[HN Gopher] Radios, how do they work? (2024)
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Radios, how do they work? (2024)
Author : aqrashik
Score : 211 points
Date : 2025-10-23 05:56 UTC (17 hours ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| giuliomagnifico wrote:
| The article is interesting but some images have Error 400 so I
| can't see them.
| mrasong wrote:
| Probably just a network hiccup.
|
| I ran into the same thing earlier, but it worked fine after a
| couple refreshes.
| tomhow wrote:
| Previously:
|
| _Radios, how do they work?_ -
| https://news.ycombinator.com/item?id=39813679 - March 2024 (109
| comments)
| psyclobe wrote:
| Cool article; I'm still baffled though.
| tiniuclx wrote:
| I've been studying for my amateur radio license recently, and
| this article is a great introduction to the basics.
|
| But really, if you want to get your hands dirty with some
| practical electronics, and also want to be able to communicate
| without relying much on nearby infrastructure, amateur radio is a
| great hobby.
| DrAwdeOccarim wrote:
| Do yourself a favor and study for both your technician and
| general at the same time (I'm assuming you live in the US). HF
| is exponentially more fun than just VHF/UHF.
| i_am_a_peasant wrote:
| it's stuff like this that make me wish i lived in the US. i
| have to memorise a long questionnaire in german if i want to
| get my license
| GJim wrote:
| You can take your licence exam in any CEPT country and swap
| it (i.e. that countries licence) for a German one under
| reciprocal licencing agreements.
|
| https://www.cept.org/ecc/topics/radio-amateurs
| hrimfaxi wrote:
| You have to memorize a long questionnaire in the US (though
| in English) to get it there, too.
| ux266478 wrote:
| Only the entry level license (Technician in the US,
| covering UHF/VHF) is substantially different from the
| German one, and it's also much more restricted. Germany in
| general is a much better country for radio, especially if
| you wanted to ever do high power broadcasting.
|
| I think the one perk to the US is that the FCC has
| basically stopped caring about all but the most important
| frequencies. This makes HF particularly fun, since HF
| pirate radios are often the best listening stations in the
| entire RF spectrum. I have no idea what that's like in
| Germany, but I would imagine given the general ordnung
| culture and veneration of rules, German hams are much less
| tolerant of flagrantly unauthorized broadcast stations and
| your regulatory bodies are more proactive in shutting them
| down.
| gwbennett wrote:
| Great advice!
| tzs wrote:
| Take a look at Extra too. You may find that it actually isn't
| much harder than General.
|
| The Extra exam is 50 multiple choice questions broken down by
| category as follows: 6 Commission Rules
| 5 Operating Procedures 3 Radio Wave Propagation 5
| Amateur Practices 4 Electrical Principles 6
| Circuit Components 6 Practical Circuits 3 Signals
| and Emissions 8 Antennas and Transmission Lines 4
| Safety
|
| You need to get 37+ correct to pass. Another way to think of
| that is you can get up to 13 wrong and still pass.
|
| Within each category there are subcategories. "Antennas and
| Transmission Lines" for example has 8 subcategories. The 8
| questions in "Antennas and Transmission Lines" are one from
| each of those subcategories. The question pools for these
| subcategories each have 10-14 questions.
|
| If you compare to the closest corresponding
| categories/subcategories from the General and Technician exam
| you'll probably find that there are a few cases.
|
| 1. The Extra is just more of the same. It's not harder per
| se. "Commission Rules" for example.
|
| 2. The Extra goes goes deeper and also adds new material that
| is more advanced.
|
| 3. The Extra is in new territory.
|
| If you get to the point where the Technician and General are
| going to no problem, then you will probably have no trouble
| getting to the point where case #1 is also no problem, and
| case #2 is also well in hand. It is #3 where you might have
| trouble.
|
| But remember that you can get 13 wrong and still pass!
|
| Pick say 10 subcategories that are in case #3 that look like
| they would be the hardest to get good at and just write them
| off.
