How do antennas work?

Question

I know the basics about em radiation. However, I’m not in college, and I’m not a genius. I’m really looking for someone that’s willing to help explain this to me in a relatively simple way, without long equations and over complicated phrasing.

I thought that antennas had to be part of a closed circuit, like alternating current circuits...sure maybe there’s a capacitor in there, but it essentially is closed(for ac, not dc necessarily). I’ve seen laptop antennas that appear to have a signal wire and ground wire. The ground wire covers most of the signal wire. Only the ‘antenna’ length is exposed..not covered by ground. It appeared to be a closed circuit(kind of like a loop)..

Well, an rc car/copter antenna appears to be a single rod. I don’t see a ground at the tip or anything. It appears that it’s open circuited. The only thing I’ve gathered from googling for hours is that the em waves(lots of photons) hit the antenna and cause electrons to gather at one end, then the other end of the antenna. This causes a potential difference . I’ve read this only works at high frequencies, although A.M radio also uses a single rod antenna, and it’s only in the kz or mz. I’m just so lost. I’m not trying to claim I know everything I said for a fact. This is just how I interpreted it.

Also, why does the length of an antenna matter so much? I get that if the em wavelength is short, using a smaller antenna is best because it doesn’t receive longer wavelength waves as well. I think that the resonance frequency of the antenna has something to do with this(I’m still a bit confused about how resonance works with an antenna..basically what decides the resonance frequency?). But if photons are striking individual electrons(at least that’s what I’ve learned, although I’ve heard that as a wave, a single photon can strike multiple electrons at once, pushing, then repelling them, which I also don’t understand) But how does the resonant frequency of the antenna have anything to do with the frequency of photons? Why does the resonance frequency correspond directly with the length of the antenna relative to the wavelength of the em wave? Is there something I’m missing here? -Does the length of the antenna have anything to do with the probability of absorbing photons at that same wavelength, without having as high of a probability of absorbing longer photons? But then it would absorb shorter wavelength photons easier based on that logic..

Guys, I’d like to say I’m sorry for the way I word things. I’m not smart. I have a really hard time expressing my questions using conventional terms. I haven’t gone to college or anything, so I don’t know a whole lot. If anyone is willing to try to help me out, I’d really appreciate it. Please feel free to ask me to clarify any of my questions in a way that you’d understand better.

Answer 1

Your question is somewhat meandering, but I'll try to answer what I can.

First, I advise you to stop thinking about antennas in terms of photons, or quantum mechanical phenomena in general. The first antennas were built using only a classical understanding of electromagnetism, and such an understanding is all that's needed to understand most of their properties today. This will only end up confusing you further. Simply imagine that there are some electrons inside a conducting material that are subject to electromagnetic forces generated by incoming electric fields.

The antenna works because an incident electromagnetic wave generates an oscillating electric field which pushes the electrons in the antenna around. An antenna doesn't have to be a closed circuit. Theoretically, a Hertzian dipole with two conducting balls at the end linked together by a conducting wire could also be an antenna, since an incident electric field will still push electrons around inside. In fact, such a design can be refined and made fairly practical using a half-wave dipole antenna.

Using RLC circuits, you can construct an antenna which responds very strongly to an electric field at a specific oscillation frequency (the resonant frequency of the circuit) and not so strongly to other frequencies. Many such circuits combined effectively split the incoming signal to its separate frequency components, a useful operation in signal processing known as the Fourier transform.

The resonant frequency of an antenna is determined by its constitution. Mathematically speaking, this is a general property of second order differential equations; but in down-to-earth terms, any AC circuit with some inductors and capacitors in it has a "resonant frequency": this is the frequency current would circulate through the circuit at if you supplied some DC voltage to it and let the current run its course.

It turns out that the mathematics of the situation works out in a way that if you supply an AC power source to such a circuit at a frequency very close to its resonant frequency, its effective impedance is much lower, i.e. you get a much larger mean current running through the circuit even though you're supplying it the same amount of power. This is because the counteracting effects of the capacitor and the inductor are cancelling each other and eliminating a lot of power dissipation. At exactly the resonant frequency, the impedance of the capacitors and inductors of in your circuit exactly cancel, and only the resistors contribute to the impedance, which raises the circulating current for fixed voltage.

You can easily construct an RLC circuit with a given resonant frequency by taking a low resistance wire and hooking it up to an inductor (say, a loading coil) and a capacitor whose inductance $ L $ and capacitance $ C $, respectively, have been chosen such that $ \omega = \sqrt{1/LC} $.

For a half-wave dipole antenna, the resonant frequency is determined by the length of the wire. This is because the current induced inside the dipole by an electric field at some wavelength must have the same wavelength, and the length of the wire constrains what wavelengths current can flow through the wire at while retaining their phase. (Mathematically, we would say there is a boundary condition imposed by the ends of the dipole.) Generally such dipoles are also sensitive at odd multiples of their resonant frequency, unlike an RLC circuit whose resonance is sharp inside a single frequency region.

If this explanation isn't clear enough, you can ask for further elaboration in the comments.

Answer 2

An antenna is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an oscillating radio frequency electric current to the antenna’s terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves).In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified.

What is an antenna and how antenna works