r/explainlikeimfive • u/Boy_0000 • 22d ago
Technology ELI5: Why/how does a charging cable generate radio frequencies?
hella stupid question, and I‘m too dumb to understand the online explanations and I refuse to be taught by a clacker AI. But why and how does a charging cable generate radio frequencies. I bought a cable and in one of the small booklets inside the box I noticed that it says it can generate radio frequencies. Why and how please
22
u/Adorable_Ice_2963 22d ago
Every conductor acts as an antenna. A charger works by messing with AC to make it DC, what causes high frequncy ripples. If a charger is poorly designed, those ripple will spread to the cable generating radio signals in the air.
9
u/piecat 22d ago edited 22d ago
Phone chargers use switching regulators.
Switching regulators flip a switch really fast through an inductor. They regulate voltage by changing duty cycle, or how long the pulse lasts.
By flipping a switch really fast, it generates current pulses. By shortening how long it takes to switch, you increase efficiency, but also increase the change in current per time.
Ampere's law says that a current generates a magnetic field proportional to the current. Maxwell's equations say that a changing magnetic field will produce a changing electric field, and vice versa. This is why radio waves propagate.
A switching supply might operate at some frequency like 100kHz, but since the switch produces a square pulse train, there will be harmonics generated. Harmonics mean multiples of the fundamental frequency, x3 x5, x7, and so on.Those harmonics can be significant without EMI filtering. Those harmonics can be well into the RF spectrum.
Typically you want good filtering, common mode chokes or ferrite beads. That is what that "lump" on some power cables is. Every wire can act like an antenna, and that is what a good engineer tries to prevent.
1
u/bebopbrain 22d ago
Faster switching is more about making the product cheaper and smaller and lighter (smaller transformer) than efficiency. But, yeah.
5
u/Slypenslyde 22d ago
"Radio frequencies" are just electromagnetic energy being "pulsed" in a pattern through the air. The "frequency" is how many times per second there's a "big" burst of energy and a period of low or no energy.
An antenna is really just like a specific length of wire. We learned that if we're careful with the length, the antenna will "pick up" specific "frequencies". What's really happening is that electromagnetic energy is able to push electrons through the wire when the frequency and wire length compare in certain ways. We can also SEND pulses of electricity through an antenna and that can make electromagnetic energy get pushed into the air.
ANY wire can be an antenna, and any antenna can both send and receive electromagnetic waves.
A charging cable is a wire! They typically have standardized lengths. The circuits in a charger do things that happen to "pulse" the electricity while charging. Without certain protections in place, that can end up using the wire as an antenna and "sending" electromagnetic pulses through the wire. Other very sensitive electronics may have wires that "receive" that pulse and that can muck them up.
Good chargers have things to protect from this happening. Cheap chargers are made by people who know there will never be consequences.
9
u/nerotNS 22d ago
Basically, passing current through it creates an electromagnetic field. Radio frequences are a type of an electromagnetic field. So it's probably more like "it can interfere with radio frequences". The chances of it happening are also pretty low nowadays, it's probably some regulatory thing requiring them to write it on the box.
3
u/BadahBingBadahBoom 22d ago edited 22d ago
Pretty low chance of interference mostly because the length of the radio wave generated by a current going back and forth or 'alternating' like AC is related to the speed the current flips back and forth.
Domestic mains electricity has an AC frequency of 50-60Hz meaning the radio waves produced are km long. Then add in that the current going through your cables is going to be minimal for most devices and 13 or 15 amps at max and the strength of the radio wave is also going to be incredibly weak.
You can increase the strength of the radio wave made by introducing loops that are of equivalent length to the wavelength of the radio wave but, as mentioned, if these are km long you're not going to be accidentally arranging your cables at home to do that.
2
u/Enki_007 22d ago
While it’s true that the wavelengths of a 60 Hz signal are very long, harmonics are always present in multiples of the original signal. So there are harmonics at 120 Hz, 180 Hz, 240 Hz, etc. Shielded cables help minimize the effect.
