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How is portable AM radio possible?
- wglb 5y agoThe first sentence is fully wrong. Receiving antennas can be any length. Doesn’t improve from there.
- mikece 5y agoAM is a type of modulation; commercial aviation uses AM in the 108 to 130 MHz range; the military uses AM in the 300 - 400 MHz range; I'm sure there are amateur radio operators using amplitude modulation at SHF frequencies and higher for grins if nothing else. A better title is "How is a portable medium wave or longwave radio receiver possible?"
- munchler 5y agoThe article is about "AM radio", which I think is commonly understood to be medium wave (also known as "AM band") transmissions. https://www.fcc.gov/general/am-radio https://www.fcc.gov/general/am-radio
- TurkTurkleton 5y agoIt must pain you deeply whenever you look at your car radio and it says "AM" and "FM" instead of "medium wave" and "ultra-short wave".
- implements 5y agoIt’s not a deep pain - but it jars a bit once you’ve learnt you can broadcast AM on high frequencies and FM on low. For a domestic radio, AM / FM just means low / high fidelity, and users just see two sets of numbers 520 to 1602, and 88 to 108 (in the UK). I doubt most people belly feel the physics of it, or that the latter is higher frequency than the former.
- RF_Enthusiast 5y agoThe band name is actually a recent issue with broadcasting on the AM Band in the USA. The FCC now allows for AM medium wave broadcast stations to convert to digital (HD) only. This causes many engineers to cringe at stations using branding such as "Digital AM"[1] or "AM HD" since there's no AM involved. Even the FCC calls it "All-Digital AM Broadcasting"[2]. [1] https://www.am1230digital.com/ https://www.am1230digital.com/ [2] https://docs.fcc.gov/public/attachments/FCC-20-154A1.pdf https://docs.fcc.gov/public/attachments/FCC-20-154A1.pdf
- munchler 5y agoIt’s fun when a technology evolves beyond its original name. E.g. Taping a song, filming a movie, dialing a phone, etc.
- exabrial 5y agoI have a [dumb] question for EEs: How does an AM Radio work if no part of the radio is earthed? Basically, how can an AM radio detect a difference in electrical fields if there is no earth reference from which the transmitter is attached?
- Enginerrrd 5y agoNot an EE, but my guess is Capacitors. Or at least one big enough capacitor. I think the capcitor responds to the EM fields out-of-phase with the rest of the circuit creating voltage differentials that can be measured. Not certain though.
- klodolph 5y agoIt's not entirely wrong to say that an antenna is a capacitor, but it's not a good way of thinking about antennas. The problem is that once you start working with radio waves, everything is a capacitor, everything is an inductor, and everything is an antenna. With a capacitor, you have two plates and you design the structure of the plates in order to create a strong electrical field between them without leaking current between the plates or leaking the electrical field into free space. Antennas are designed the opposite way---not as a "big enough capacitor", but as an incredibly bad capacitor. Instead of holding a charge, the electrical field created by the antenna leaks into free space and is lost as electromagnetic radiation, or in reverse. A good antenna is a bad capacitor. That said, the antenna does have non-trivial capacitance and inductance.
- exabrial 5y agoIt irritates me when people get downvoted for attempting an explanation from what they understand.
- Dylan16807 5y agoDownvotes aren't a punishment. Marking wrong-ish things in gray is useful for discourse!
- klodolph 5y ago
- postit 5y agoI like radio, I grew up with short wave and still remember picking up stations late in the night in languages I’d never imagined existed. I recently bought a cheap SW radio to play with my kid and I got surprised by the amount of stations from china broadcasting strong signals over basically anything.
- helloguillecl 5y agoI’d also do the same! Back in the 80 in Chile meant no access to foreign languages so SW was incredibly amazing for me. What radio do you suggest so I can show it to my children?
- AnonymousPlanet 5y agoI recently got an SDR dongle, remembering similar experiences from my childhood. I noticed the exact same thing about Chinese radio stations. At first I thought I was lucky to find some reflections from the other side of the world. But globally tuning around via kiwisdr.com made me realise that these must be locally repeated. This is a remarkable phenomenon. Together with a load of other government supported stations from the US, Britain and France and the occasional apocalyptic warnings by Christian radios, this makes the air full of someone's propaganda. Realising that was a bit sad.
