[HN Gopher] Radios, how do they work?
___________________________________________________________________
Radios, how do they work?
Author : todsacerdoti
Score : 544 points
Date : 2024-03-25 07:56 UTC (1 days ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| codyd51 wrote:
| 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!
| Animats wrote:
| > I was also delighted by the point about how all methods of
| modulating a wave can be recontextualized as frequency
| modulation!
|
| That's the classic way to think about it. Another way is to
| view the input as simply a sequence of voltage readings.
| Extracting a useful signal from that is an exercise in
| exploiting redundancy in noisy data. [1] Software defined
| receivers work that way.
|
| Analog radio (AM, FM, etc.) is a hulking big carrier weakly
| modulated by the signal. Analog TV, which was AM video with FM
| audio, had 80% of the power in the carrier. Analog UHF TV
| stations often had multi-megawatt transmitters to overpower
| noise by sheer RF output. Digital broadcast TV transmitters
| output maybe 150KW, because the modulation is more efficient.
|
| Modern modulation techniques are insanely efficient. It's
| amazing that mobile phones work.
|
| [1] https://ocw.mit.edu/courses/6-450-principles-of-digital-
| comm...
| somat wrote:
| I may not be understanding what part of the operation you are
| talking about. but radiated power? no transmitter had
| megawatts of radiated power. 50Kw for a fm broadcast antenna
| is a common number passed around. The huge "voice of america"
| shortwave station was 300Kw.
|
| I have heard of military radars having megawatts of radiated
| power. but even then it was in the low megawatts.
| Hikikomori wrote:
| The goat testicle doctor did get permission for running his
| border blaster at a million watts.
|
| https://en.wikipedia.org/wiki/John_R._Brinkley#Brinkley_and
| _...
| drmpeg wrote:
| For UHF television stations, the effective radiated power
| (ERP) is typically 1 Megawatt. That is accomplished (for
| example) with a 57 kilowatt transmitter and an antenna with
| 12.44 dB gain.
| Animats wrote:
| Up in the UHF TV bands, huge transmitter power was
| required. The FCC allowed 5 megawatts.[1] Few stations
| actually used that much power, but 1 MW was not uncommon.
|
| Amusingly, over-the-air digital TV is making a comeback.
| The cable industry pushed prices up too high. But the
| "comeback" is to only 14% of the viewer base.
|
| [1]
| https://en.wikipedia.org/wiki/UHF_television_broadcasting
| hilbert42 wrote:
| _" "...all methods of modulating a wave can be
| recontextualized as frequency modulation!"
|
| That's the classic way to think about it. Another way is to
| view the input as simply a sequence of voltage readings."_
|
| Right. And modulation of any type produces sidebands as per
| Fourier! Do anything whatsoever to disturb a pure sine wave
| then math and physics dictates it so.
| gxs wrote:
| 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.
| duped wrote:
| 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.
| duped wrote:
| And for those even _deeper_ into the theory, one question you
| might ask is, if we can divide spectrum and time to get some
| bandwidth B per channel, how many bits can we send /receive
| over a distinct channel?
|
| The answer is C = Blog2(1 + S/N) where B is the bandwidth and
| S/N is the signal to noise ratio determined by the environment
| (how much noise is present relative to the signal being
| transmitted). The crazy thing is this was proven in the 1940s
| and everyone interested should go read _The Mathematical Theory
| of Communication_ by Claude Shannon. This is referred to as the
| Shannon-Hartley theorem, and it determines the channel capacity
| (C, in bits /second) of any communication channel in the
| presence of noise.
|
| The math concepts might seem heady, but it's actually fairly
| approachable and available online. It's fascinating that the
| fundamentals were proven out in one work nearly 80 years ago by
| a handful of people, and the math is not that bad.
|
| The thing that makes this nuts is that if an engineer picks
| some target bitrate for a device, say a cellphone watching
| video, they can work backwards to determine the channel
| capacity they need, do some experiments to figure out noise,
| and then determine what the target their modem protocol needs
| to reach to be suitable. And this is how we get 5G and fiber or
| whatever comes next.
