Radios, how do they work?

For sure they do not work the way the "Path Loss Equation" would have you believe they do. The path loss equation violates conservation of energy ie the frequency or wavelength term depending on how it's structured cannot be in the equation. And the receiving antenna does not have any 'gain' other than physically getting bigger or smaller, though the transmitting antenna can have gain depending on shape and size. That is, the transmitting antenna and the receiving antenna work very differently. Yes, end to end the path loss equation gives the right answer but in between it's scientifically illiterate.

Would it be possible to construct a rudimentary FM radio receiver with only the most basic parts ala Masters of the Air?

I think this undersells the trick behind radio.

Say we have the technology to broadcast a signal from an antenna to receivers, with some bandwidth B. Without getting clever, we can only send or receive one signal, since any others would interfere with each other.

The trick is, can we do something to shift the bandwidth B to some other base frequency F such that B + F > B? Or B + (N - 1)F > B? And if we can do that, and then downshift from B + NF back to B, it means we can broadcast to multiple channels, and receivers can tune their antennas to F and downshift the decoded signal to 0 and receive it at the original bandwidth B.

A cheap way to do this is amplitude modulation, where multiplying a signal with bandwidth B by a carrier signal of frequency F shifts it up to the range F +/- B and we can space channels apart by 2B to get however many channels our antennas allow for.

The real question is, why is it 2B and not B? Well that lies in some Fourier analysis, where the bandwidth of a signal extends into negative frequency ranges. But neverless, there is another trick, called single-side-band modulation (SSB) where we can shift a signal into the range F + B instead of F +/- B, and demodulate it into -B, B to get the original.

And that gets us to the 1950s in terms of radio technology.

The trick behind FM is to understand we can get more bandwidth by shifting the frequency response not into a series of non-overlapping channels centered at carrier frequencies like AM, but to distribute most of the information across many non overlapping bands over the entire spectrum of the antennas. To do this we don't modulate the amplitude of the carrier, but its frequency. This makes it possible to distribute far more bandwidth across a wide range of frequencies, and it's how FM radio works today.

These concepts create the foundation for modern radio communication, We can modulate data signals to different bandwidths and receive them, provided we know where to tune to. And these bandwidths can either be continuous chunks of spectrum (AM), or interleaved (FM). The next step is to think in terms of time, which is to say that we can have receivers negotiate not only which ranges in frequency they care about, but which time frames they want to listen before waiting for their next time slot.

For those interested in the theory, the fundamental problem is that we can design antennas that can transmit or receive at some fixed maximum bandwidth, bounded by physics. The engineering problem is to find out how to share that bandwidth to maximize the number of receivers and/or senders by sharing the same bandwidth. Amplitude modulation is excellent, but it divides the bandwidth up into a fixed number of channels of maximum individual bandwidth. FM is a bit more efficient in how it can allow many broadcasters to even more receivers choose which channels they receive. But for modern communications, where we need high bandwidth for distinct transmitter/receiver connections, we need protocols to figure out how to share the bandwidth over the air and the two tricks are to divide that bandwidth by frequency (like AM and FM) or time (sharing the same frequency channels, but only picking the frames that we care about), or both.

This is an excellent article, thank you for submitting it! I love how effortlessly this article delivered an intuition for why an ideal antenna length would be half of the wavelength of the signal you want to receive. I was also delighted by the point about how all methods of modulating a wave can be recontextualized as frequency modulation!

I'm intrigued by things like this that used to be high technology but now are mature and pushed way down into the infrastructure. No one is going to make much money being really good at radio, any more than they will be really good at machining steel, but it's still necessary for higher levels of the tech stack to function.

Their primer article [1] is also really nice.

Today, I’d like to close this gap with a couple of crisp definitions that stay clear of flawed hydraulic analogies, but also don’t get bogged down by differential equations or complex number algebra.

Related: many, many years ago, when Facebook didn't exist yet, Google still passed as a "good" company, and hobbyist electronic geeks had almost only PICs to choose from, I found online a very long and complete electronic course that went from 0 to basic R/C concepts, to transistors, up to pretty advanced topics like magnets/transformers and IIRC radio too.

It was made of pretty raw HTML pages and images, and what was most peculiar about it was that it managed to explain a lot of concepts up to an applicable level (as in, actually designing analog circuits) without (any?) calculus at all.

Some of those may be false memories, but if I remember correctly:

* Its HTML style had a yellowy background * It was taken from an old-ish (US?) navy electric engineer-focused applied electronics course for training naval engineers. * It was more focused on analog circuits

I remember I downloaded it all but after all those years who knows where it could be. Maybe in some 1GB disk of my first Pentium PC, so it's basically lost.

Does anyone in HN knows what I'm talking about? I was never able to find it again.

[1] https://lcamtuf.substack.com/p/primer-core-concepts-in-elect...

A perfectly uniform waveform is still not useful for communications...

It is if you encode information by switching it on and off in standard patterns. These uniform waveforms--or "continuous wave" (CW)--allow very simple devices with very little RF power to be used to communicate with Morse Code.

Also, I’m not sure if people are aware of the number of radio systems that enable their smartphones.

NFC (eg. Apple Pay) is a radio, range a few cm. Bluetooth is a radio, a few meters. WiFi is several radio systems, range tens of meters. Cell phone is several radio systems, range up to kilometers. GPS (and rival systems) range up to thousands of kilometers.

Magnets

I work RF world pretty regularly, and I still consider the Superheterodyne Receiver to be tantamount to magic.