- dylan604 5y agoSounds similar to Radio Free America via Man In The High Castle vein. Broadcasting "freedom" around the globe kind of a thing only in reverse.
- trasz 5y agoCan also be done with WebSDRs, like http://websdr.ewi.utwente.nl:8901 http://websdr.ewi.utwente.nl:8901. Regarding China: I've recently discovered that Chinese radio has Polish language programme. It's mostly (Chinese) music, but it's the first time in my life I've heard Polish spoken with Chinese accent, and it's quite unique. (Not "unique" like engrish or other stereotypes; it's just that Polish isn't particularly popular as a second language.)
- deleted 5y ago
- 1970-01-01 5y agoFun link time: With enough power, literally anything can become an AM radio. No tuning or filtering is required. https://www.youtube.com/watch?v=uo9nGzIzSPw https://www.youtube.com/watch?v=cAn_7vutwxM
- deleted 5y ago[deleted]
- jbothma 5y agoI was literally just posting a story to that effect :)
- fuzzfactor 5y agoGuitar amplifiers are often quite high gain, and the cables sometimes get kind of dirty or bits of adhesive on the connections which can act as a diode. You can also get corrsion on the spring contacts of the input jack. When the ground shield is not making good contact the cable itself can make a better receiving antenna, and you sometimes hear a station when all the elements align. Lots of guitar pickups are ferritic coils too. But there usually has to be a strong station nearby broadcasting at the lucky frequency. Wasn't able to read the article but for me the two major portables were the automotive and the handheld. Car radios were AM for decades before FM got popular. Originally there weren't that many stations across the country so there was less interference, but car radios were really top-shelf items so they could be sensitive enough, to pick up weaker nearby signals as well as sometimes hear powerful stations a hundred miles away or more. By the 1960's they were even more modern, more sensitive, and expensively made, but the financing made it only a few dollars more on the car payment. If you didn't have a sensitive reciever, driving across the country was lots of radio silence. Take a look at a 1966 Mustang with its factory AM radio. These were very sensitive recievers with a very extensible antenna, the taller the better for AM because you're working around buildings and geography to pick up from far away pretty much relative to line-of-sight. This wasn't a directional antenna so it didn't matter what direction the car was going, what mattered was the station's power and the obstacles between you and it. With hand-held or portable household radios having the internal ferritic AM antenna, these are highly directional so you have to carefully position the radio for best reception and often reposition the radio when changing to a different AM station. For FM radios added to '60's cars you would retract the antenna down to a couple inches less than an American yard to match wavelengths better, these were not directional either. Eventually there were lots of aftermarket affordable AM/FM tape players after FM took off, not as many people listened to AM and these recievers had an AM section that was usually not so great electronically. With the antenna all the way up it still wouldn't pick up what the factory AM radio could do. Once the dipoles embedded in the windshields came along, you then had more directional reception and it was tuned for FM by design, so AM gets even less love. The modern vehicles which do have a conventional fender-mounted whip antenna are usually non-extensible and fixed at the short height that's best for FM.
- jbothma 5y agoThis reminds me of when I built an amplifier around 2002. Just before this I built a crystal radio - basically a very long wire, a diode, and a crystal earphone. No amplifier.needed because the earphone is so sensitive. As I got the amplifier all soldered up, I heard radio on a speaker without any signal connected if I touched a metal tool on some contact. I asked about this on an electronics mailing list and if I recall correctly people were mostly annoyed that I was talking rubbish. But I'm pretty sure something was acting as the diode, I was the antenna, and the amplifier put the signal out.
- trasz 5y agoCan be done even without an amplifier: https://mythbusters.fandom.com/wiki/Tooth_Fillings_Radio_Myth https://mythbusters.fandom.com/wiki/Tooth_Fillings_Radio_Myt...
- jfk13 5y agoPerhaps some contact involved was acting as a "cat's whisker" detector. https://en.wikipedia.org/wiki/Crystal_detector https://en.wikipedia.org/wiki/Crystal_detector
- molticrystal 5y agoWell one type of foxhole radios had oxidated razorblades touching graphite from a pencil as the detector, I imagine the class of objects that might be too difficult work with practically unpowered expand greatly when you have powered amplification available, and perhaps your tool had a coating or something that acted in a similar way.
