how radio waves are broadcast out of an antenna.
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Radio waves are waves (like water waves) in the electromagnetic field of the universe. If you move electricity or magnets back and forth in space, it will send ripples through this field, and we call these ripples radio waves.
An antenna is simply a length of metal through which you can move electricity back and forth.
I am telecommunication engineer and this topic was difficult for me to understand when I was studying. This is how I build an intuition about it.
The simplest kind of antenna is a wire, just a piece of metal. Hence it is conductive, you can have an electrical current though it. Electrical current is just movement of free electrons in the wire. For example, if you have an AC current you can imagine electrons going back and forth along the wire.
Here comes the interesting part. How this current is radiated out of the wire? It is simpler than it seems, the effect of moving charges in the antenna (electron flows) generates charges in the air around it (what is called an electromagnetic field). This charges as they move back and forth from the antenna stimulate the air around it and so on and so forth. The farther you go, the smaller this effect is (power decrease rapidly with the distance). However, if the current in the antenna is powerful enough and the antenna is cleverly design (not just a wire) you can reach interstellar distances with this method.
An antenna is just a fancy looking electric wire. Radio waves are electricity that is so excited it has the ability to leave the wire.
To add to the antenna size part of it, the antenna size is a function of the wavelength, high freq = smaller. think of it like matching the difference between the wire and the air. Once you get the wavelength match perfect the efficiency goes up, and more of the electricity leaves the wire vs bouncing around the wire, oh yeah there’s that part where if the antenna doesn’t match the frequency the radio waves have the ability to bounce back down the wire.
take a tuning fork and strike it.
like that, only instead of hitting it, you’re pumping an electrical current through it to give it energy.
Any alternating electric field travelling in any metal generates an electromagnetic field around it. This alternative field propagates as a “wave” at the speed of light.
A single straight piece of wire with a changing current flowing through it works as an antenna, coupling the line to the EM field.
We can even pick up the “hum” of the 60hz power lines in your house with unprotected sensitive devices.
Antenna design is all about efficiently transfering as much power to or from the em field and the transducer a.k.a. antenna, including limiting directionality.
You ever take a bath and swoosh your body backwards and forwards to make the water start a wave that goes back and forth? It’s like that … but in the *fluid* of the electromagnetic field.
Find a thing, bendy wire/stick/thing. Something that you can shake really quick and make it vibrate.
Vibrate the “thing” as close to a constant frequency. You will notice that there are points (nodes) that stay in place, and other parts that move at a maximum distance.
You can change the distance between nodes and other aspects by changing the frequency you shake it, or the material it’s made of.
This is *somewhat* analogous to RF radiation. The frequency moves down the antenna and bounces back at the end. This forms electromagnetic nodes along the antenna. The points of maximum movements is where the actual radiation happens; throwing off photons at those points that have the same frequency.
Also, I’m pretty sure RF propagation is like, 10% Voodoo.
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