> 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.
Yeah, sort of. One of the more interesting things about radio is that everything is an "antenna" :-). When I got my amateur radio license I found in talking with other HAMs that there is a lot of "lore" and not nearly enough "science" in those discussions.
The ARRL antenna book does a pretty good job of pulling out the basics from electrodynamics to the point where you can use simplified "rules" to build antennas to achieve various results. One of my favorite type of antenna is a "magnetic loop" because they are smaller and more easily portable.
>When I got my amateur radio license I found in talking with other HAMs that there is a lot of "lore" and not nearly enough "science" in those discussions.
I was taught the same in my Electrical Engineering degree! Antenna design is apparently very much a black art, even at the professional level.
> I was taught the same in my Electrical Engineering degree!
That's true. I'm not implying at all that in Engineering they do stuff the same way ham radios do, but sometimes we see Engineering building successfully upon certain phenomena, even though the comprehension of the underlying mechanisms is still evolving.
A good example of that are the explanations for how an airplane wing works. It's been 20 years or so that the Coanda Effect is accepted as the explanation (afaik), but that didn't prevent engineers from building airplanes before that.
Many, many years ago when I was young, poor and a newly minted extra class operator (back when 20 WPM was the requirement), I had used electric fences, gutters, bed springs, file cabinets, etc... as HF antennas. Some did not work for crap, but others were surprisingly good. Anything and everything can be an antenna.
The main issue with "way longer than that" is that the impedance varies a whole lot with small changes in frequency when the antenna is much longer than the wavelength, and this is inconvenient. (Along with all the pattern oddities you can get).
> The main issue with "way longer than that" is that the impedance varies a whole lot with small changes in frequency
It could be, but I'm not sure. Beverages (receive-only), and travelling-wave antennas in general, I'm not sure if they suffer from that. And even if they do, you may compensate this disadvantage with tunning/matching devices. That's what we do with magnetic loops, a price to pay for their reduced sizes and noise immunity.
> And even if they do, you may compensate this disadvantage with tunning/matching devices.
Yes, kind of. But if the impedance is really really high or really really low, then your receive efficiency will be really bad.
> That's what we do with magnetic loops, a price to pay for their reduced sizes and noise immunity.
Yah, and typical magnetic loop matched bandwidth is ~30KHz. Works fine for a narrow band of SSB or CW, or even a single AM radio station. But this is impractically small for a lot of use cases.
> Yes, kind of. But if the impedance is really really high or really really low, then your receive efficiency will be really bad
I must strongly disagree. A "really really high" or "really really low" impedance won't be a really really big problem by itself. Look at end-fed long-wires for instance. They have high impedances at the feed point (which is expected, since you are feeding them at their ends (well, almost.. because there must be a small counterpoise forming another "leg", otherwise you would have an almost infinite, unmatchable impedance at the feedpoint)). You need a 9:1 balun to bring its impedance closer to 50 ohms. And that doesn't turn them into bad antennas. On the contrary, if you have the luck of living in a high apartment and having some place down there to string it, you are going to have a GREAT antenna for long-distance communications, especially if the soil provides good reflection.
> Yah, and typical magnetic loop matched bandwidth is ~30KHz. Works fine for a narrow band of SSB or CW, or even a single AM radio station. But this is impractically small for a lot of use cases.
The bandwidth can actually be narrower than 30KHz, but don't forget that magnetic loops have variable capacitors for EXACTLY that reason: you must tune them to the frequency you are operating on.
> I must strongly disagree. A "really really high" or "really really low" impedance won't be a really really big problem by itself.
450 ohms isn't really high. That's easily manageable with a balun, as you state. It even matches some feedlines.
But random wires end up with impedances that vary considerably from crazy high to crazy low across relatively small changes in frequency, and this in turn sucks. You can't fix it with a fixed balun or reactance. And if your impedance is 15k ohms, you're going to need to reach a rather ridiculous voltage to get any power out.
