[HN Gopher] How Antennas Work
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How Antennas Work
Author : codesuki
Score : 150 points
Date : 2020-04-05 11:21 UTC (11 hours ago)
(HTM) web link (www.antenna-theory.com)
(TXT) w3m dump (www.antenna-theory.com)
| thelazydogsback wrote:
| I think the biggest take-away, that especially Hollywood needs to
| take note of, is what everyone gets wrong -- you don't point a
| whip antenna _at_ something to get the highest gain, you want to
| be at a right angle to the source. Remote controls often have the
| antenna oriented the wrong way for distance /gain, and I've seen
| people orient wifi router antennas to "point" to usage areas,
| etc.
| jacquesm wrote:
| Depends on the polarization. You want the antenna at 90 degrees
| to the direction of the field. A vertical whip radiates
| horizontally, a horizontal whip radiates vertically!
|
| The reason you usually put them vertical is so that you can use
| the Earth as a ground plane, leading to less wasted power. A
| horizontally positioned whip antenna will lose a fair bit of
| power to the ground.
| nerdbaggy wrote:
| It all depends what the polar patterns look like for the access
| point. In a omni directional antenna being 90 degrees is
| generally the best because it puts out a donut shaped pattern.
| But with a directional antenna you want to be inline with it,
| or whatever the direction is pointing.
| katmannthree wrote:
| If by remote control you mean TV remotes, those usually work
| via an IR LED. The radiation pattern is pretty much what you'd
| get with visible light, i.e. the highest intensity zone is
| directly in front of the LED.
| chrisseaton wrote:
| > If by remote control you mean TV remotes...
|
| They don't mean that.
| katmannthree wrote:
| Do you know what they meant? Absent any context I don't
| think ``remote control'' has another well known device. I'm
| assuming that the person I responded to doesn't have a
| background in this stuff as this is was their first
| introduction to antenna radiation patterns.
| chrisseaton wrote:
| Not sure if you're joking or trolling, but for example
| 'remote control car', 'remote control helicopter',
| 'remote control boat', 'remote control bomb disposal
| robot.' The handset you use to control them is itself
| called the 'remote control.' That's what they mean.
| katmannthree wrote:
| No, I'm serious. Thank you for explaining.
|
| I've heard people talk about tv remotes as ``remote
| controls'' several orders of magnitude more times than RC
| gear.
|
| I've never seen someone have to reorient their RC
| transmitter to get a better signal. Modern RC
| airplane/car/helicopter/boat transmitters have multiple
| fixed antennas (as do the receivers) and have for many
| years (since people switched from 50MHz radios to 2.4GHz
| spread spectrum radios). The range is long enough that it
| you'll generally lose sight of your vehicle before you
| lose the radio connection.
| neves wrote:
| wow, I thought I would get some nice tips to position my wifi
| routers and repeaters antennas, but it too much information.
| Useful for who wants to get deep knowlegde, but I think I still
| need an easier guidance for my routers.
| kawfey wrote:
| To be fair, it's antenna theory, not propagation theory. That
| by itself is another realm of electrophysics.
|
| https://en.wikipedia.org/wiki/Radio_propagation
|
| There are some ray-tracing and wave theory solvers out there
| that will readily accept a 2D floor plan, but 3D full wave
| solutions are intensely complicated computational problems that
| probably won't even resolve to real-life just because of the
| complexities of the real world. The biggest problem for AP
| (router) placement is multipath interference, which either
| creates a null or node at any given location, compromising of
| basically infinite paths between the AP, bouncing off of walls,
| furniture, people, animals, and everything,
|
| The network engineer's rule of thumb is to ignore multipathing
| completely, and start at 0dB a foot from the router, and
| subtract 10dB for passing through drywall, 15dB for brick
| walls, 8dB for glass, and 6dB for every doubling of distance
| between the wireless AP and the desired client locations. As
| long as you stay above around -20- -30dB, you should have good
| signal. I've come up with these numbers in my own experience as
| an RF engineer. Typical software usually uses n-bounce ray
| tracing to determine deadzones or optimum placements, but that
| stuff is expensive and only as accurate as the 2D or 3D model
| of your space.
|
| An even less intense rule of thumb is place as close to common
| client locations then move them by trial and error until it
| works best. I place a single AP on the ground in the center of
| my home, and another mesh node for my back yard, and my whole
| house is covered. (I use ubiquiti unifi gear, which are far
| more powerful than a router/AP-in-one, and it gives you a lot
| of insight on how well the clients are connected, interference,
| and other useful data).
|
| This is also why mesh wireless is getting popular, so you can
| dot mesh APs around the home in just about every room, and
| ignore the whole problem.
| nerdbaggy wrote:
| The big thing to see if you can find is the polar pattern for
| your access points. That shows how the signal radiates and you
| can aim your stuff better.