| rcxdude wrote:
| Shannon was pretty ridiculous. He basically invented
| information theory, proved all the major theorems involved,
| and applied it to communications and error-correction codes.
| If you work in RF you can't do much without encountering his
| work. (It did take a while before anyone figured out how to
| get close in practice to the limits he proved, though)
| scrlk wrote:
| And before his work on information theory, his master's
| thesis showed that Boolean algebra could be used to design
| digital circuits and invented logic gates:
|
| https://en.wikipedia.org/wiki/A_Symbolic_Analysis_of_Relay_
| a...
|
| https://spectrum.ieee.org/claude-shannon-information-theory
|
| One of the all time greats.
| femto wrote:
| > divide that bandwidth by frequency (like AM and FM) or time
|
| Ah. The real magic is when we separate by space (beyond just
| frequency or time). The ability to do this was discovered
| relatively recently, in 1996, by a guy called Foschini, though
| radio astronomers will say "Meh". By adding multiple antennas
| and doing space-time coding engineers found they could pump an
| order of magnitude more data through a radio channel. The maths
| involved is high school level (linear simultaneous equations),
| and it's magic to understand Foschini's work and think "Why
| didn't we do that before?"
|
| The other bit of radio magic is error control coding. This is
| the stuff that lets us reliably talk to Voyagers I and II.
| touisteur wrote:
| Fascinating how we keep being inspired by fundamental physics
| and astronomy to keep cramming mode information in our
| channels. I'm still trying to understand Orbital Angular
| Momentum multiplexing https://en.m.wikipedia.org/wiki/Orbital
| _angular_momentum_mul...
| femto wrote:
| I'd agree with the Wikipedia article, that it sounds like
| MIMO, in that it requires the beam to have a spatial
| extent.
|
| From the Wikipedia article:
|
| > can thus access a potentially unbounded set of states
|
| That's what people originally thought about MIMO. MIMO's
| not unbounded. The limit to the number of states is related
| to the surface area of the volume enclosing the antenna,
| with the unit of distance being the wavelength. A result
| radio astronomers already knew when the comms people
| derived it. With absolutely no evidence to back it up, I'd
| guess that the same limit applies to OAM multiplexing.
|
| As an aside, when one expresses physics in terms of
| information theory my understanding is that the maximum the
| number of bits that can be stored in a volume of space
| (also the number of bits requited to completely describe
| that volume of space) is related to the surface area of the
| volume with the linear unit being Plank lengths. Is MIMO
| capacity in some way a fundamental limit in communications?
|
| [1] https://physics.stackexchange.com/questions/497475/can-
| anyon...
| leoh wrote:
| Magnets
| dsiegel2275 wrote:
| Came here to see this. Thank you.
| scovetta wrote:
| Same here, glad I'm not the only one.
| Simon_ORourke wrote:
| Would it be possible to construct a rudimentary FM radio receiver
| with only the most basic parts ala Masters of the Air?
| moi2388 wrote:
| Yes, we did this in middle school. Was great fun :D
| lormayna wrote:
| AM is quite easy (a diode and a capacitor can be enough), an FM
| receiver need a local oscillator that require some active
| elements (transistors) and a more complex circuit.
| sebcat wrote:
| Under what circumstances is a diode and a capacitor enough to
| make a radio receiver?
| danbruc wrote:
| If you are building an AM crystal radio. [1] You will also
| need a high-impedance speaker [2] if you want to operate it
| without a power supply, otherwise you will need an
| amplifier. You can avoid using a commercial diode by making
| your own point contact diode as done in Foxhole radios [3]
| and you can make your own piezoelectric speaker from
| Rochelle salt [4]. Here [5] is one personal projects site
| touching all those topics.
|
| In conclusion, you should be able to build a simple radio
| from copper wire, aluminium foil, a pencil, a razor blade,
| and baking powder.