> The bandwidth can actually be narrower than 30KHz, but don't forget that magnetic loops have variable capacitors for EXACTLY that reason: you must tune them to the frequency you are operating on.
Yes, which is fine as long as your signal comfortably fits in the bandwidth, as mentioned. Note that e.g. a reasonably wide commercial AM station might -not- and have rolloff of the high audio frequency response.
> But random wires end up with impedances that vary considerably from crazy high to crazy low across relatively small changes in frequency, and this in turn sucks.
Could you send a link with a practical case? I'm not implying I don't believe you, it's just that I've never seen such complain. Personally, I don't use random wires. But I never see someone complaining like you about how wildly the characteristic impedance varies. Granted, it varies, but well within the range of most couplers. Besides, this is another source of myths: you often encounter folks excited about how a given coupler "tune everything" and "turn any piece of metal into an antenna", but they are never concerned about things like radiation patterns, radiation resistances, losses from the antenna, the cables, the coupler etc. They just see a SWR close to 1:1, celebrate and go to the internet complain about how empty are the bands.
> And if your impedance is 15k ohms, you're going to need to reach a rather ridiculous voltage to get any power out.
This is another case I haven't faced so far, neither myself or in the literature. 15k ohms? If the impedance really reached that high, maybe it's for the lack of a counterpoise, even if a small one?
In a real-world end-fed, if you don't go high power (low power DX is perfectly feasible on days with good propagation and narrow bandwidth modes), you can keep the voltages at the feed point within the limits of commercial connectors, so that shouldn't be a problem.
> Yes, which is fine as long as your signal comfortably fits in the bandwidth
I can't remember of any mode demanding more than 30KHz (again, I'm not implying they don't exist). Sure, you have Amateur TV, but this mode is reserved for clubs and, if they are really meaning business that hard, they will certainly not use a magloop. Ah, I've also seen some guys transmitting signals to generate pictures at waterfalls of web SDRs. They were using a HackRF and probably a very high bandwidth signal (I'm not sure), but again, if you are getting into experimental modes, you are probably not going to use a small, portable antenna anyway
For a long time I had a long random wire up and had a lot of success on it. But it would not tune on small segments of the bands because the impedance was very high or very low. And it was relatively deaf near those points, too.
I shortened it slightly until those dead zones were in places I didn't care about.
As the wire gets longer, those scary multiples of half wavelength become more common (though narrower). E.g. if I have a 150m electrical length antenna, around 17.987MHz, 18.986MHz, 19.986MHz, 20.985MHz, 21.984MHz, are really bad. This is if the wire is straight and far above the ground. If it also has random bends, you get even more places where funny things happen.
And then, of course, really crazy things happen when you start trying to use this as a receive antenna up on UHF-- 399.7, 400.7, 401.7, 402.7, etc, not to mention the unpredictable, strong, and varying directionality.
> This is another case I haven't faced so far, neither myself or in the literature. 15k ohms? If the impedance really reached that high, maybe it's for the lack of a counterpoise, even if a small one?
" A multi-band end fed wire antenna presents a potential problem in that a ¼ wave length antenna cut for say the 80 metre band will present a feed impedance of about 70 ohms however when the same antenna is used on the 40 metre band the feed point impendence will be several thousand ohms and therefore difficult to achieve a match even with a quality tuner. The trick is to making this type antenna an easier match by avoiding the lengths that are 1/2 wavelength and harmonically related to 1/2 wavelengths or simply multiples of 1/2 wavelengths that will present the most extreme rang of impedances."
4NEC2 graphs are in the article. Admittedly, 15k ohms is an extreme case: at some point, earth losses, etc, prevent you from getting all the way to infinity, and ohmic losses of the wire and feedline stop you from getting all the way to 0 ohms. Of course, neither of these mechanisms makes you better at radiating power or receiving.
> I can't remember of any mode demanding more than 30KHz (again, I'm not implying they don't exist).