- deleted 5y ago[deleted]
- buescher 5y agoYou probably got your amplifier into regeneration. As they say, amplifiers oscillate and oscillators don't. No rectifier needed.
- zh3 5y agoIn a 'loopstick' antenna, all that's going on is that one side of the coil is receiving the signal before the other. This is what there's a strong null when the coil points towards the source. A little surprised by one point in the article, as I generally find his articles well-informed. In particular, from the purely theoretical point of view there is no difference between a antenna for tranmitting and receiving - however, from a practical point of view if you try and feed 500KW into a little loopstick antenna it (sans safety circuits) will be quite pretty for a second or so (i.e. transmitting antenna have to be large, just to handle the power). The problem is even greater at longwave, where amateurs on 137KHz [0] are well aware of the crazy voltages that develop when trying to feed a 'short' antenna at these frequencies (a 1/4 wave is over 500 metres). [0] https://en.wikipedia.org/wiki/2200-meter_band https://en.wikipedia.org/wiki/2200-meter_band
- aj7 5y agoYes.
- wrycoder 5y agoThe loopstick is much shorter than a wavelength, so the phase of the received EM field is the same over its length. However, the magnetic field can induce an RF current in the loop, which is a rather long wire tightly wound over a ferrite core to increase its inductance. The coil is shorted with a capacitor. Together, they form a resonant circuit at the desired frequency (which is selected by making small changes in the capacitor). Energy is transferred back and forth between the magnetic field of the loop and the electric field between the plates of the capacitor. The EM field reverses around a million times per second, and so does the current in the resonant circuit. The overall effect is like pushing a child on a swing. If the period of the swing is synchronized with the timing of the push, the amplitude of the swing will increase. Because of this resonance effect, it is possible to transfer energy from the EM field into the resonator and develop enough voltage to drive an amplifier chain. It's even enough to drive high impedance earphones in a crystal radio set. It also helps that local radio stations are very powerful - several kilowatts, and they are close.
- pxmpxm 5y ago
- aj7 5y agoIn the case of an AM radio, the electrical length is orders of magnitude LONGER than the physical length.
- foobarbecue 5y agoYeah... he got this backwards I guess? Is anything correct in this article?
- simne 5y agoI'm electric engineer for first education, and length of wave is very dependent from environment electromagnetic properties. And ideal antenna is dipole (two sticks) of quarter wave length. In short, vacuum have coefficients 1 for electric and for magnetic properties (Earth atmosphere is very close to vacuum) and in medium with higher coefficient (all other mediums have coef 1 or more), speed of light is smaller and wave length is also smaller. Most known for e-m props are glass, water and glycerine (or just commodity oils), I at the moment lazy to look for glass properties (they vary), but for glycerine, electrostatic 80 times more than vacuum, so antenna will be 80 times smaller. And after Second World War found (invented) new materials, which have e-m coef-s of 1000-10000 and even more, so could make much smaller antennas than for free air. Also exists lot of variants of spiral antennas, which are less effective than ideal dipoles, but good enough for practical purposes. In most radical forms, used some sort of spiral coated with material with very high e-m coefficient, so antenna could be very small.
- mlyle 5y agoThe number one reason that an inefficient, small antenna works well for AM radio is that even with an awful antenna, AM radio performance isn't limited by receiver sensitivity. Atmospheric noise and nearby EMI will dominate over any receiver noise.
- simne 5y agoIn reality, it depends on channel width. Exists UWB radios with 10th MegaGertz channels and even more (from zero), and they have extremely good performance even with relatively low transmit power.
- blamazon 5y agoIt's worth checking out Andrew's comment at the bottom of the blog which explains the following: "The reason why transmitting is different from receiving: you want a transmitting antenna to be efficient, but a receiving antenna can often be very inefficient and still do its job well..."