|
| [1] https://en.wikipedia.org/wiki/Crystal_radio
|
| [2] https://en.wikipedia.org/wiki/Crystal_earpiece
|
| [3] https://en.wikipedia.org/wiki/Foxhole_radio
|
| [4] https://en.wikipedia.org/wiki/Potassium_sodium_tartrate
|
| [5] https://rimstar.org/science_electronics_projects/index.
| htm#S...
| KRAKRISMOTT wrote:
| By coiling wires separately to form an inductor
| f1shy wrote:
| Very high impedance transducer, and very low forward
| voltage diode.
| helsinkiandrew wrote:
| A Foxhole radio was often made from a coil of wire
| (inductor), a razor blade and pencil lead (diode):
|
| https://en.wikipedia.org/wiki/Foxhole_radio
|
| > The aerial is connected to the grounded inductor. The
| coil has an internal parasitic capacitance which, along
| with the capacitance of the antenna forms a resonant
| circuit (tuned circuit) with the inductance of the coil,
| resonating at a specific resonant frequency. The coil has a
| high impedance at its resonant frequency, and passes radio
| signals from the antenna at that frequency along to the
| detector, while conducting signals at all other frequencies
| to ground. By varying the inductance with a sliding contact
| arm, a commercial crystal radio can be tuned to receive
| different frequencies. Most of these wartime sets did not
| have a sliding contact and were only built to receive one
| frequency, the frequency of the nearest broadcast station.
| The detector and earphones were connected in series across
| the coil, which applied the radio signal of the received
| radio station. The detector acted as a rectifier, allowing
| current to flow through it in only one direction. It
| rectified the oscillating radio carrier wave, extracting
| the audio modulation, which passed through the earphones.
| The earphones converted the audio signal to sound waves.
| lormayna wrote:
| AM peak detector is probably the easiest and primitive AM
| demodulator: it's basically made by a diode, a capacitor
| and a resistance. I implemented it when I was in the high
| school and I was making the first physics experiments.
|
| The idea behind this demodulator is quite easy: the diode
| filters out all of the negative part of the signal, then
| the positive signal charge the capacitor and the energy is
| released in a quite constant way (R*C must be several order
| of magnitude higher than 1/f where f is the carrier
| frequency) during the negative signal "hole".
| sebcat wrote:
| I was trying to say that a capacitor and diode (detector)
| is not a complete receiver.
| lormayna wrote:
| You will need also a resistor otherwise the capacitor is
| not going to discharge, but the resistance is the easiest
| component :)
| wkjagt wrote:
| I read "AM is quite easy" as "AM demodulation is quite
| easy".
| stavros wrote:
| 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.
| Gibbon1 wrote:
| Not an RF engineer. But I mess with radio's for a living.
|
| Most of the time when people try to explain antenna's they
| start talking resonance. Which really describes a 'good'
| antenna.
|
| What an antenna does is create a alternating magnetic field
| with a alternating electric field 90 degress out of phase with
| each other. Blah blah blah quantum electrodynamics blah blah
| blah radiates photons.
|
| Resonance means the antenna stores energy as resonance. That
| increases the electric and magnetic fields making the antenna
| radiate more efficiently. Some antenna's are very wide band and
| 'flat' and used for cough cough military cough cough and other
| applications.
| jayknight wrote:
| >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.
| roelschroeven wrote:
| One could argue that technically it's no longer uniform if it's
| switched on and off, though.
| jdietrich wrote:
| It is no longer uniform. It's counter-intuitive (unless
| you've really internalised the Fourier transform and/or the
| Shannon-Hartley theorem) but a pure sine wave stops being a
| pure sine wave if you key it on and off and occupies
| progressively more bandwidth as the keying rate increases.
|
| An even less intuitive result is that you can decode a signal
| that is weaker than the noise floor if the data rate is
| sufficiently low and/or the bandwidth is sufficiently high.
| This has practical applications in amateur modes like JT65,
| ultra-wideband communications and even GPS.
| jayknight wrote:
| You can see it happening in! [1] is a waterfall display
| (time is vertical axis, frequency is horizontal) of a few
| CW signals and compare the harsh braodband clicks on the
| right to the nice dotted lines on the left. That kind of
| broadband noise happens when your signal goes from on to
| off too fast (or something else like just not generating a
| clean sine wave). If your radio can shape your keying to
| have a little ramp-up/ramp-down you get a much cleaner
| looking signal like those on the left.