Lots of exotic, interesting modes, but also --- AM radio. 15KHz is in the human audible range (of younger people) and some AM stations carry audio this high.
Occupied bandwidth of an AM radio station is 2 times highest audio frequency conveyed.
Also, you probably are a few KHz off in tuning of your magloop, so you may have e.g. 20KHz on one sideband and 10KHz on the other within the bandwidth of the magloop. In turn you get the weird phasing artifacts and attenuation of higher frequencies that comes from receiving only 1.5 sidebands.
Thanks for the references. As I suspected. You take an antenna cut for a band, try to use it on others and find its limits -- nothing abnormal there. Still, maybe something you could circumvent with additional devices to transform the impedance? I can't check the article now, but maybe some changes to form a counterpoise, or feeding the antenna at a different point, could adapt it without big investments -- if you're really willing to spend resources on such improvisation.
And about AM, it doesn't transmit the full human-audible spectrum, so the channel is not that wide. Currently standards result in ~20KHz IIRC. BTW, some amateurs use receiving tunnable loops to listen to distant AM stations -- the antenna's narrow bandwidth is actually an advantage in such cases. And, again, someone interested in transmitting in AM or using some of those exotic modes have no reason whatsoever to worry about loop's bandwidths -- just look for an appropriate antenna and stop worrying.
It doesn't make sense to try to hammer a nail with a slipper and complain about its sole not being hard enough.
> You take an antenna cut for a frequency, try to use it on others and find its limits
The whole idea with a long, random wire is that it's not cut for a frequency, and happens to work well for most. This is about the most common case that exists, IMO :P
There's nothing you can do for it to make it work well on those places where it's worst-- other than shorten or lengthen it. Yes, you can make it tune up by e.g. increasing ground losses, but that is not a win. (E.g. the T2FD does this deliberately, adding a shorting resistor to make the antenna tune anywhere).
> Currently standards result in ~20KHz IIRC.
AM is enormously variable depending upon band and operator. Yes, MW AM in the US is about 16KHz wide. Ecos del Torbes was frequently 35KHz wide, and sounded fantastic (better than broadcast FM) when it was strong and there weren't terrible atmospheric crashes. CRI is usually 30KHz wide.
Witness the R390A, with a bandwidth KC (audio bandwidth) switch that goes to 16KHz. That's the -3dB point, too, and it rolls off relatively gently past where you have it switched. High fidelity AM was and is a thing. https://upload.wikimedia.org/wikipedia/commons/1/12/R390A.pn...
> It doesn't make sense to try to hammer a nail with a slipper and complain about its sole not being hard enough.
No one has said that magnetic loops or long wire antennas are bad. I like them. I just pointed out the limitations and you disagreed.
So, 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?
One extremely important factor is that the physical length affects the radiation pattern. Roughly speaking: as the EM waves go back and forth through the antenna wire, the electrical current will reach different peak values in different parts of it. The parts where the current is higher are the ones that irradiate most. Sometimes you want to adjust and interfere in this process to create certain patterns of interest. E.g.: an antenna to talk to other ground-based stations should irradiate more to the sides, while one for satellites should irradiate more to the sky. There are many ways of building patterns of interest, and tweaking the antenna length is one of them.
On RX/TX antennas, those patterns apply both to TX and RX, i.e., they will pick more signals coming from the same directions they irradiate more strongly.
It's a fascinating field. There are many things I don't quite understand yet, and I see some controversy even among professionals, it seems that sometimes they can't settle on a single explanation for certain phenomena.
It’s related to the time it takes light (EM wave) to travel down the length of the antenna and back. The speed is somewhat slower when guided by the wire. The time should be one period.
Indeed, this is one of the factors. This is represented as the velocity factor and depends on the metal used to build the antenna, and maybe other factors I'm not aware of.
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:
This 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.
>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.
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).
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.
