One insight that may help non EE educated people understand what an antenna does: The simplest form of an antenna would be just a simple point emmiting electromagnetic waves. These waves would be transmitted all around that point distributed as sphere. All points of a sphere will have the same power. Now, an antenna has a different geometry than a point that helps somehow "focus" the EM waves, so their power is not distributed around the sphere uniformally but some directions get more power depending on the antenna design.
Now the thing to keep in mind is that an antenna is a passive device. It does amplify the signal but it does not add any power to it, it just collects the power to specific points. This may be easier to understand with a receiving antenna (which collects the signal).
For example, consider the satellite dish which is of course an antenna. Due to its design it should be conceptually easy to understand that the power of the transmitted field is all gathered in a very small area in the front of the dish. The largest the dish, the smaller the point where all the transmitted power is pointed, so less power would be needed to cover largest distances (and more difficult finding where the dish needs to point).
Great explanation! Maybe I'm asking a dumb question but I'll shoot anyway: what is power? I mean, when I change the power of a signal what physical variable am I playing with? Amplitude, frequency, or wavelength are intuitive to visualize, but power?
Power is how "strong" a signal is. I think it's the easiest one to understand, that's why everybody's talking about dbm!
In all electric circuits to change the power a component will consume you either change the voltage you apply to it or change its resistance, based on the P=VI & R=V/I equations.
I really cannot get my head around it. What is "strong" in a signal? What physical variable is stronger? I assume "strong" or "more power" means that the signal attenuates less over time/distance, but what is causing this lesser attenuation?
No, the attenuation is exactly the same no matter the power and is analogous to the square of the distance. So a signal that has power X in a point 10 meters from the source it will have X/100 20 meters from the source.
The analogy of the waves that another comment mentioned is a nice one (since we talk about EM waves).
Another thing that may help you understand that more power means that the electrical charge to the protons/electrons in the air where the EM field is transmitted will also be more.
The amplitude of the wave is bigger to begin with. I think the waves in water analogy is suitable here: drop a stone in water and there'll be very big ripples just around the point where it drops, but they'll get smaller the further out they are. Drop an even bigger stone, you'll get bigger ripples. That's power.
An antenna doesn't amplify a received signal. It takes an actual electrical amplifier to do that. When an antenna's gain is being mentioned, that gain is actually how much better the antenna is at taking in more of the signal than the standard candle isotropic antenna, e.g., one that transmits and receives equally in any direction.
Well it amplifies the signal by the Q of the antenna. The issue is that high Q resonators make lousy radiators, as the radiation resistance is very low compared to the reactance, and bandwidth is 1/Q. But you can certainly get amplification by a high-Q resonator inserted in a field.
This is why electrically small antennas have an effective aperature much, much larger than their physical size. That ferrite rod antenna in an AM radio can have a massive electrical aperature (antenna gain) since it is narrow band (high Q). The resonance is amplifying the signal.
You can also transmit through it if you can keep it cool. It may have 0.1% efficiency, radiating 1W for 1 kW of input power, if you have a nuclear reactor (say a VLF antenna on an aircraft carrier where you can’t have wire antennas), you can trade power for size.
Yes I tried to make that clear by mentioning that an antenna is a passive device. I just wanted to clarify that antennas help both at the transmitting and receving end. English are my 2nd language so maybe I could have explained it better...
Now the thing to keep in mind is that an antenna is a passive device. It does amplify the signal but it does not add any power to it, it just collects the power to specific points. This may be easier to understand with a receiving antenna (which collects the signal).
For example, consider the satellite dish which is of course an antenna. Due to its design it should be conceptually easy to understand that the power of the transmitted field is all gathered in a very small area in the front of the dish. The largest the dish, the smaller the point where all the transmitted power is pointed, so less power would be needed to cover largest distances (and more difficult finding where the dish needs to point).