|
| The noise is effectively AM, since you are modulating the
| signal from 0 to full amplitude, and with the very fast
| amplitude change you get what looks like characteristic AM
| signal with a center carrier and symmetric sidebands.
|
| [1] https://imgur.com/BnagQzb.jpg
| nonninz wrote:
| 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...
| helsinkiandrew wrote:
| I guess the original "NEETS" content is this:
|
| http://compatt.com/Tutorials/NEETS/NEETS.html
|
| Content updated in 2011.
| nonninz wrote:
| THAT'S IT!!!
|
| Thank you so much! I've been looking for this for at least
| fifteen years!
|
| And there are even links to previous HTML versions (this one
| is PDF)... amazing!
| albert_e wrote:
| Amazing thanks for remembering a great resource that you
| saw years ago. Seems very comprehensive. Bookmarking this.
| Isamu wrote:
| >flawed hydraulic analogies
|
| I want to say that's cool, avoid common pitfalls in
| explanations, but I want to to point out that all analogies
| fall short, otherwise they would be the same thing, and not an
| analogy.
|
| That is, if the hydraulic analogy were perfect, then that would
| mean that electronics would just behave as a fluid and we could
| teach it an a part of fluid dynamics.
|
| But instead it is an analogy, electronics is not a part of
| fluid dynamics, there's just a few similarities that can be
| used for teaching.
|
| It's not unusual to teach an imperfect simplistic model at
| first that you intend to supplement later with more details
| that break the analogy.
| helsinkiandrew wrote:
| 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
| threeio wrote:
| I can't recommend this entire series enough, Hunkin's work is a
| masterpiece.
| TrackerFF wrote:
| 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.
| bitcharmer wrote:
| What's worth mentioning is that shortwave offers much lower
| transfer latency than optic fibre so it's possible to establish
| faster cross-continental communication over radio than trans-
| oceanic optic fibre cables.
| Aloha wrote:
| 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.
| wkjagt wrote:
| Not that it matters much, but it seems to be somewhat unclear
| who came up with the idea for the superheterodyne receiver
| first. Could be Armstrong, or Levy, or even Schottky. The
| patent in the US was eventually awarded to Levy.
|
| Armstrong definitely was a genius though. Before the
| superheterodyne receiver he also invented the regenerative
| receiver.
|
| And you're right, the superheterodyne is such a marvelous
| technology. The principles it's based on aren't super complex
| in itself, but the combination of them is genius.
|
| https://en.wikipedia.org/wiki/Regenerative_circuit
| https://en.wikipedia.org/wiki/Superheterodyne_receiver
| hilbert42 wrote:
| Ha, not magic but conceptually the superhetrodyne is an
| absolutely brilliant design and it's still not lost is 'magic'
| even after a hundred years, and likely never will despite newer
| digital concepts (they being more complex to implement).
|
| _" Edwin Armstrong was a brilliant brilliant man."_
|
| Right! ...And as you'd likely know, Armstrong's tormentor and
| nemesis was an arrogant, despicable bastard of the first order!
|
| (Believe it or not, but decades ago I worked in a prototype lab
| at RCA and actually met David Sarnoff albeit briefly. That
| never changed my opinion of him.)
| Aloha wrote:
| It was in part reference to "Any sufficiently advanced
| technology..."
|
| So, you worked for RCA.. here is a question that there is no
| good book on, but what killed RCA, and what was it like
| working there?
|
| I dont know what I think of Sarnoff - not sure I'd use evil -
| he was a "no niceties" fiercely competitive capitalist for
| sure - and how you feel about that may vary, Armstrong was
| also an extremely hard headed man, and thats not something
| that result in successful litigation - even if I am normally
| biased towards the underdog, he isn't always the most
| sympathetic underdog.
| Painsawman123 wrote:
| 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
| your_challenger wrote:
| This has potential to be an interactive topic like one of
| https://ciechanow.ski 's topics.