| walshemj wrote:
| Home routers are dipoles - if you want to get into wifi look at
| the cisco press books
| iamhamm wrote:
| It's definitely more of theory site. This talks about
| enterprise WiFi, but it might be helpful:
| https://www.accessagility.com/wifi-design-guide
| sunstone wrote:
| It's mildly irritating that the concepts of explanations like
| this (and almost all others) are based on a false understanding,
| through the path loss equation and the law of reciprocity, of the
| underlying physics of antennas.
|
| While this kind of approach allows for the proper engineering of
| antenna systems it is at least 50% wrong regarding the underlying
| physics.
| syphilis2 wrote:
| Is there an index of all the pages on this site?
| 205guy wrote:
| This is a neat website Y an antenna engineer with lots cool info,
| but the big picture is buried. From that page, click on "antenna
| basics" then scroll all the way to the bottom where you can find
| "why do antennas radiate?"
|
| http://www.antenna-theory.com/basics/whyantennasradiate.php
|
| That answers the fundamental question of how antennas work.
|
| Edit: looks like kawfey's comment answered the issue already, I
| didn't scroll down far enough:
| https://news.ycombinator.com/item?id=22787249
| brudgers wrote:
| some past comments, https://news.ycombinator.com/item?id=19708982
| peter_d_sherman wrote:
| Excerpt:
|
| "Specifically, consider this statement: _Complexity is not a sign
| of intelligence; simplify. I have found this to a priceless
| amount of wisdom._ "
| supernova87a wrote:
| One thing I have always struggled to understand about antennas --
|
| Of course I know that an antenna is most efficient at radiating
| power when its length is some fraction/multiple of the emitted
| wavelength. But I cannot for the life of me intuit how the
| electrons are being excited and behaving.
|
| If I use the bathtub analogy of sloshing water, it cannot be (I
| believe) that the electrons are sloshing in bulk up and down the
| antenna and "accumulating" at one end at the speed of light.
|
| On the other hand, if each electron along the length of the
| antenna is oscillating in its own relatively stable position,
| what then does the length matter to the electron at one end
| versus the other?
|
| Or should I understand it as, energy is being transmitted out of
| the antenna, like it is in a flute being played, and the
| electrons are most constructively-interference being reinforced
| to resonate at the frequency desired (by standing waves in the
| conductor) if the length of the material matches the wavelength?
|
| This has always been hard to visualize.
| madengr wrote:
| Need to think about charge displacement and fields rather than
| electrons. The electrons moving in wire (direct current)
| circuit only move, on average, a few cm/sec. Now the charge
| displacement moves at the speed of light.
|
| Think of a tube stuffed tightly with marbles. Push the marbles
| at one end, and they move near instantly at the other end. The
| force transfers instantly, but the marbles may hardly move.
|
| Now imagine the marbles connected with stiff, little springs.
| Push on one end, and the compression wave moves through the
| charges quickly (the speed depends on the spring constant). It
| hits the other end, yielding a little more charge accumulation,
| then bounces back, yielding a little less charge. Do that a 2.5
| billion times a second and you have a WiFi antenna. The charges
| don't move much all; it's the charge displacement that moves,
| which is the E and H field. The key is also that the
| displacement must have acceleration (harmonic motion) to have a
| derivative. Accelerating charges radiate; constant velocity
| charges (direct current) don't.
|
| The jist of it is that all the energy is contained in the
| fields. That metal rod has charge that is easily displaced.
|
| Now as to how an accelerating charge radiates; I can't
| remember, but Feynman covers it in his 3 volume lectures.
| dang wrote:
| A thread from 11 months ago:
| https://news.ycombinator.com/item?id=19708982
|
| This topic is uncommon enough that we won't call this a dupe
| (this came up yesterday:
| https://news.ycombinator.com/item?id=22781498).
| dbcurtis wrote:
| How antennas work, very high level:
|
| 1. Accelerate an electron, get a photon. A good transmitting
| antenna is something that is an efficient structure for
| accelerating electrons.
|
| 2. Antennas are reciprocal. They receive as well as they
| transmit.
|
| 3. Resonant structures are often used because you can keep more
| electrons accelerating with less energy -- the damp finger on the
| rim of a wine glass effect.
|
| 4. Power can be directed by appropriately phasing the radiating
| sub-structures to create constructive and destructive
| interference in the radiated energy.
|
| The rest is modeled simply with a set of simultaneous three
| dimensional second-order partial differential equations.
| autonoshitbox wrote:
| Wow, you're so smart. You've educated so many with your idiotic
| expansion of the phrase "3D 2nd-order PDEs".
| Enginerrrd wrote:
| >2. Antennas are reciprocal. They receive as well as they
| transmit.