- aidenn0 5y agoThat also coupled with this explanation of why an AM radio antenna can be inefficient; an inefficient antenna isn't going to be great for VHF bands like FM radio. > The number one reason that an inefficient, small antenna works well for AM radio is that even with an awful antenna, AM radio performance isn't limited by receiver sensitivity. Atmospheric noise and nearby EMI will dominate over any receiver noise. https://news.ycombinator.com/item?id=30859968 https://news.ycombinator.com/item?id=30859968
- raverbashing 5y agoA bit of a disappointing article The TL;DR is actually: the loop antenna captures the magnetic part of the signal. Not the electrical part. That's how you get it (Also, that store where your antenna "has" to be a multiple of the wavelength is not really true. It is better if it is, but it is ok if it isn't)
- deleted 5y ago[deleted]
- lil_dispaches 5y agoWas the answer harmonics?
- mc4ndr3 5y agoNo one has precisely one quarter wave length antennas. Half or full length would be more ideal, save for resistance. Again, no perfect antenna. SWR changes with weather conditions. The antenna just has to be good enough for the job. Receiving can be accomplished with much weaker antennae than transmission.
- deleted 5y ago[deleted]
- mhh__ 5y agoIf you hook an SDR up to basically anything you can pick up all kinds of crap, engineers like rules of thumb but they are usually sufficient rather than necessary.
- Rebelgecko 5y ago>People are not transmitting AM signals from portable radios. That's what I did before I got a car with Bluetooth :(
- dspillett 5y agoWas that not FM rather than AM? And the power & signal quality requirements to broadcast to your car radio are quite different from trying to be heard over a significant distance.
- foobarbecue 5y agoI've had those but I've only ever seen FM ones
- sobriquet9 5y agoTransmitting is not very different from receiving. In fact, there is reciprocity property.
- smm11 5y agoAM radio is neck-and-neck with morse code as the only things that have not evolved or improved one single iota in a million years.
- zw123456 5y agoIt's all about the watts baby. When I was a kid I made an AM radio receiver with a safety pin and a piece of zinc and a crystal earphone. Later in life, working on an experimental MW system at Bell Labs, a fun gag was to put a Neon lamp in front of the transmitter horn antenna and watch it light up when you switched on the transmitter, then nonchalantly say, oh ya she's working.
- deleted 5y ago[deleted]
- hilbert42 5y ago"...a fun gag was to put a Neon lamp in front of the transmitter horn antenna" Years ago, I used to go on field trips in a convoy of vehicles with a group of radio amateurs and we used 2M (146MHz) TXs into 5/8 whip antennas on the vehicle roofs and attached to their tip were NE2 neon indicators which would glow whenever the TX was keyed on. At night the neon glow seemed to hover above the vehicles as if suspended in mid air (it was difficult to see the antenna in the dark). A light suddenly appearing out of nowhere used bemuse other motorists. Trouble was it also attracted the cops although they never took any punitive action.
- zw123456 5y agoYup, it's an age old ham gag for sure. We were doing it to show off the power of our invention of course. The funny part was how when we demo'ed it for some exec's they sort of covered their privates when they saw that neon glow. It was in an anechoic chamber so there was of course no danger anyway. But it was still good fun.
- CamperBob2 5y agoFirst, for reasons I don’t understand very well yet, transmitting is very different than receiving. People are not transmitting AM signals from portable radios. Not actually correct at a conceptual level. Antenna reciprocity is as close to an immutable law of physics as we tend to get. However, numerous practical concerns will interfere if you try to use a small ferrite loopstick antenna in a high power AM broadcast transmitter. In principle, a transmitter could match the E field of a small metal terminal to much longer wavelength using series inductance (more or less amounting to a Tesla coil), or it could match a ferrite inductor to the H field with parallel capacitance, as an old-school pocket transistor radio does. But the field components near the antenna would be insanely high, causing dielectric losses, corona discharge, or induction heating of everything in the vicinity. So there's often no practical way to take advantage of the theoretical equivalence of transmission and reception. At low power levels it will work as expected, but things fall apart in a hurry as the power goes up. (And yes, a portable AM radio like the one in the article does transmit. Get two of them, tune in a weak station on one, then tune the other one 455 kHz below the first, and you'll hear a beat note. This will be noticeable with the radios positioned several meters away from each other.)