| javier_e06 wrote:
| I truly enjoyed the article. When I played the vimeo video of 1/2
| l 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.
| hoseja wrote:
| 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.
| entropicgravity wrote:
| 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.
| bigbillheck wrote:
| > 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.
|
| Why is that?
| nestes wrote:
| Short answer: it doesn't, though I understand why it's
| misleading. Read my response above.
| entropicgravity wrote:
| It's because energy created by the transmitter _must_ degrade
| as one over R squared in the far field. The frequency (or
| wavelength, have your pick) has nothing to do with the energy
| transmitted because energy must be conserved. Putting in the
| frequency term then violates conservation of energy between
| the antennas. Then, at the receiving antenna the error of
| conservation of energy is then patched up by assigning a
| bogus 'gain' at the receiver. The transmitter and receiver
| are asymmetric but the path loss equation pretends that they
| are because that's easier for most people to understand and
| it works out 'end to end'.
| nestes wrote:
| Absolutely I agree that the geometry of the problem
| dictates 1/R^2 dependence, regardless of frequency. The
| gain, which I agree is a misleading way to think about the
| area, is related to the area of receive through the
| frequency terms. If you don't like that form of the path
| loss equation, I understand (I don't either!), but physics
| is not broken.
|
| Where the "bogus" gain really shines, though: I can take my
| original receive antenna, operate it as a transmitter (so
| gain is now relevant), receive with my original transmit
| antenna (where I now care about area) and get the exact
| same result in terms of loss!
| bigbillheck wrote:
| The formula on wiki has a distance squared term in the
| denominator tho?
| nestes wrote:
| Yes and no. I emphatically agree that the way the Path Loss
| Equation (Friis) is taught is misleading. I much prefer the way
| you interpret it, with the transmit antenna represented with
| gain and the receiving antenna having only an effective receive
| area. It's much more intuitive because I can visualize a
| spherical shell of power radiating outward.
|
| That said, a receive antenna does absolutely have "gain", which
| is evident by the antenna receiving a stronger or weaker signal
| depending on its orientation with respect to the transmit
| antenna. The key is this: for an arbitrary antenna, the
| (transmit, if you like) gain has a one-to-one relationship to
| the "effective receive area" at a given frequency, so talking
| about area and gain are equivalent, if not intuitive. We
| usually assume for point-to-point links that the antennas are
| oriented at each other, and in such cases (for good aperture
| antennas), you are absolutely right that the physical area and
| effective area are approximately equal. For ideal wire
| antennas, however, the physical area of the antenna is 0, but
| the effective area is nonzero (because of magic).
|
| Now, I disagree that the path loss equation violates
| conservation of energy. The link to the effective area and gain
| depends on the wavelength. When I increase the frequency of
| operation but I keep the gain of the antennas constant, the
| areas decrease, so my receive antenna is physically smaller and
| the power goes down. Not breaking physics. A lot of people will
| say "path loss gets worse as you go up in frequency", and this
| is extremely misleading if not "scientifically illiterate" as
| you pointed out. Sure, there are molecular absorption bands
| from oxygen/water that literally dissipate power in the
| atmosphere, but generally speaking, the path loss didn't get
| worse, your receive antenna just got smaller.
|
| Now wait a minute, what if I just made my receive antenna
| larger? Well, you can do that! The problem is that because gain
| and area are linked, efficiently receiving power in a given
| LARGE area (with respect to the wavelength) implies high gain.
| High gain implies a very narrow beam (more like a laser pointer
| than a normal dipole spilling energy everywhere). So it becomes
| really important that I "point" my receive antenna perfectly at
| the transmitter. Satellite dishes are really big, and they
| absolutely have to be pointed accurately at the satellite.
| fourier54 wrote:
| How can an equation that does not represent a balance of energy
| violate energy conservation?
|
| With path loss equation I assume you refer to Friis equation
| which is just the ratio of power received at an antenna to
| power given to the transmitter. It is correct and does not
| violate conservation of energy since it says nothing about the
| power not received at the receiver
| nestes wrote:
| What they're saying is that the geometrical interpretation of
| an outwardly expanding spherical shell of power shouldn't
| depend on frequency. In this respect they are correct and
| they have a good intuition for the problem.
