Eli5: What component in eletonics like cellphones and computers actually “receives” and “emits” wifi signals? What is wi-fi in terms of physics?
In: Physics
WiFi signals are just a specific wavelength of radio waves, which is a specific range of electromagnetic waves, the same as light. You can think of devices communicating through WiFi as a bit like using a flickering lightbulb to transmit messages in Morse code, except that the code it uses is more complex, and the light is in a color that is invisible to the human eye but for which most things like walls are transparent like glass.
All wireless technology needs an antenna of some type to transmit and receive signals.
WiFi is controlled fluctuations of the electromagnetic spectrum, radio waves, in the 2.5 and 5 ghz range. EM radiation is made of photons, massless particles, that travel at the speed of light.
The electronic component that receives and emits signals in therms of physics is called antenna. The space around the antenna is filled with electromagnetic waves. If WiFi is receiving then those waves manifest themselves on the antenna as a changing voltage pattern – this voltage pattern is being read. Received signal is very weak and needs amplification as well as many processing and analysis steps to be understood.
If WiFi is emitting then the voltage pattern on antenna is being forced by the WiFi chip itself. The power of emitted signal must be very large, because the signal will loose much of it as it traverses space in the form of electromagnetic wave.
Honestly, in terms of physics WiFi is not much different that a basic FM radio. The real power of WiFi protocols are in the way that they form EM signals – those voltage patterns – which allows them to stuff as much information in them as possible.
When you apply a voltage to an open wire, the electromagnetic field around it changes a little. This change propagates to infinity, diminishing in magnitude as distance to the wire increases. Do that many times per second, and you have a radio wave. By superimposing additional voltages onto a radio wave of known frequency, a message (such as sound, or digital information) can be sent. This is called modulation.
Electromagnetic waves induce voltages in wires (the reverse of the process above). Precise filtering for specific frequencies allow a receiver to reproduce the signal encoded in the radio wave. This is called demodulation.
A radio antenna is just a wire of a specific length that corresponds to the wavelength (or even fraction) of the intended radio frequency. This causes it to resonate electrically at that frequency, which increases transmission and reception efficiency by attenuating frequencies other than the intended frequency.
WiFi frequencies are ultra high (UHF), meaning their antennas can be very short. This is useful in cellphones and other small devices.
In terms of physics, Wifi is exactly the same stuff that your microwave uses to heat up food…microwaves. Same frequency too. Wifi devices just use way less power making them safe. Microwaves are a frequency of electromagnetic radiation also known as light which have less energy than what is needed for us to be able to see them.
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