[HN Gopher] How is portable AM radio possible?
___________________________________________________________________
How is portable AM radio possible?
Author : _Microft
Score : 85 points
Date : 2022-03-30 18:20 UTC (4 hours ago)
(HTM) web link (www.johndcook.com)
(TXT) w3m dump (www.johndcook.com)
| mc4ndr3 wrote:
| No 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.
| Rebelgecko wrote:
| >People are not transmitting AM signals from portable radios.
|
| That's what I did before I got a car with Bluetooth :(
| dspillett wrote:
| Was 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.
| simonblack wrote:
| The 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 signals 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.)
| blamazon wrote:
| It'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 wrote:
| That 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
| zh3 wrote:
| In 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
| aj7 wrote:
| Yes.
| wrycoder wrote:
| The 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 wrote:
| "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."
|
| That's a fantastic analogy!
| teraflop wrote:
| The other way I've seen this phrased is that unlike a
| transmitter, it's OK for a receiving antenna to be incredibly
| thermodynamically inefficient, because a weak signal can always
| be amplified -- as long as it's not swamped by noise generated
| within the receiver itself.
|
| Let's say an AM radio station transmits a 1kW signal at 1MHz,
| with a bandwidth of 10kHz. If you have a receiver 100 miles
| away, and ideal isotropic antennas at both ends, then by
| plugging some numbers into a link budget calculator, it looks
| like the received power is about 12 orders of magnitude above
| the thermal noise floor. That means it's plausible that you
| could still get a clear signal even if your antenna only
| manages to capture a millionth or a billionth of the incoming
| energy.
| hilbert42 wrote:
| Ferrite antennas are remarkably efficient considering their
| size. If one were to examine the magnetic flux around a
| ferrite antenna one would find that the flux is channeled
| from the surrounding area into the ferrite rod (the rod
| effectively concentrating the field).
|
| In effect this makes the antenna's effective size (its
| aperture) much bigger than its actual physical size.
|
| Also, remember broadcast services are designed to put
| sufficient field (signal) strength into a 'notional' receiver
| that's operating within the rated service area of the
| broadcast. Both the field strength and the receiver
| sensitivity are designed to provide a worthwhile listening
| signal. The designed signal-to-noise is dependent on the type
| of service and for AM B/C band it is in the order of 40dB or
| so for the primary service area.
| [deleted]
| smm11 wrote:
| AM 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.
| aj7 wrote:
| In the case of an AM radio, the electrical length is orders of
| magnitude LONGER than the physical length.
| postit wrote:
| I 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.
| tzs wrote:
| Another interesting way to explore radio from around the world
| is via the streaming feeds that many stations now provide.
|
| A great site for this is http://radio.garden/ which shows a
| globe with lights showing stations. Click on a station and it
| starts streaming. It is quite fascinating.
|
| Radio.garden has been discussed a few times on HN. Here is one
| such discussion [1] that contains useful links to other world
| radio exploration resources.
|
| [1] https://news.ycombinator.com/item?id=23477771
| dylan604 wrote:
| While I'm sure this has some great stations that would
| normally be beyond the range of people listening, there's
| something that feels like cheating (for lack of better word).
| It's just not the same as spending time making "small moves"
| to dial in by hand a signal and then figure out where it is
| originating. I have a feeling there's a lot of that kind of
| nostalgia that makes SW/HAM radio ops interested vs just
| clicking a preset. Then, to hear a station one day but not be
| able to pick it up again for some time because the one time
| you did get it had some specific atmospheric condition that
| isn't always present. Of course I'm projecting my own
| personal feelings about it, but I doubt I'm alone.
| AnonymousPlanet wrote:
| I 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 wrote:
| Sounds 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 wrote:
| Can also be done with WebSDRs, like
| 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]
| helloguillecl wrote:
| I'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?
| CamperBob2 wrote:
| _First, 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.)
| mikece wrote:
| AM 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 wrote:
| The 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
| TurkTurkleton wrote:
| It 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 wrote:
| It'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 wrote:
| The 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/ [2]
| https://docs.fcc.gov/public/attachments/FCC-20-154A1.pdf
| munchler wrote:
| It's fun when a technology evolves beyond its original name.
| E.g. Taping a song, filming a movie, dialing a phone, etc.
| raverbashing wrote:
| A 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)
| mhh__ wrote:
| If 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.
| sytelus wrote:
| Do 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.
| exabrial wrote:
| I 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?
| curiousfab wrote:
| A radiated EM wave at far-field distances has both a magnetic
| and an electric field component.
|
| You can receive the magnetic field component with a - surprise
| - magnetic antenna, that has an inductor/coil, through which
| part of the magnetic field passes and thus induces a current
| (like in a classical transformer):
|
| You can receive the electric field component with an electric
| antenna, like a dipole. The electric field gradient causes a
| voltage to be present at the antenna terminals, somewhat like
| an open capacitor.