|
| For example in "Antennas and Transmission Lines" you might
| decide that the "Smith chart" subcategory, which has a pool
| of 14 questions, would take a lot of time to get good at. So
| skip it. That's 14 less potential questions you have to be
| prepared to answer, leaving more time to study for things in
| class #2 and the class #3 things that look most doable.
|
| It doesn't cost extra to take the Extra test at the same
| session that you take the Technician and General tests, and
| there is no penalty for failing, so might as well go for it.
| jandrese wrote:
| As I understand it General gets you most everything and
| Extra is mostly useful for people who want to teach amateur
| radio classes.
| royskee wrote:
| Huh, I never heard that one. Extra gets you more
| frequency privileges (nice not having to worry so much
| about band edges) but IMO the real benefit is being able
| to enjoy reciprocal operating under CEPT when traveling
| abroad.
| akvadrako wrote:
| You can operate abroad with General in most countries.
| akerl_ wrote:
| A random plug for https://hamstudy.org/
|
| The US ham test question pools are fully public. Your test
| will be a mixture of questions from the pool. HamStudy
| basically lets you churn the question pool, and then will
| offer explainer text / references to back up each question
| and correct answer.
|
| I went on a vacation and used their phone app any time I was
| standing in a line. You can set it to just keep spinning
| through the questions, with a bias towards ones you're
| getting wrong.
| gtsnexp wrote:
| This reminds me of a beautiful book written by Paul Nahin, 'The
| Science of Radio'.
| frobisher wrote:
| Bought it but never read it - is it worth pushing through?
| gtsnexp wrote:
| I did many years ago. Absolutely worth it.
| dd_xplore wrote:
| To get an idea about radios, I made a crystal radio when I was in
| 7th grade, I only had few components. The only component I had
| difficulty in getting was the crystal oscillator (I was living in
| a rural town).
|
| It was mind blowing when I first heard the audio through IEMs !
| It felt magical that this contraption was working without any
| battery source.
| immibis wrote:
| The crystal radios I know don't have crystal oscillators - the
| word "crystal" refers to the diode.
| gtsnexp wrote:
| https://www.youtube.com/watch?v=-GxL13rid1w
| kragen wrote:
| This is a video about the diode, in this case a Schottky
| diode made with a coil of copper wire filed to a fine point
| touching a piece of galena sitting loose in a bottlecap
| found by the side of the road.
|
| The video is notable for demonstrating the original
| "breadboard" technique, where you connect your wires and
| components by clamping them to a wooden board with the head
| of a wood screw. The book I learned the technique from
| recommended using a dished washer under the head of the
| screw so that the screw head doesn't push the wires
| sideways as you tighten it.
| pkolaczk wrote:
| You can use a germanium diode or even Shottky now instead of a
| "crystal".
|
| Such a simple radio can be a gateway drug to a very complex and
| deep hobby. In my case it went like that:
|
| 1. Built a simple radio
|
| 2. Could hardly hear anything, need to add an amplifier to it
| 3. Now it's better but captures a lot of noise
|
| 4. Design a filter to select just that one station
|
| 5. Now I want to listen to more stations.
|
| 6. Ugh, you can't design a good filter with variable frequency.
| Enter the superheterodyne world.
|
| 7. Now finally got something that resembles a tunable AM radio,
| but it kinda whistles / hums a lot. Ah, so the mirror image is
| a real thing?!
|
| 8. Need a higher IF to be able to better reject the image
| before the mixer. Ok, let's make a double conversion superhet
| then.
|
| 9. Buy a set of ceramic filters and play with them to get the
| best selectivity.
|
| 10. Try to add more amplification only to learn if you go too
| far you get an oscillator instead of an amplifier.
|
| 11. The sound level is not stable. Add AGC.
|
| 12. Pick up some stations from 5000+ km away. Nice. But there
| is some weird distortion. Oh, I've been a culprit of frequency
| selective fading...
|
| Fast forward and now I'm building a PLL synchronized AM product
| demodulator with a squaring loop for carrier recovery.
|
| Fun. Lot of fun! Wholeheartedly recommend!
| Aloha wrote:
| I have used a synchronous AM detector once in a radio, its a
| game changer for listening to shortwave.