In the analogy of pushing the child, the length of the swing determines the resonant frequency. In that case, is the LC (length*capacitance?) circuit the equivalent of swing length?
Also, with pushing a child, I don’t need to push every cycle, I can push every other cycle (1:2), every third/4th cycle (1:3,1:4), or 2 out of 3 (2:3), etc.
Is there an equivalent with radio? Can the LC circuit be tuned to half the signal frequency and still pick up?
The swing length, to first order, is independent of the period, as Galileo noticed while watching a swinging lantern during a church service. He timed it with his pulse. It's the period (or its inverse, frequency) which is characteristic of a resonance, not amplitude.
Interestingly, the period of the resonance is proportional to the geometric mean of the capacitance and the inductance. The proportional factor is 2pi, which is there because of the "rationalized MKS" units chosen. (If it wasn't for that, one Farad resonating with one Henry would have a period of one second.)
You're quite right: pushing every other cycle is analogous to pulsing an LC in phase every other cycle. This is done using a class C amplifier[0], which puts out short pulses at the input frequency. The parallel LC in the amplifier load resonant at twice the input frequency sees in phase current pulses in its inductor every other cycle, which add energy. That's how a frequency doubler works, and the technique works for higher harmonics, also, as you thought.
> LC circuit be tuned to half the signal frequency
In the case of a swing, that would work, because the child isn't there for half the pushes. But in the case of an LC tank, the "pushes" always inject current into the coil in the same direction, so half if them will be out of phase and cancel the previous one.
Sorry, by “swing length” I meant the length of the rope of the swing, not how far it swings (amplitude). You’d agree that Galileo saw lanterns on longer ropes swing slower?
> In the case of a swing, that would work, because the child isn't there for half the pushes. But in the case of an LC tank, the "pushes" always inject current into the coil in the same direction, so half if them will be out of phase and cancel the previous one.
Thank you for clarifying that for me! So, no “AC” antennas?
"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."
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.
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 transmitter field strength and the receiver sensitivity are designed in combination to provide a worthwhile listening signal. The designed signal-to-noise ratio is dependent on the type of service and for the AM B/C band it is in the order of 40dB or so for the primary service area (the secondary service area typically 10dB less).
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.)
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.
Richard 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
The 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.
if the most resonant one is not the one you want, that would be a noise you have to filtered.
Which is where the tuned-circuits of the radio's front-end come into play. In a sense the locations of the tuned circuits are not especially critical, in that the tuned-circuit filtering can be at the antenna itself, at the radio-frequency amplification front-end stage, or even at the intermediate-frequency amplification stage. In really good radios you would find tuned-circuits in all of those locations to increase the level of wanted signals and the attenuation of unwanted signals.
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.
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.
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.
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.)
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.
"...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.
Yup, 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.
There 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.
Forgetting 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.
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.
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.
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.
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.
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.
I love radio too. I still remember the time I got to use a super het Zenith AM / SW radio. It was tube type, sensitive as heck, and had a few nice features, including three bandwidth presets: 3.5khz, 5khz and 8khz.
My grandfather had it connected to an excellent outdoor antenna too!
At night, there was not a single spot on the dial where no station could be heard. It was amazing!
One could hear "the noise of our world" and listen to stations from all over the place, and it would vary some due to propagation changes naturally occurring.
SW was as good and the difference being stations from all over the world. You know this, of course, but I wanted to paint a picture for others.
So there I was listening, and grandma would run the mixer, and the neighbors electric fence, a car starting, maybe a bit of atmosphere noise from a small lightning discharge all combined into the program, plus the world piped right into my ear.
Most of the time this sounded way better than I am writing here.
It is like having another sense! And it present, immediate, right here with me, right now.
And in my mind, I pictured the world, far away places like points of light. On a good night, it was like no light pollution where one can see so many stars!
And one of those little portables might be like in town, not so many stars, but plenty of bright ones.