- upofadown 5y agoThe short answer is that a loopstick antenna receives the magnetic field (the "M" in EM (electromagnetic)) while a dipole antenna receives the electric field. A discussion here: * https://radio-timetraveller.blogspot.com/2011/01/unassuming-antenna-ferrite-loopstick.html https://radio-timetraveller.blogspot.com/2011/01/unassuming-... So not only do they work, but they work well. They are not a compromise antenna.
- hatsunearu 5y agoThis is a one-dimensional take. All antennas receive both the electric and magnetic field. You can think of a magnetic field as being equivalent to current in a transmission line, and the electric field being equivalent to voltage in a transmission line. Each antenna has a quantity called wave impedance that tells you what the ratio of electric and magnetic current it couples from and to (reciprocity) free air EM propagation (free air has a certain radiation resistance, and the antenna has its own radiation resistance). A dipole antenna has high-ish wave impedance, a loop antenna or magnetic antenna has a low wave impedance. It isn't the only factor to whether it is efficient or not efficient. The actual reason why transmitter antennas have to be pretty good at being efficient--receive performance is much much greater than transmit performance. Receive performance is basically limited by only the noise that comes from its various components. Transmit performance is limited, essentially, by how much power it can pull from the wall and how much power the final transmit stage can take before catching on fire. Every little bit counts for the transmit side, because increasing the power by two costs $$$$$$ whereas decreasing the receive noise floor by half isn't that big of a deal, and half the time isn't even necessary because of atmospheric noise.
- upofadown 5y ago>All antennas receive both the electric and magnetic field. A loopstick antenna used for AM receives only a negligible amount of electric field. That simply is not possible due to very small size of the wire coil vs the 300 or so meters of the wavelength used. Electric fields can not induce magnetic flux in the ferrite bar as it is non-conductive. As mentioned in the article, this is a feature. Most electrical noise comes in the form of an electric field, not a magnetic one.
- sytelus 5y agoDo you remember those giant cell phones? It was mainly because of antennas and batteries. Fractal antenna is one of the most magical innovations that has enabled today's smartphone age but its universally ignored.
- buescher 5y agoThose aren’t fractal antennas, which don’t really work very well anyway. Fractal antennas are effectively kind of like trying to tap a horn or discone antenna at the widest point. If you work in the biz there will always be people pitching purportedly novel antennas to you - a good reference is Hansen and Collins’ Small Antenna Handbook
- myth2018 5y ago> The length of antenna you need to receive a radio signal is proportional to the signal’s wavelength, typically 1/2 or 1/4 of the wavelength Without wanting to be too pedantic, but this particular piece of misunderstanding is what makes things like portable antennas to seem miraculous, when they aren't. The typical 1/2 wavelength size is a convention for dipole antennas. For a number of reasons -- e.g., they have a characteristic impedance of ~50 ohms at the feedpoint, which happens to be an impedance close to the ideal for maximum power transmission + minimal losses, a desirable feature for TX/RX antennas, but not so desirable for RX-only antennas (like TV antennas, which are usually designed to operate with coax cables of 75 ohms -- not so capable of transfering high powers, but they provide lower losses). Those values were found experimentally. That doesn't mean however that an antenna of a different size won't work. Different sizes (either physical or electrical) will present different characteristic impedances, and that's OK as long as it's matched to receiver's. Also, electrically short antennas like the ones in portable AM receivers are actually better than longer ones (for the intended use), as they collect much less noise due to their narrow bandwidths. Another evidence that this strict "1/2 or 1/4 wavelength rule" doesn't hold is that some antennas are way longer than that -- the resulting mismatched impedance (which probably won't be 50 ohms anymore) can be easily corrected and this disadvantage is compensated by the larger physical area covered -- it then collects more electromagnetic energy, making a great receiving antenna for weak signals.
- anyfoo 5y agoSo, dumb question, what is the length of an antenna dependent on? Just desired frequency band and impedance, or is there more? And why, i.e. what physics result in that?
- simonblack 5y agoThe missing word is 'efficiency'. To TRANSMIT a radio signal (and here we don't differentiate between AM, FM, PM or any of the other ways of modulating a radio wave) most efficiently and most cheaply the antenna must be tuned to the signal's frequency/wavelength. To RECEIVE a radio signal, while it's most efficient to have the antenna tuned to the signal's frequency/wavelength, it's mostly not as convenient to do so. If the antenna only picks up 20% of the possible signal, so what? As long as enough signal is received for you to listen to, that's all that matters. (In some cases, where we're not needing portability, like permanent radio-receiving stations we usually do see tuned antennas in use.)