|
| Magnetic and electric antennas can be build completely
| independent of any ground reference. Of course, when they're
| used above ground it will have an influence on the radiation
| pattern (due to interference between the incoming direct wave
| and ground reflections), but they don't need "earth" to work.
| CoastalCoder wrote:
| If we're having a basic-physics-for-adults, I have a follow-
| up question...
|
| Suppose I have a permanent magnet, and I blast it with an EM
| wave like you describe.
|
| Assuming that the magnet and EM waves were very strong, could
| we (in principle) observe that magnet being pushed towards /
| away from the radio signal source?
|
| And do radio waves have some kind of stronger effect on
| molecules that are dipolar?
| jonsen wrote:
| > ...effect on molecules that are dipolar?
|
| That's exactly what happens in a microwave oven. Water
| molecules are dipolar and vibrates with the microwaves.
| klodolph wrote:
| > Assuming that the magnet and EM waves were very strong,
| could we (in principle) observe that magnet being pushed
| towards / away from the radio signal source?
|
| You would need radio waves with impractically low
| frequencies (and therefore impractically large antennas).
|
| > And do radio waves have some kind of stronger effect on
| molecules that are dipolar?
|
| Radio waves interact very well with metal, which is not
| dipolar. If you want to poke around at dipole moments with
| EM radiation, you're more likely to do it in the IR region,
| from what I remember from my instrumentation class in
| chemistry.
|
| EM fields interact with substances because the EM field can
| be absorbed and make electrons or other charges in the
| substance move around. Different types of motion require
| different amounts of energy, and this puts them at
| different places in the EM spectrum.
|
| In the middle of the EM spectrum you can excite a lot of
| various vibrational modes. With longer wavelengths, you can
| interact with things like unpaired electrons. With short
| wavelengths, you can interact with electrons deep inside a
| substance. Each different part of the EM spectrum lets you
| probe a different aspect of the substance you're testing...
| in one part of the spectrum you might be able to see
| resonant peaks corresponding to specific structures or
| shapes, in another part of the spectrum you can see peaks
| corresponding to specific elements.
|
| Take what I'm saying with a grain of salt because this is
| all half-remembered from chemistry classes a long, long
| time ago.
| simne wrote:
| That's easy. Earth is basically very large capacitor (could
| think of about 1 Farad).
|
| If you will feed AC current via smaller capacitor, it will just
| have larger resistance, but will also conduct.
|
| And with higher frequency need smaller capacitor to achieve
| same resistance. Practically, antenna size for 50Hz is about
| 10th of kilometers, but for Kilo-Hertz, need much smaller
| antennas and smaller capacitors, and in hundreds Megagertz
| band, current electronics could easy make radio of match size
| or even smaller.
| gotaquestion wrote:
| There are some great answers here. I'd like to add a little.
|
| You used the term "earthed", which means you are searching for
| a reference to something. An antenna has its own reference: one
| side is at a different potential than the other due to
| induction from the receiving RF wave. It's easier to think
| about with a dipole antenna, where each half of the antenna
| receives a different phase of the wave: the relative difference
| between the poles of the antenna is the voltage induced by the
| changing wave. It is its own reference.
|
| Now in any circuit that is not literally grounded, there is
| always a reference voltage that is called ground. Everything is
| referenced and built to this. Whether or not it is "zero volts"
| is immaterial, since it is the reference: we can make it zero
| volts by analysis. The potential induced on the antenna is a
| quantifiable charge can be compared to this reference, and
| amplified.
|
| Hmmm. That sounded clearer in my mind, but I just wanted to
| point out the relative nature of a reference ground in a
| portable circuit.
| klodolph wrote:
| Radios use electromagnetic fields.
|
| The EM field can induce a current in an antenna. This current
| is what is received and amplified. Recall that an antenna, at
| its simplest, is just a length of wire.
|
| Keep in mind that when you are working with antennas, you have
| to forget about Kirchoff's laws, because the simplifying
| assumptions that make Kirchoff's laws work aren't true for
| antennas. By Kirchoff's laws, you can't have current flowing
| through a wire that isn't connected to anything. However, the
| charge density of the wire is changing under the influence of
| the radio waves--the radio waves are pushing the charges in the
| wire from one side to the other, and then back again.
| HPsquared wrote:
| In essence they pick up on waves. An analogous question would
| be, "How can a ship detect waves on the sea if it isn't
| anchored?" - The ship is moved up and down, and that
| acceleration is sensed inside the ship.
|
| In an analogous sort of way, electromagnetic waves induce
| movement in the charged particles (electrons) in the antenna.
| They are swept up and down the antenna by the waves.
| jjoonathan wrote:
| > earthed
|
| The entire concept of "earth" and "ground" is inherently sort
| of DC and not RF, because RF is when wavelengths get short
| enough that you expect voltage and current to vary over the
| dimensions of your system, even if you have a perfect
| conductor.