|
| While true, this could be really misleading in practical
| application. A really good transmitting antenna doesn't always
| (or even usually) make a really good receiving antenna.
| wl wrote:
| I would say it's the other way around: A really good
| transmitting antenna is necessarily a really good receiving
| antenna (at least at the frequencies it's tuned for), but a
| purpose-built receiving antenna might make a poor
| transmitting antenna because of power handling and matching
| considerations.
| Nux wrote:
| Yep, like smartphones wifi antennas, get a strong signal,
| bur experience is bad due to weak transmitter.
| dbcurtis wrote:
| In terms of the properties of the antenna itself, it is
| reciprocal -- the "How Antennas Work" part. You comment is
| more along the lines of "How to Use Antennas", that is a
| different question.
|
| The places where there is a benefit to having different
| antennas for receive and transmit depends on the
| characteristics of the channel and on the application. The
| classic example being where a receiving antenna that reduces
| reception of local noise can give a better signal-to-noise
| ratio than an antenna that has been optimized for the best
| transmitted signal footprint at the location of the other
| station.
|
| So, while I agree that in practice there are plenty of times
| where separate receive and transmit antennas have a benefit,
| it isn't because of antenna physics.
| madengr wrote:
| Case in point is my ham radio station.
|
| I have an active magnetic loop for HF RX. This allows me to
| null out near field interference (a few dB from the
| transformer in the back yard) and provides very broad
| bandwidth (VLF thru HF). It is 75 feet from the house, away
| from that near field interference.
|
| TX is an inverted L, up the side of the house, with a tuner
| at the base. I don't have electrical length, but I do have
| power, so I can trade off efficiency for a power amp, and
| the bandwidth with the tuner. Interference is irrelevant,
| but I can see some 60 Hz cross modulation on a monitoring
| receiver, due to coupling to the house wiring.
| Youden wrote:
| Honest question: in what way? As in it might make sense to
| have an omni-directional antenna for the transmitter but a
| directional antenna for the receiver rather than omni-
| directional for both?
| esmi wrote:
| The correct answer is, it's complicated. (Of course) It
| depends greatly on the definition of the output, for
| example, which can vary by application.
|
| Easy answer is, for a fixed load (I.e. a resistor) and
| frequency, one varies amplitude with power. (perhaps
| absolute value of area under the curve might be a better
| description)
|
| For more information see here: http://hyperphysics.phy-
| astr.gsu.edu/hbase/Waves/powstr.html This is for a physical
| string, which I think is easier to wrap your head around.
| The math is very similar for electrical waves.
| madengr wrote:
| Ha ha, well I take issue with the "simply" part. There are
| several expensive, EM field solvers. If it were simply, they
| would be cheap.
| sizzzzlerz wrote:
| If you're more interested in a practical approach to designing
| and building antennas for radios, find a recent copy of the ARRL
| Handbook, the source of information about ham radio. In it,
| you'll find some theory, a little math, but mostly, how to build
| and deploy a wide variety of antennae to cover frequency ranges
| from 2 MHz to 10 GHz. They also offer a stand-alone book on
| nothing but antennas with more details and examples.
| h2odragon wrote:
| > and deploy
|
| Thats one of the things that made the ARRL Antenna handbook so
| valuable to me; they have some solid advice on weatherproofing
| and dealing with things like wind and snow loads.
|
| Its easy to make an antenna; its just a bent length of wire at
| the end of the day. Making it be the _same_ length of bent wire
| today, tomorrow and beyond, when its mounted outside and /or
| in harsh conditions, thats difficult.
| Avamander wrote:
| Had to turn the zoom to 66% to make it readable, yikes.
| meonkeys wrote:
| Brace yourself for an onslaught of garish advertisements. If the
| content is truly exhaustive, a book would be a far better
| presentation format. I didn't survive long enough to find out.
| iamhamm wrote:
| Did we go to the same site? I didn't get ads and the material
| I've been clicking through is pretty good.
| madengr wrote:
| You mean the gal in the bikini right above the diagram of a
| dipole?
| iamhamm wrote:
| Wow! My blocking is doing great! :-)
| neves wrote:
| You must install uBlock Origin and live in a completely
| different and serene internet. :-)
| amelius wrote:
| Can somebody explain why I can send high definition video over
| WiFi, but not over an average quality USB 3 cable that is
| extended to a 6m length?
| madengr wrote:
| In a nutshell:
|
| If you have to guide an EM wave without dispersion (e.g. TEM
| propagation), it takes at least two conductors; those have
| loss. The smaller they get (think a thin coax) the more lossy
| they become.
|
| If you need a wide bandwidth, you need a smaller conductor
| arrangement to keep it from "over-moding" (becoming non TEM).