| pkolaczk wrote:
| Oh, thanks, good to know. Now I feel more motivated.
| Because actually it's not as easy as it looks from the text
| books. It's like with drawing an owl. Yeah, pass the signal
| through a mixer and feed recovered carrier to its LO port
| and you're done. Sure. Simple. Now just recover the
| carrier. So far I have built a PLL that locks to a clean
| signal but stops locking when the signal is modulated too
| much. Aargh.
| jacquesm wrote:
| > You can use a germanium diode or even Shottky now instead
| of a "crystal".
|
| Or a razor blade or a piece of roofing lead and a needle.
| wingspar wrote:
| Now I'm interested...
| jacquesm wrote:
| Diodes occur naturally, all you need to do is find them!
|
| Good places to look for them are conductors and crystals.
|
| https://en.wikipedia.org/wiki/Crystal_detector
|
| https://rimstar.org/science_electronics_projects/razor_bl
| ade...
|
| Good luck! And don't sneeze once you've found a good
| spot.
| peterfirefly wrote:
| > Diodes occur naturally, all you need to do is find
| them!
|
| Even true for LEDs!
|
| https://en.wikipedia.org/wiki/Silicon_carbide#Natural_occ
| urr...
|
| https://en.wikipedia.org/wiki/Light-
| emitting_diode#History
| whartung wrote:
| There's something to note here.
|
| As someone who's always dabbled in electronics, skimmed and
| read some books, my primary complaint abot most electronics
| texts is that they just talk about individual topics:
| oscillators, amplifiers, etc.
|
| What they never talk about, is putting them all together.
|
| But as witnessed by this list, that's what a radio is. A
| collection of these "meta" components into a whole to get a
| better radio experience.
|
| A radio built like this, with individual subsystems connected
| together, is much more understandable. Many (not all) radio
| schematics are presented as a whole, rather than the parts,
| or why you might (or might not) want to change one part or
| another (not components, but one, say, filter circuit to a
| different one).
|
| It just seems to me that once you get past some basic theory,
| starting with a radio, and then systematically taking it
| apart is a better way of approaching electronics education.
| mickduprez wrote:
| "A radio built like this, with individual subsystems
| connected together, is much more understandable."
|
| Yes! this has been my experience too, building something
| from first principles and given some tools and direction to
| experiment you get the chance, and experience, to really
| learn. I've been looking for resources like this for
| building amps but they're either small signal or the whole
| design. You understand how they work but not where and what
| to change if you wanted to tinker or build your own.
| kragen wrote:
| Crystal radios don't normally contain crystal oscillators,
| which do require a battery source. Perhaps you mean a crystal
| rectifier.
| hshdhdhehd wrote:
| Never thought about the AM bandwidth thing before that is
| interesting and seems obvious now (from Fourier Transform point
| of view)
| drmpeg wrote:
| Here's a video of AM modulation. SDR transmitter (running GNU
| Radio) connected to an SDR receiver.
|
| https://www.youtube.com/watch?v=9LsJn0CyyZI
| randyrand wrote:
| needs more pictures!
| Liftyee wrote:
| I've been learning about radios for a while, and this article
| explained one of the key questions I had: why can't you turn on
| and off some single frequency waveform faster to transmit data
| faster? (answer: changing amplitude messes with the spectrum and
| makes it no longer a single frequency...)
|
| I tend to prefer these visual and intuitive explanations to the
| mathematically based ones usually given in lectures. The "open
| capacitor" example was something I hadn't thought of before.
| tigereyeTO wrote:
| > In today's article, I'm hoping to provide an introduction to
| radio that's free of ham jargon and advanced math
|
| ...(scroll)...
|
| > The identity for cos(a + b) can be trivially extended to cos(a
| - b), because subtraction is the same as adding a negative
| number:
|
| ...(scroll)...
|
| > From the formula we derived earlier on, the result of this
| multiplication necessarily indistinguishable from the
| superposition of two symmetrical sinusoidal transmissions offset
| from a by +- b, so AM signals take up bandwidth just the same as
| any other modulation scheme.
