Wifi: “beamforming” only begins to describe it
apenwarr.ca
apenwarr.ca
For example, the last page of a datasheet [1] for a ceramic chip antenna (size of a capacitor or resistor commonly used on small electronics like smartwatches or glasses with -0.5 dBi gain) shows the sphere in various cross sections. The last page of a datasheet [2] for a "whip" antenna shows an E plane pattern (looking at the side of the antenna as it stands up) that looks like a pattern from a Rorschach test but the H-field (looking down at the antenna) is almost a perfect sphere.
Most (all?) of these diagrams come from RF testing rooms like the one in this shot [3]. You can't see it in the photo but usually in the floor is a rotating platform with a coax cable carrying an RF control signal coming from the testing instrumentation.
[1] http://www.johansontechnology.com/images/stories/ip/rf-anten...
[2] ftp://ftp2.nearson.com/Drawings/Antenna/SG102N-2450V2.pdf
[3] http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Dri...
http://www.nec2.org is a very popular program.
A more descriptive listing of some of the issues with the article, or links to better resources, would rock.
Here is a good tutorial on phased arrays. It's actually a lot easier to think of it when receiving a skewed wavefront, and the phase shifters (delay elements) compensating for the delay.
For a better source, I'd start with the wikipedia articles on beamforming, phased arrays, and interferometry (http://en.wikipedia.org/wiki/Beamforming, http://en.wikipedia.org/wiki/Phased_array, http://en.wikipedia.org/wiki/Interferometry).
"(There's another concept called "antenna efficiency" which basically says you can adjust your scoop to resonate at a particular frequency, rejecting noise outside that frequency. That definitely works - but all antennas are already designed for this. That's why you get different antennas for different frequency ranges. Nowadays, the only thing you can do by changing your antenna size is to screw up the efficiency. You won't be improving it any further. So let's ignore antenna efficiency. You need a good quality antenna, but there is not really such a thing as a "better" quality antenna these days, at least for wifi.)"
Here you are conflating the issues of bandwidth and efficiency. Efficiency is how much RF power is lost to dissipative mechanisms in the antenna compared to how much is actually radiated. What you go on to describe is the bandwidth of the antenna. Your explanation that the bandwidth of the antenna is somehow chosen to reject noise for the receiver also demonstrates a misunderstanding of how all modern receiver architectures work. The noise bandwidth is either set by an IF filter in a super heterodyne receiver, or in the case of a synchronous digital receiver, in the matched filters. Antenna bandwidth may occasionally be chosen to reject interferes, but this is a different concept entirely. I suspect in consumer wi-fi antennas, if bandwidth is designed at all beyond "enough" it would be to maximize impedance bandwidth to allow for loading. It is also patently incorrect to say that "the only thing you can do by changing your antenna size is to screw up the efficiency". Larger antenna apertures provide more focusing, and therefore more directivity, and for a given efficiency more gain. Considering an array of antennas as a single antenna is one example of this. Another would be dish or horn type antennas which can be made with larger and larger apertures for more and more gain.
There are other issues throughout the article, but it's really not my intention to go line by line. It's also not my intention to discourage you from investigating this stuff further. I'm a practicing electrical engineer, and I feel every day all I learn is how much more I have yet to learn. Suffice it to say many of these concepts are very nuanced and complex (more so than your article reflects), so I applaud you for trying to figure this out on your own. Just don't think it will be nearly as simple as learning a new programming language. A couple basic things I would recommend you look up: the difference between directivity and gain, and the difference between coherent vs non-coherent combination. If you can, find someone who really, really knows this stuff to bounce your thoughts off of. Best of luck!
I'll leave the reply as an example of some of the factual problems with the article.
MIMO is more like vision. With a 2D array of pixels and a 2D array of photocells, you can transmit a vast amount of information through the same volume of space, and nearby equipment can reuse the same colors without much interference.
With smarter radios and more antennas, radio still has a long way to go before we hit the universe's data cap.
Off topic, but... this has been done. Amazes me to this day.
"Optical Time-Domain Eavesdropping Risks of CRT Displays" http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf
(Of course, given the extremely wide bandwidth of visible light and the high SNR between your TV and other light sources, you don't need that many spatial streams to beam the information representing HDTV across space. But humans.)
So while not a 100% perfect example, I don't think the original comment is all that terrible.