| Once it's non-TEM, you get dispersion and corrupt your signal.
|
| So there is a fundamental trade-off of bandwidth versus loss.
| Free space propagation is always TEM, so plenty of bandwidth,
| but now you must direct it with antennas as opposed to guiding
| it with conductors.
|
| You can get 110 GHz of bandwidth on a 1 mm coax, but it is very
| lossy, so much so that the microwave industry/research is
| looking into non-contact wafer probing for mmWave and THz
| applications.
|
| Fiber has similar issues. It's extremely low-loss, and non-TEM,
| but that dispersion is small enough you can multiplex in
| multiple channels without much dispersion across a single
| channel. Over long runs, still the dispersion is large enough
| that it needs to be compensated with various tricks.
|
| A 10G Ethernet copper cable is at most 3 meters, and hard-wired
| to the SFP modules. But you can buy a 10G mmWave radio and get
| 10 km. Now several km of that copper cable would be hundreds dB
| loss.
| spapas82 wrote:
| One insight that may help non EE educated people understand what
| an antenna does: The simplest form of an antenna would be just a
| simple point emmiting electromagnetic waves. These waves would be
| transmitted all around that point distributed as sphere. All
| points of a sphere will have the same power. Now, an antenna has
| a different geometry than a point that helps somehow "focus" the
| EM waves, so their power is not distributed around the sphere
| uniformally but some directions get more power depending on the
| antenna design.
|
| Now the thing to keep in mind is that an antenna is a passive
| device. It does amplify the signal but it does not add any power
| to it, it just collects the power to specific points. This may be
| easier to understand with a receiving antenna (which collects the
| signal).
|
| For example, consider the satellite dish which is of course an
| antenna. Due to its design it should be conceptually easy to
| understand that the power of the transmitted field is all
| gathered in a very small area in the front of the dish. The
| largest the dish, the smaller the point where all the transmitted
| power is pointed, so less power would be needed to cover largest
| distances (and more difficult finding where the dish needs to
| point).
| rambojazz wrote:
| Great explanation! Maybe I'm asking a dumb question but I'll
| shoot anyway: what is power? I mean, when I change the power of
| a signal what physical variable am I playing with? Amplitude,
| frequency, or wavelength are intuitive to visualize, but power?
| spapas82 wrote:
| Power is how "strong" a signal is. I think it's the easiest
| one to understand, that's why everybody's talking about dbm!
|
| In all electric circuits to change the power a component will
| consume you either change the voltage you apply to it or
| change its resistance, based on the P=VI & R=V/I equations.
| vvanders wrote:
| Power roughly correlates to amplitude.
|
| On the transmitter side keeping the signal linear means you
| generally don't see great efficiency in terms of power in ->
| power radiated.
| sizzzzlerz wrote:
| An antenna doesn't amplify a received signal. It takes an
| actual electrical amplifier to do that. When an antenna's gain
| is being mentioned, that gain is actually how much better the
| antenna is at taking in more of the signal than the standard
| candle isotropic antenna, e.g., one that transmits and receives
| equally in any direction.
| spapas82 wrote:
| Yes I tried to make that clear by mentioning that an antenna
| is a passive device. I just wanted to clarify that antennas
| help both at the transmitting and receving end. English are
| my 2nd language so maybe I could have explained it better...
| madengr wrote:
| Well it amplifies the signal by the Q of the antenna. The
| issue is that high Q resonators make lousy radiators, as the
| radiation resistance is very low compared to the reactance,
| and bandwidth is 1/Q. But you can certainly get amplification
| by a high-Q resonator inserted in a field.
|
| This is why electrically small antennas have an effective
| aperature much, much larger than their physical size. That
| ferrite rod antenna in an AM radio can have a massive
| electrical aperature (antenna gain) since it is narrow band
| (high Q). The resonance is amplifying the signal.
|
| You can also transmit through it if you can keep it cool. It
| may have 0.1% efficiency, radiating 1W for 1 kW of input
| power, if you have a nuclear reactor (say a VLF antenna on an
| aircraft carrier where you can't have wire antennas), you can
| trade power for size.
| degski wrote:
| The only thing not on the web-site is how antennas work, even in
| theory.
| kawfey wrote:
| As an antenna engineer, I've referred to this site hundreds of
| times. To be fair, it's almost an eli5 version of how antennas
| work, but it's been a good reference for basics.
|
| http://www.antenna-theory.com/basics/whyantennasradiate.php is
| as deep as it gets.
|
| For DEEP antenna theory, I can't recommend the Balanis book
| high enough. https://www.amazon.com/Antenna-Theory-Analysis-
| Constantine-B...
| amai wrote:
| Not complete without atomic antennas:
|
| https://www.technologyreview.com/s/611977/get-ready-for-atom...
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