| foobarian wrote:
| To be fair, they did qualify it as "advanced" math :-)
| mholt wrote:
| I got my amateur radio license last year, and this is precisely
| why I haven't been able to do much with it: seemingly all the
| guides, even the license study materials, use vocabulary I'm
| not familiar with. I have two CS degrees and a solid foundation
| in math, but I can't understand how radios work because of
| _vocabulary_ more than the math.
|
| My uncle who's been building his own radios for over 60 years,
| tried to explain to me how antennas work, and even to him it
| comes down to "black magic".
|
| I'm told the way they work is not really intuitive, so you just
| have to math it out.
|
| Maybe I should have gotten an EE degree.
| esseph wrote:
| It gets crazier when you start talking about things like a
| Tarana BN. The amount of processing in them is pretty insane.
|
| But yeah, black magic is right!
| hedgehog wrote:
| Even just the theory is kind of mind expanding. I've done a
| little signal processing and ideas like "negative frequency"
| sound absurd up front and then seem reasonable once you've
| worked with them.
| _whiteCaps_ wrote:
| I'd love to see more people on the air. My advice is to get a
| radio with a good tuner, build a simple dipole with the
| online calculators, and try to make contacts that way.
| ux266478 wrote:
| I assure you, that doesn't go away in electrical engineering
| or even theoretical physics. Electromagnetism exhibits a
| large degree of behavioral emergence. It's one of the most
| well studied aspects of physics, but remains a rather
| convoluted and seemingly arbitrary puzzle box of nonsense
| especially at a macroscopic level.
| mkipper wrote:
| I work in software now, but I have an electrical engineering
| degree and started my career on a project developing a radio.
| Our project probably had ten or more electrical engineers on
| it, and only one or two of them _really_ understood the RF
| side of it. It 's a _very_ specialized skill -- even EEs with
| >20 years of experience would describe things as black magic.
|
| So I don't think you're alone feeling this way. Even with a
| good foundation in the theory and math, I think most people
| hit a wall with radios at some point. All the people I worked
| with who intuitively "got" RF stuff had been doing nothing
| else professionally for over a decade.
| cfraenkel wrote:
| A long time ago, worked on a comm satellite program. It
| used a whack of tuned cans to combine high powered transmit
| signals with harmonics in each others' frequency bands to
| feed into the antenna. I once asked how they worked. The
| answer was 'magic'. I mean, they were physical RF filters,
| but no one could explain or reproduce how they worked.
| There was this one guy who could tighten the screws that
| adjusted the inside baffles so they 'worked'. No one else
| could.
| lormayna wrote:
| Antennas are really black magic: optimizing an antenna
| requires stocastich method like genetic algorithms, simulated
| annealing, etc. Moreover if you want to model the radiation
| patterns and the electrical characteristics you need to use
| finite element calculation methods. So, you need a lot of
| computation power as antenna are not a problem that can be
| solved in a closed form.
|
| Source: I almost burnt my PC on simulating a dipole array
| while studying for the antennas course at the university
| zppln wrote:
| Related: https://en.wikipedia.org/wiki/Evolved_antenna
| richiebful1 wrote:
| I've had my ham license for ten years, but I've only ever
| used a basic car-based mobile setup and my handhelds. My
| morse code speed is abysmal. QRP and all that are really
| cool, but I just use ham radio to supplement my fire/ems
| handheld in a natural disaster.
| peterfirefly wrote:
| Are you objecting to the _trivial_ extension to cos(a - b)? The
| cos(a + b) identity itself is nicely explained in the text.
|
| The third thing you quote is the result of fairly simply symbol
| manipulation that requires no new knowledge apart from the
| original cosine identity and the obvious corollary about
| subtracting an angle being the same as adding a negated one.
|
| There is zero advanced math there. No complex numbers, no
| calculus, no limits, no Fourier, no "functions are vectors,
| too".
| Isamu wrote:
| To be fair, I think most people regard "not advanced math" to
| exclude trig, and maybe algebra too. Fear and loathing of
| math is a thing.