Edwin Armstrong was a brilliant brilliant man.

Antennas have always been black magic for me, and this article blew my mind with the "capacitor you pull apart". Thank you for posting this article, this is fantastic.

In today’s article, I’m hoping to provide an introduction to radio that’s free of ham jargon and advanced math.

Sounds great! Let’s dig in.

… the fundamental mirroring behavior is still present, but it’s usually managed pretty well. Accidental mirror images of unrelated transmissions can be mitigated choosing the IF wisely, by designing the antenna to have a narrow frequency response, or by putting an RF lowpass filter in front of the mixer if needs be

Mission failed. Ah well.

Tim Hunkin has posted a remastered version of his "The Secret Life of the Radio" TV program (from 1987) which recreates some of Hertz and Marconi's experiments with spark gaps and coherers.

https://www.youtube.com/watch?v=LMxate9gegg

I think it is also worth mentioning the role of the ionosphere - which is the (charged) part of the atmosphere that will reflect radio/EM waves, and make it possible to communicate with someone on the other side of the globe. The ionosphere has different layers, and is quite dynamic - depending on the sun and its activity.

Basically, imagine a charged shell around the earth that reflects electromagnetic waves back, and that the properties of said shell is constantly fluctuating. Solar storms (and following northern lights) are bad news for radio communication.

That's the very, very ELI5 version.

This has potential to be an interactive topic like one of https://ciechanow.ski 's topics.

This reminds me of a really cool video on superheterodyne receivers that Technology Connections did. https://www.youtube.com/watch?v=hz_mMLhUinw

An old and very accessible classic for the "general audience" to understand the theory behind "Radio Science" is Jim Sinclair's How Radio Signals Work All the Basics plus where to find out more.

I truly enjoyed the article. When I played the vimeo video of ½ λ dipole antenna electric field propagation I reached for my headphones hoping to hear dark side of the moon. No dice. I get antennas and their physical characteristics and I am always intimidated by the math behind digital signal processing (DSP). Again, great article.

Another trick, which I haven't really appreciated for a long time, is that it's VERY dark in the radio frequencies. Black bodies radiate barely any energy there. It's quiet so if you shout even moderately loudly you can be heard halfway across the globe. It's permanently night and even small lamp shines quite far.

"Radio communications play a key role in modern electronics, but to a hobbyist, the underlying theory is hard to parse."

I don't believe radiocommunications and the electronics of radio is hard to understand—at least that's so at a level where a hobbyist can gain enjoyment from the subject.

I say that as someone who obtained a radio amateur's license in junior highschool at age 15.

Yes, radio engineering and its physics does get very complicated at the high end, and for a good understanding one requires advanced math including partial differential equations such Maxwell's equations and their SR/Special Relativity extensions, and beyond that one needs to understand the physics of electrodynamics and that requires knowledge of quantum mechanics including QFT (Quantum Field Theory), which is top-echalon physics and close to as complex as physics gets.

However, the hobbyist doesn't need to know an iota of that advanced complex stuff to enjoy radio as a hobby. Absolutely none of it.

All that he/she needs to know are very basic principles such as how antennas receive and radiate signals, how radio signals are amplified and detected, and later on how signals are mixed, multiplied and hetrodyned, and how radio transmitters and receivers work—even the principles behind how the common superhetrodyne receiver works is pretty standard knowledge for a radio hobbyist.

Back when I was learning about radio I doubt very much if an article would have been written in the tone of this story, especially so one that implied that to understand the subject could be difficult even at a hobby level. Why, you may ask? Well back then, if anyone had a hobby interest in electricity and electronics then essentially the only outlet for their interest was radio and perhaps television, as the other branches of electronics would not have been as readily accessible to hobbyists.

Nowadays, that's changed, there's much more to keep a hobbyist's interests such as programming, computers, computer games, and other electronics not based on radio technology—digital electronics for instance, so knowledge about radio tech and radiocommunications theory have become much less commonplace having been diluted amongst all these competing interests. Obviously, the knowledge is still out there but it's more widely dissipated and not as easily accessible in the practical sense, especially so for hobbyists of a young age.

When radio was essentially all that there was around there were many more elementary books on radio available for younger readers and these increased in complexity as the hobbyist gained practical experience. For instance, when I first became interested in radio my first introduction to the subject—like most others—was building crystal set radios, and from there we advanced to incorporating tubes and transistors into our more advanced designs. For beginners, hands-on practical books such as how to build crystal sets which included many different designs were commonly available.

(Back then, a well known author of books on crystal sets and basic radio was Bernard B. Babani, an unforgettable name if ever there was one. His books are still available but you'd never know to look for them unless told about them.)

Today, many have never heard of crystal sets let alone their 'cats' whisker' detectors, so when they become interested in the subject they're thrown in at the deep end. And not having the basics already under their belts, the more advanced radio theory comes as a bit of a shock.

I went to my friends eecs graduation a long time ago at ucla and the founder of Qualcomm talked about how what drove him to get his phd was his curiosity and determination to understand truly how radios worked.

He said that he got his phd because that’s pretty much how long it took him before he felt like he really understood how his radio worked, and even then sometimes wasn’t sure.

Was a good speech that this article reminded me of.

If anyone enjoyed this article, then i'd recommend reading this one as well[1], it's an interesting article with a focus on the relationship between radio and probabilistic reasoning in the early 1900s. https://www.argmin.net/p/the-spirit-of-radio