at the speed of light, but the magnet is changing extremely fast.

This makes it so that if you take a snapshot of the magnetic field at a single instant, you will see the field shift from positive to negative to positive to negative with distance from the antenna. If you measure the distance it takes to go from positive to negative to positive again, that's your wavelength.

Here's a gif of an ANSYS simulation I made of the Electric Field of a simple dipole antenna over time. You can see the wave-like pattern in the field magnitude.

And here's one I made of a Yagi Antenna.

TwentySevenNihilists t1_j8lznkg wrote on February 15, 2023 at 7:53 AM

Is what I'm seeing in these animations two/eight smaller antennae arranged such that their wavefronts(?) are in phase with each other, so they amplify to create a much stronger wave with a flatter curve?

I've worked with wireless hardware of various types most of my adult life, but I've still always had that disconnect with trying to visualize a wave in 3d space; especially when looking at some of the antennae I've run across. Would it be accurate enough to say that the "waves" expand in a shell from a point of origin, and each "shell" is more or less the location of the peak field strength at the moment you're measuring it?

SilentHunter7 t1_j8mpg55 wrote on February 15, 2023 at 1:17 PM

So these animations are only showing you a quarter of the picture. It's a plot of the absolute value of the Electric field. It doesn't tell you anything about the direction of the field, nor does it show the Magnetic field.

So for the dipole, all it is is two straight wires, about a quarter wavelength long connected to a transmission line. Imagine the top connected to the center of the coax, and the bottom connected to the shield.

When a wave coming down the coax hits the antenna, it causes a current in the wires. Electrons will be pushed into the top wire and pulled out of the bottom wire. This creates a charge on the wires, negative on top, and positive on the bottom, and you can see that in the animation.

But because waves reverse, soon you'll get a reverse current and the top will become positively charged and the bottom negatively charged. This can happen billions of times a second for something like 2.4GHz wifi.

And also, current creates a magnetic field. So when current is flowing in the wires, there is a magnetic field wrapping around them. This current hits zero when the wires are fully charged, and is at it's maximum right when the wires are neutral.

So now you have Electric and Magnetic fields all swirling around each other at a constant frequency. This is you get EM radiation.

If I haven't lost you yet, you should consider going to school for electrical engineering; antenna theory is some of the most esoteric shit this side of Quantum Mechanics.