| peterfirefly wrote:
| skill issue
| andoando wrote:
| How I wish I could quit my job and go back to school.
| kragen wrote:
| This is basic math, not advanced math. If you don't know it,
| don't worry, you can learn it pretty quickly. Maybe watch some
| 3Blue1Brown videos and do some textbook exercises and/or game
| programming. Git gud. There is no royal road to radio
| modulation.
|
| I learned that subtraction was the same as adding a negative
| number sometime around second grade, and I learned (then
| forgot) the trigonometric angle-sum identities in tenth grade.
| And that was even with the handicap of having to attend school
| in the US.
|
| And, just above the text you're complaining about, he even
| provides a straightforward geometric proof of the angle-sum
| identity! So you don't even have to know it to read the
| article! You just have to know what a cosine is! I learned what
| a cosine was in eighth grade because I wanted to program a game
| where objects would fly across the screen at a constant
| velocity but a varying angle. You can learn it too!
|
| He's not, like, invoking the convolution theorem or anything in
| those quotes. Although he does get into it a bit.
|
| I think that, if you know the convolution theorem and Euler's
| formula, things like the production of sum and difference
| frequencies from the multiplication of sinusoids start to seem
| obvious rather than sort of random. When I was in high school
| they seemed sort of random. My uncle had tried to explain
| Euler's formula to me, along with the Taylor expansions for
| sine, cosine, and the exponential, but I hadn't really
| understood, because I didn't have the background knowledge to
| appreciate them then.
| anthomtb wrote:
| > Euler's formula
|
| So much EE-related math becomes trivial (or at least not-
| hard) once you've internalized this formula.
|
| What I am trying to decide is 1) Did I zone out in class when
| Euler's formula was introduced or 2) Did my secondary school
| mathematics classes just kind of gloss over it?
|
| I lean towards 2 but unfortunately none of my college classes
| reintroduced the formula and I ended up making a lot of
| problems harder than they should have been (I have an EE
| undergrad).
| kragen wrote:
| I'm not sure any of my secondary-school classes taught it,
| but I dropped out of secondary school to go to the
| university, so maybe they would have in the following year.
| In secondary school I was in the "smart kids" math track,
| though, and they also had an "average kids" track and a
| "dumb kids" track, and I really doubt those tracks covered
| it.
| pengaru wrote:
| What do you consider _advanced_ math? This isn 't solving
| differential equations...
| supportengineer wrote:
| EDIT: I am offended that you guys think my awesome explanation is
| from GenAI.
|
| Imagine a circuit. Like a flashlight. The electrons flow from the
| battery to the lightbulb and back. It's like a race track. They
| proceed in an orderly fashion.
|
| There are some other electrical components. If you hook them up
| in just the right way, you get something called an LRC circuit.
| The electrons don't flow in an orderly way now. They go back-and-
| forth. In spurts and fits. You're making them wiggle. There are
| some very nice equations that allow you to specify exactly how
| much and how fast the wiggling is.
|
| One cool thing about a circuit with wiggling electrons is that if
| you put some wires close by those electrons will also start
| wiggling.
|
| This is called radio.
| neduma wrote:
| I was listening to story of QualComm in Acquired podcast. This is
| highly related.
| bdcravens wrote:
| My grandfather, when he was a boy in the 1910s or 1920s built a
| radio out of a round oatmeal box, copper wire, and a crystal. I
| never saw the original, but he built a replica of it (featured in
| a book in the 1970s) that I saw as a child.
|
| https://sg30p0.familysearch.org/service/records/storage/dasc...
| tonymet wrote:
| This is a great review, but don't let theory interfere with
| practice. There's an art and feel to radio and antenna building.
| I've built antennas with scrap wire and baluns without double
| checking . Just a rough length of wire tweaked until you hear
| someone.
|
| The theory seems overly complex (because it is). but for
| practical radio you can tune up nearly anything and chat -- and
| be way more productive with it.
|
| Imagine obsessing over the mechanics of baseball or fishing
| without ever tossing a ball or casting a reel.
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