In my mind they are all transmitting and we get to listen and get news, happenings, brought to us as fast as possible.
There are few effective differences. Noise being one. It will be what the world is doing there, not here. Other things may crop up. It may be night there.
AM radio was the first tech I considered magic. The world is alive and so are we! And that radio being like another sense invokes strong feelings in some people. Did in me.
There was a time when more households had am radios than electricity. A time when hams constructed their own rigs in a shack in the back yard - the ozone and noise from the spark gap and the stink of the battery acid was not tolerated in the house.
Many hams still construct their own equipment, especially for low power (QRP) operating [1].
I was captured by the same magic as you were. When you are in that dark, with only a kerosene lantern, to be able to listen to the all sounds carried through the ionosphere and bouncing off the Heaviside layer is a transformative experience.
Another thing you can listen to is the "dawn chorus"[0].
It's astonishing that, at any instant, there is a single voltage to be measured at the terminals of an antenna. Yet, through the magic of resonance, it is possible to separate all the manifold signals that have additively combined to produce that single, complicated waveform.
Makes me want to get the tube radio I scored at the yard sale last year tuned up. It works, but needs caps and an alignment. Maybe this summer.
In this way, I feel fortunate to have experienced those old radios. A good one, restored has a compelling sound that I consider timeless.
It is astonishing! And yes, additive is where the magic is at, at least where I feel one can experience it most directly. Great call on the lantern. That is a compelling experience worth having period.
Thanks for the samples! They are pretty good ones though hearing them through earbuds is a bit different experience. I feel they have a bit more presence when delivered by dual paper cone in a solid cabinet and the pre-emphasis in play. Don't get me wrong though. They are accurate and well produced.
I held a license for a while. May get another one in the future when I really feel that itch. My gear was all older school stuff and it all needed a repair or two. Rite of passage. Contacting the people who set me up on air was the point of it all.
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.
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.)
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?
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..."
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.
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?
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.
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.
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.)
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.
Needlessly snippy response to someone trying to help, especially when your initial comment definitely comes across as a complaint about the unclickable links.
Indeed, simply providing clickable links would have been a better response to the original deliberately unclickable links than spawning a giant ridiculous thread to consider the topic of link clickability. This is generally true.
I was working in a lab one day by myself and kept hearing a faint voice somewhere in the room; this turned out to be an AM station that was being picked up by the drive coils of an old seismograph-type pen plotter and vibrating the mechanism enough to produce sound.
Guitar 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.
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?
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.
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.
Thanks for explaining that. I always had a bit of trouble understanding how wifi and portable RF worked in general, I only accepted that it worked out of magic!
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.
In my minds eye, It seems a ship can detect motion relative to the earth... it's the earths gravity thats holding both the sea and the ship down... argh. Sorry, I'm still struggling.
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.
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?
> 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.
I thought microwave heating had more to do with inducing electric currents in the water and the resistance losses heating the food?
That being why they’re bad at defrosting ice unless there’s already some liquid water present, and why they can heat molten glass but not (directly) melt solid glass?
(I’m neither a chemist nor a physicist nor a radio frequency electronics engineer, so I may be way off here).
"Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most effectively on liquid water, and also, but much less so, on fats and sugars.":
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.
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.
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”
For the exact same reason a transformer works when it isn't grounded. Electromagnetic fields do not require a ground. But adding a ground to an AM radio does help to boost the signal (as does a long wire antenna to the top of the hot side of the ferrite coil).
An antenna has a lot of electrons in a conductor. Any passing electrical field can bully these electrons around, because it’s a conductor. If you apply an electrical field you’ll start to push electrons up the wire, where they’ll start collecting, eventually reaching an equilibrium where they are repelling each other hard enough to fight back.
Antennas use passing electromagnetic fields to allow electrons to be sloshed around like that. Then, the buildup of charge is measured and amplified to recreate the signal.
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.
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.
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.
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.
Those 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
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.
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?"
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].
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.
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.