|
| Now here's the catch: If the receive area were not changing
| as a function of frequency when the receive antenna gain is
| kept constant (it does), this would break physics (it
| doesn't). However, the effective area of an antenna with
| fixed gain varies as 1/lambda^2. In effect the geometric
| interpretation is still correct, but the variation of antenna
| area with gain resolves the seeming paradox and saves
| physics.
| fourier54 wrote:
| > the geometrical interpretation of an outwardly expanding
| spherical shell of power shouldn't depend on frequency
|
| I think nobody says that is does. I believe the problem is
| to call Friis transmission equation "Free-space loss".
| Actually the Friis formula is composed of 3 terms: the
| receiving and transmitting antennas gain and the actual
| free space loss which has the 1/R^2 dependency (which
| actually isn't a "loss" in energy balance terms, since it's
| not lost energy, just energy not received at a certain
| point, so we could argue about that term too...)
| nestes wrote:
| Yep! Fully agreed with all your points, I was just trying
| to get at the original poster's line of thinking.
| sobriquet9 wrote:
| Transmitting and receiving antennas work the same way. Flip the
| sign of time in Maxwell's equations, and radio waves will run
| perfectly backwards.
| hilbert42 wrote:
| _" And the receiving antenna does not have any 'gain' other
| than physically getting bigger or smaller..."_
|
| Well, it depends on one's definition of gain! If you were to
| say to the designers of the ELT (the Extremely Large Telescope)
| that it had no gain over isotropic then they'd fall about
| laughing (remember, its method of operation also relies on
| collecting and concentrating incoming EM radiation as do RF
| antennae). An antenna's effective gathering aperture and
| directivity for both RX and TX is just about everything, and
| the coupling efficiency from the antenna to the feeder and
| RX/detector, and vice versa for the TX just about covers the
| rest.
|
| _"...though the transmitting antenna can have gain depending
| on shape and size. "_
|
| Uh? How? What's the difference? Physics says the law of
| reciprocity applies, a good transmitting antenna also makes
| just as good a receiving antenna. The only proviso being that a
| transmitting antenna has to be designed to withstand high RF
| power levels (even then, this only applies to TX power levels
| where I2R losses can cause enough heating to damage the antenna
| and feed lines, similarly, high power TX levels can lead to
| very high voltages which can arc over; TX antennae are designed
| to handle this.)
|
| I used to work with microwave transmitters and receivers and my
| microwave dishes and other types of antennae were directly
| interchangeable--in fact, they were identical.
|
| Re the Path Loss Equation, it works in the practical sense and
| is used everywhere. Fighting over technicalities here is akin
| to arguing the difference between laws of motion under Newton
| and when they're subject to the rules of Einstein's Relativity.
| It's damn obvious when one's applicable and the other is not.
| wkjagt wrote:
| This reminds me of a really cool video on superheterodyne
| receivers that Technology Connections did.
| https://www.youtube.com/watch?v=hz_mMLhUinw
| rramadass wrote:
| 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_.
| hilbert42 wrote:
| _" 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.
| pfdietz wrote:
| 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.
| chemeril wrote:
| I promise people still make piles of money being really good at
| radio and really good at machining steel. The complexity of the
| deliverables has increased, yes, but the expertise and
| technical skill to do modern radio and machining is very much
| rewarded in the marketplace.
| martinky24 wrote:
| The US (and Chinese, and Russian, and European...) government
| spends billions a year on companies that are good at radio.
| Radar, satellite communications, 5G, etc, etc. are all critical
| parts of modern technology stacks, that are "high technology",
| and key for forward innovation. If you think it's a solved
| problem, why doesn't every telecom company have nationwide 5G
| deployed yet?
|
| There is A LOT of money to be made in the space, if you're
| good.