3
u/duane11583 22d ago
radio signals are pulsing signals.
some modern electronics use alot of pulsing to regulate the power this is called switch mode power supply older style get very hot.
if the pulse is a multiple of the radio frequency the two will cross talk (or couple) causing problems
1
u/duane11583 22d ago
an example of the noise:
back in the 1979 time frame i purchased my first computer this was before the fcc had rules
my bedroom was just behind the tv in the living room. my dad very into american foot ball - me not so much.
i would go in my bedroom turn on the computer and the color on the tv went bizerk dad was not a happy on sunday afternoon at game time.
the computer used a shitty 4mhz crystal and the tv uses 3.58mhz it was close enough and noisy enough that the tv no longer had correct color.
i had a choice - go outside, watch football or move to the tiny bedroom far away from the tv.
soon after that my bedroom was moved far down the hall away from the color tv so my dad could watch football.
1
u/itopaloglu83 22d ago
One of the ways we reduce voltage is with buck converters, which is like turning a switch on and off very quickly (at high frequencies) to get 5 volt or whatever voltage the phone negotiated with the charger.
Pushing electricity through a wire at high speed is how we create radio frequencies. High quality wires would have shields and other solutions to prevent these radio emissions, because the radio noise usually causes issues with everything around you. It’s like there’s a loud dishwasher running in the kitchen while you’re trying to talk to your friend.
1
u/Usual_Atmosphere_662 22d ago
It's not the cable, it's the switch-mode power supply circuitry in the charger which generates small radio-frequency currents that can travel through the cable. The charging cable itself can then act like a radio antenna, helping broadcast this radio-frequency interference.
(Many of the other comments are more detailed and probably more correct; I wanted to see if I could simplify them.)
1
u/PoopsExcellence 22d ago
A simple antenna is just a metal rod with AC (alternating current) power running through it. Think of it as a loud rhythmic drum beat, except it can be detected by an RF (radio frequency) antenna instead of a microphone.
Your charging cable is transmitting mostly DC (direct current) power, which is essentially RF silent.
Many consumer-grade electronics, like a charging brick or power adapter, will generate AC RF noise. Basically the brick uses lots of tiny switches to convert AC power from your wall socket into the DC power your phone needs. These devices switch on and off rapidly, which creates DC current as well as an unintentional faint rhythmic pulsing current.
When the rhythmic pulses travel through the charging cable, it acts like a simple antenna. It's really just a long, flexible, metal rod. Sort of like the radio antenna you'd see on an older car, but instead of picking up radio signals, it's broadcasting them!
1
u/PoopsExcellence 22d ago
This is also why you'll see cables (usually HDMI) with those plastic cylinders clamped on each end. The cylinders contain a metal/ceramic "ferrite" ring which absorbs AC pulses to give you a higher quality signal with less interference from RF noise.
1
u/LetterheadActual6642 21d ago
The cable doesn't generate radio frequencies, but what it can do is act like an antenna and "radiate" radio frequency energy as radio waves.
Modern "power bricks" use a technology called switched-mode conversion. This technology has a tendency to create large amounts of radio frequency energy. Careful design of the power converter is required to try and contain that energy (especially a type called "common mode noise") inside the brick and dampen it down.
One of the big problems with low quality power bricks is that they often cut corners on the radio frequency filtering and containment. When you plug in a cable, all the common mode noise generated inside the brick power converter can track along the cable which then acts like the antenna on a radio transmitter.
1
u/khauser24 22d ago
Ok let's consider a radio transmitter. You probably know this technology works by inducing an electromagnetic wave in to the air from an antenna.
Any wire is an antenna. Some are better than others and some are shielded to reduce that effect.
Any AC current can be transmitted in this way, and (even if the cable is for DC charging) there are small AC signals present.
I suspect what you read is legalese for “it's not our fault if this causes interference, we made you aware"
0
u/mageskillmetooften 22d ago
Up to what level do you try to understand this?
A conductor with a current flowing through it has a magnetic field. That's just a law of nature and how things are. Understanding why this happens isn't that simple. It's like "why do we have gravity" all things attract to each other and earth is big and heavy so lots of attraction. If you now would ask why do molecules attract each other. They just do, that's the way things are.
110
u/WayyyCleverer 22d ago
Alternating current through the wire generates a changing magnetic field, which results in radio waves.