A quick workaround is to run the following in the error console:
ctx.ellipse = function (x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise) { ctx.arc(x, y, Math.max(radiusX, radiusY), rotation, startAngle, endAngle, anticlockwise); }The thing to notice about SNR is that you can increase it by increasing amplification at the sender (where the background noise is fixed but you have a clear copy of the signal) but not at the receiver.
This presumes that the signal that you are looking for is above the detectability threshold at the receiver. It may well not be, if you are trying to capture a weak signal.
Let's put aside the concept of a MASER used in radio astronomy and the size of their antennas. Or the fact that your wifi router or dongle has an amplifier inside of it. Let's do an experiment.
I have two instances of kismet running in my lab here in the leafy suburbs. One has a 3.5 inch antenna attached to a fit pc with internal wifi, the other has a 14 inch antenna attached to an Alfa with kismet running on kali in a vm on my mac laptop. The 14 inch antenna is successfully decoding 167 wifi networks and the 3.5 inch antenna sees 35 networks. I don't know what the relative gain rating of these antennas are, or what the relative quality of the radios is, but the dramatic difference on the receive side is pretty telling.
That (EDIT: The analogy used in the article) is not just a good analogy, but summarizes our best understanding of both light and quantum physics. (Feynman knew exactly what he was talking about -- he invented/discovered some of the fundamentals). Whether you'd use quantum physics as an analogy to explain light, or light as an analogy to explain quantum physics is a matter of taste -- which one you find more intuitive and which less intuitive -- which depends on what you've been told by others previously.
I disagree with this statement. If you try to learn everything from first principles, you'll never get anywhere. Students need analogies to understand the big picture while learning the details.
"That is not just a good analogy, but summarizes our best understanding of both light and quantum physics"
And the Lesbeque integral is a more rigourously defined operator that the Riemann integral, but would you use it to introduce calculus to first year students? Of course not - you teach the basics, and in grad school, you let the students that need to worry about the more complicated stuff take Real Analysis.
There are certainly connections between interferometry and quantum physics. However, I don't see how the author's explanation of the subject is enhanced by using quantum physics. For the purpose of this article, classical physics explains the phenomenology just fine. As I said in another comment below, and wglb said above, appealing to quantum physics when it is entirely unnecessary is a bad smell, and is usually nothing more than a distraction to make the author sound more sophisticated.
The author explains in the article how to understand the behaviour of light. It turns out quantum particles behave in exactly the same way. Feynman (building on Dirac's observations) built up the theory for quantum particles. With regards to light, this understanding comes from a couple of centuries ago (Ref: [Huygens principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...) and [relations to quantum mechanics](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...)
That isn't the argument. What I'm arguing is the (lack of) utility of the explanation. In what way does it enhance, support, or in any way contribute to the article? His argument is essentially:
A. We think of light as traveling in straight lines.
B. But because quantum physics, they don't! They travel along infinitely many paths.
C. But all of these paths cancel out except the straight line path.
D. And that's how beamforming works. Except it's beam-un-forming.
So he brings up a quantum phenomenon to complicate the scenario, then immediately reduces it back to the original scenario, without ever explaining how that original scenario works (i.e. how waves add together to create constructive and destructive interference: note that the words "wave" and "phase" never occur in same paragraph), then claims that this is somehow elucidating. So what's the benefit?
This coincides with the Pilot Wave model, where particles travel along paths modeled by the peaks of mathematical waves.
tl, dr: One puzzler is that antenna radiation is frequency dependent, while that of Johnson noise in the microwave regime is flat with frequency. Then, considering detailed balance, how does an antenna matched to a terminated coax cable establish thermal equilibrium with space? What happens is that the 1/f^2 of the antenna pattern is cancelled by the f^2 (in the long-wavelength regime) of the Rayleigh-Jeans blackbody radiation formula.
http://www.rearden.com/DIDO/DIDO_White_Paper_110727.pdf
as cited also in this talk:
The best explanation I've found relates to quantum mechanics, in a lecture I read by Richard Feynman at some point.
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The Feynman Lectures on Physics can be found online here, you probably want to read part Ⅲ.
In case anyone else is interested: the distinction between phase delay and time delay comes into play if bandwidth is large as you say, but in one more case as well. If the array is large enough that the time delay across the array is on the order of a symbol period you must use a true time delay. If you use a phase delay instead of a true time delay, energy from separate symbols will be added together to make the final bit decision and the resulting ISI will increase BER.
That's all anyone needs to know. Move along.