| Enginerrrd wrote:
| Not 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.
| exabrial wrote:
| It irritates me when people get downvoted for attempting an
| explanation from what they understand.
| Dylan16807 wrote:
| Downvotes aren't a punishment. Marking wrong-ish things in
| gray is useful for discourse!
| klodolph wrote:
| It'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.
| skeptikal wrote:
| The answer to your actual question is rather simple - Maxwell's
| law. But there's a more interesting point in your question "how
| do you reference a signal?". If you want to really understand
| this, Id read about diff mode and common mode signals.
|
| Layman answer:
|
| the signal is not DC so you can reference it to itself.
|
| More specific:
|
| The EMF must cause a current in the antenna (Maxwell's law).
| This current causes the antenna to have a potential gradient
| along the length of the antenna. You can define any point along
| the antenna to be "ground" and take opposite ends of the
| antenna. (or not; ground is a conversion)
|
| The receiver itself is just a fancy amplifier, it doesnt need a
| reference (you can keep everything as a differential value) but
| if you want to keep things simple, you can transform a
| differential signal to one referenced to an arbitrary DC point.
| Choose a battery terminal and call that "ground"
| topspin wrote:
| Misusing terms in a concrete manner to make this conceptually
| obvious:
|
| Imagine a receiver that is, with respect to some reference, at
| 100V, receiving an RF signal with a peak to peak voltage of 1V.
| The receiver is detecting the signal alternating between 100.5V
| and 99.5V with respect to ground. The receiver only 'cares'
| about the -0.5 +0.5 part. From the perspective of the receiver
| there is a signal oscillating around 0V; the local reference
| (Earth or otherwise) doesn't matter.
|
| In the real world RF systems may need, or at least benefit
| from, earth reference. Particularly at lower frequencies where
| antennas are large, transmission lines are long and power
| supplies are imperfect common mode currents emerge as a problem
| and it becomes necessary to tie down these elements to a
| reference. That doesn't change the nature of the RF signal
| however; it's an alternating current that appears 'on top of'
| whatever the prevailing reference happens to be.
| simne wrote:
| I'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 wrote:
| 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.
| simne wrote:
| In 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.
| 1970-01-01 wrote:
| Fun 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
| belter wrote:
| Absolutely: "System Bus Radio"
|
| https://github.com/fulldecent/system-bus-radio
| dylan604 wrote:
| You go to the trouble of providing links, yet you've done
| something to make them not links. Is there a purpose to this,
| and if so, mind sharing what it is?
| mannykannot wrote:
| It looks to me like the two leading spaces after a blank line
| caused the URLs to be displayed verbatim [1], in which case,
| it might be accidental. (Update: apparently not.)
|
| Testing...
| https://news.ycombinator.com/formatdoc
|
| [1] https://news.ycombinator.com/formatdoc
| c22 wrote:
| I don't claim to speak for or know the author's intent, but
| letting hn linkify your url means losing the end of it to
| ellipses. In the case of youtube, some people might like to
| copy and paste the video identifier and access the content
| through some third party app. I, for one, found the approach
| convenient.
| 1970-01-01 wrote:
| yes, this is it
| Dylan16807 wrote:
| Well for everyone's future reference, youtube links are
| 43 characters, short youtube links are 28 characters, and
| HN leaves URLs alone up to 60 characters.
|
| Also HN will apparently _expand_ URLs that are 61 or 62
| characters, ha.
| dylan604 wrote:
| thanks for actually providing a plausible response to
| something I would not have considered.
| jessaustin wrote:
| here you go:
|
| https://www.youtube.com/watch?v=uo9nGzIzSPw
|
| https://www.youtube.com/watch?v=cAn_7vutwxM
| dylan604 wrote:
| totally missed the point. it's not like i was unable to
| copy&paste. i asked a specific question and your "here you
| go" comes no where near being a response to it.
| [deleted]
| jbothma wrote:
| I was literally just posting a story to that effect :)
| zw123456 wrote:
| It'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.
| hilbert42 wrote:
| _"...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.
| [deleted]
| [deleted]
| lil_dispaches wrote:
| Was the answer harmonics?
| myth2018 wrote:
| > 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, since their
| narrower bandwidths make them to collect much less noise.
|
| Another evidence that this strict "1/2 or 1/4 wavelength rule"
| doesn't hold is that some antennas are actually 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.
| jbothma wrote:
| This 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 wrote:
| Can be done even without an amplifier:
| https://mythbusters.fandom.com/wiki/Tooth_Fillings_Radio_Myt...
| jfk13 wrote:
| Perhaps some contact involved was acting as a "cat's whisker"
| detector. https://en.wikipedia.org/wiki/Crystal_detector
| [deleted]
| molticrystal wrote:
| Well 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.
| upofadown wrote:
| The 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-...
|
| So not only do they work, but they work well. They are not a
| compromise antenna.
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