- JKCalhoun 5y agoYeah, that was how I understood radio. An antenna picks up all radio signals, but it does a much better job of picking up frequencies that are "resonant" with it (based, I had assumed on length alone — it sounds though like I may have been under-informed in this regard). The coil + tuning capacitor (the tank?) is the adjustable filter that selects the frequency from the antenna we want to "tune in to". As you say, crappy antenna will still get something.
- martyvis 5y agoRichard Feynman gets very excited about this very fact, that at any point it has ALL of the radio waves, amongst other electromagnetic radiation, that just has to be tuned in or focussed on. https://youtu.be/FjHJ7FmV0M4 https://youtu.be/FjHJ7FmV0M4
- simonblack 5y agothat at any point it has ALL of the radio waves, Which is where the Software-Defined Radio comes into its own. https://en.wikipedia.org/wiki/Software-defined_radio https://en.wikipedia.org/wiki/Software-defined_radio
- ngcc_hk 5y agoThe issue would then be if the most resonant one is not the one you want, that would be a noise you have to filtered. Whilst practicality mean we can’t be optimal due to size, is it a major matter. Can the filter be efficient or make efficient by electronic. At least in 70s when radio listening is peak, the antenna is key to good reception.
- saul_goodman 5y agoForgetting the differences between TX and RX, a loop antenna is a transformer and works on a different principle from what most muggles think of as an antenna. The "radio" part of a wave is one half E-field, the other half of the wave is the H-field or the magnetic component of the wave. What you think of as frequency is the EMF field resonating between the E and H fields. An E-field antenna is what most people think of as a "normal antenna" and it resonates with the E-field. However you can just as easily resonate and capture the H-field or magnetic component of an EMF signal. A loop antenna is half of a transformer and it freely resonates with EMF that it comes in contact with. The tuning capacitor if your AM radio tunes the loop to resonate with the H-field. Loop antennas have different characteristics from E-field antennas, one of which is the ability to be much more compact for mediumwave and shortwave signals. This is why loop-bar antennas are used for AM radios and not much else in everyday life. http://www.emagtech.com/wiki/index.php/File:EM_wave.png http://www.emagtech.com/wiki/index.php/File:EM_wave.png
- fortran77 5y agoThis is a magnetic loop antenna; it works on a very different principle than a dipole antenna. The author of this piece is very misinformed.
- xyzzy21 5y agoThere is a "dual" of electric dipoles: the magnetic loop (or magnetic dipole). The rule of wavelengths doesn't apply as it does for electric dipoles. The latter is true only because the tips of the dipole must be at zero (0V) at the tips to maintain proper boundary conditions to allow a voltage to apply at the dipole terminals. There is no such boundary condition for a magnetic loop. Low frequency (medium wave AM and below) thus can use magnetic loop antennas without this limitation. The wavelength of LF and VLF can be kilometers and that can never be a practical radio antenna. So it's not used - loops are used. Note that this electrical wavelength does match power transmission line dimensions which is why the long wavelengths of EMP primarily match and will disrupt power grids and NOT cell phones, etc. as wrongly portrayed in movies. EMP is a pulse but not an ideal Dirac impulse thus it has a finite bandwidth. It is equivalent to a signal with a 1st dominant pole at 1 MHz (the middle of the AM band) and then another pole at 100 MHz (the middle of the FM band) and by the time you hit cellular frequencies, the signal is 80 dB down from VLF levels. Add to this the reality that ESD is a thing so all ICs used in cellular radio are pretty pulse resistant. So the power grid is really the only "piece of metal" that can or will couple EMP. And power grids also deal with impulses and have protection against then: because lightning is a routine and common thing so there are circuit protections again that which also work just find against lightning which has a very simply frequency content.
- rpz 5y agoYou can receive AM radio through the ground as well https://youtu.be/tflZIAQ6U8w https://youtu.be/tflZIAQ6U8w
- hahnchen 5y agoLOL, I read this as Apple Music Radio