|
| But, it's not AdTech, so HN isn't familiar with the field I
| guess :^)
| hilbert42 wrote:
| _" No one is going to make much money being really good at
| radio, any more than they will be really good at machining
| steel,"_
|
| How do you know? For instance, I'd suggest that not every
| method of modulation has been invented or even yet implemented.
| Also, we've hardly begun to design and implement meta materials
| into antennae and RF filters--the field's still wide open for
| innovation and invention.
|
| And new methods of 'machining' steel have recently been
| invented and are just coming into use (if I owned the patents
| I'd be sitting pretty for life).
| pfdietz wrote:
| The point I was trying to make was that these things are no
| longer what determines success or failure. Incremental
| improvements are possible, but do not dominate.
|
| Consider the fight between a company that's great at RF
| design, and a company that's on top on software. Which do you
| think will win in the market for cell phones? (This is a
| trick question.)
|
| Mature technologies tend to be things that can be outsourced.
| So one can make some money (if not a lot) as a supplier of
| these things in a horizontally integrated industry.
| Isamu wrote:
| 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.
| IndrekR wrote:
| NFC is not really a radio. Basically it uses a loosely copled
| transformer. Works much closer than 1 wavelength and only
| magnetic field matters.
| fourier54 wrote:
| 100%. Every time I read the term "antenna" when referring to
| the coil used for NFC/RFID I suffer inside...
| Isamu wrote:
| Good point, NFC is described as operating in a particular RF
| band but not through radio waves:
|
| > As with proximity card technology, NFC uses inductive
| coupling between two nearby loop antennas effectively forming
| an air-core transformer. Because the distances involved are
| tiny compared to the wavelength of electromagnetic radiation
| (radio waves) of that frequency (about 22 metres), the
| interaction is described as near field. An alternating
| magnetic field is the main coupling factor and almost no
| power is radiated in the form of radio waves (which are
| electromagnetic waves, also involving an oscillating electric
| field); that minimises interference between such devices and
| any radio communications at the same frequency or with other
| NFC devices much beyond its intended range. NFC operates
| within the globally available and unlicensed radio frequency
| ISM band of 13.56 MHz. Most of the RF energy is concentrated
| in the +-7 kHz bandwidth allocated for that band, but the
| emission's spectral width can be as wide as 1.8 MHz[57] in
| order to support high data rates.
|
| https://en.m.wikipedia.org/wiki/Near-field_communication
| dTal wrote:
| ...and yet, efficiently transferring a 1kb file between two
| physically adjacent smartphones remains an apparently unsolved
| problem.
| pests wrote:
| AirDrop it?
| jameshart wrote:
| > 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.
| hagbard_c wrote:
| Not really unless you refer to the use of 'IF' and 'RF'. Maybe
| it would have been better if they wrote these out as 'IF
| (intermediate frequency)' and 'RF (radio frequency)' with a
| link to explain in which context IF is used but for the rest
| that sentence looks OK to me.
| jacobolus wrote:
| The meaning of "mirroring behavior", "narrow frequency
| response", "lowpass filter", "mixer", "IF", "RF" are all
| unexplained both in this article and the listed prerequisite
| articles.
|
| The meaning of "mixer" might be inferred from the earlier
| "the basic operation of almost every radio receiver boils
| down to mixing (multiplying) the amplified antenna signal
| with a sine wave of a chosen frequency." And the meaning of
| "mirroring behavior" might be inferred from the earlier "we
| can see that every peak of the driving signal reaches the
| ends of the antenna perfectly in-phase with the bounce-back
| from the previous oscillation". But these are still
| explanations that rely on a good amount of past expertise and
| other jargon not covered in this or the author's other pages,
| which probably has to involve quite a lot of what most people
| would consider "advanced math", though as always "advanced"
| is relative, or alternately a lot of direct experience
| playing with analog signals.
| tonymet wrote:
| Learning radio can help improve your skills in understanding the
| analog underpinnings of networking, CPUs , electronics & circuits
| .
|
| So many issues come about by taking these for granted. EMF
| interference , a short circuit or a noisy power supply can cause
| non-deterministic issues that will drive you mad unless you are
| aware of the root cause.
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