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Comment by mrtksn | Hacker News Reader

It can work surprisingly well, I made a "dish" with an aluminum foil that was able to connect to a Romanian Wi-Fi hotspot from Bulgaria, according to Google Maps it should be something like 20KM distance. I should note that I had direct view of the town as I had significantly higher altitude.

The hotspot was some kind of public service provided by the Romanian town as it's name suggested. The ip WhoIs also confirmed that it was a Romanian one.

Every now and then the connection would have had slowed down and I would do some "Voodoo magic" on the aluminum foil to fix it(Not sure if it was me though, it seemed like if I move the dish slightly or fix a bump in the foil the connection would improve).

This was probably like 7-8 years ago and I was able to get something close to 6-7Mbps as far as I remember.

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Old trick is to use Pringles (or other) can as directional antenna: https://www.makeuseof.com/tag/how-to-make-a-wifi-antenna-out...

I remember seeing that in Mr. Robot.

Mr Robot tries to stay as true to hacking as it can.

I used one of these for six months when I moved out of home at 18. I was broke and saving for a wireless connection fee (expensive back in 2005!). When the AP I was using went secure, I'd just point to a different part of the city and repeat.

I've also achieved substantial improvement with simple parabolic shaped, ~40 cm wide aluminum foil constructions, but there always seemed a bit luck involved and it needed some tweaking. Not sure whether the foil mainly acted as shielding that reduces the noise floor from other directions. I've read in other forums that fiddling with aluminum foil is mostly luck because additional reflective surfaces also introduce interferrence patterns unless they are precisely aligned. Even without aluminum foil it often helps to move either receiver or sender by fractions of 12.5 cm (the wavelength of 2.4 Ghz radiation) or of 6.0 cm (in case of 5 Ghz) because the space is basically riddled with interference patterns: https://www.youtube.com/watch?v=aqqEYz38ens

A wok (Chinese stir-fry pan) works surprisingly well for this purpose. It's roughly parabolic, and has a smooth surface that helps you avoid irregularities in the shape of your foil.

It gets even better if you drill a hole in the center and stick an antenna vertically in there. Now you've got a wi-fi antenna with all the advantages of a satellite dish! The Chinese call it wok-fi.

Huh... no kidding. I'm going to have to try that out some day. How was the signal like on the wok with the antenna through the center?

I haven't personally tried sticking an antenna through a wok (just putting one behind the router was enough for my purpose), but rumors say that you can achieve a range of several miles if you do it right.

There are online calculators http://www.radiolabs.com/stations/wifi_calc.html note that the cable loss mentioned is the antenna feeder cable

And Cisco Press do some good books on the basics of WiFi network design.

This seems great for sending but how does it work for receiving? Or did you somehow do this on both ends?

Most antennae are retro directive. Gain is gain.

I replied here: https://news.ycombinator.com/item?id=16012484

It's not needed to do it in both ends. Naturally, improving both antennas is better than doing in only one, but every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions of communication.

So, if a new antenna in client side improves signal reception by 3dB, it also improves transmitted power by 3dB. If you used the same antenna on the other side, the 3dB would be added again in both reception and transmission, for a total gain of 6dB each direction.

> It's not needed to do it in both ends. Naturally, improving both antennas is better than doing in only one, but every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions of communication.

Does this actually make sense to you? Gain is a factor; it needs to be multiplied by something. Here's the obvious thought-experiment: consider an (ideal) parabolic mirror in (a vacuum). Put a lightbulb (infinitely tiny) at the focus. Light will come out parallel and will be just as strong no matter how far away the receiver is. i.e. the distance will be irrelevant when the sender is at the focus. Now try reversing their roles, with the lightbulb far away and the receiver at the focus. How strong is the received signal at the focus? You're claiming it won't matter how far away the sender is from the focus, which makes no sense.

Think of a parabolic dish on a microphone at a sporting event. The dish blocks noise and interference from the directions you don't want to listen to, and less noise and interference is as useful as if the sender had more power. Same result of better s/n either way.

Or a TV satellite dish. If you were correct, you could rip off the dish and point the antenna forwards instead of backwards, and still watch TV. Not gonna happen.

> The dish blocks noise and interference from the directions you don't want to listen to

You shouldn't need to talk about noise or interference for the argument to make sense. Just imagine there isn't any. The argument yields absurd results either way: the power certainly does depend on how far the sender is from the focus.

The trick here (in this Wi-Fi example) is that the end you control is the end with parabolic antenna - if you can make it to collect enough energy for RX from an omnidirectional source, you can also make it send enough energy towards that source for its antenna to pick it up.

But you're right that "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions of communication" is wrong; there are different ways of improving TX and RX, even though some overlap a bit, to some extent.

No, as long as antennas and signal power is concerned, I stand by what I said: every improvement in transmission power has a equivalent improvement in receiving power. If you have a more specific doubt, please, feel free to ask.

> If you have a more specific doubt, please, feel free to ask.

Unless by "feel free to ask" you mean "I don't plan to answer", this doesn't really seem like a sincere suggestion. I already did this and you didn't answer.

I'm still on the move, but I'll answer eventually. Your previous post did not contain any questions.

Yes, it does make sense to me. I think conclusions drawn from this thought experiment are not entirely correct. Under your experiment when sending you spatially create a beam - there is this beam as wide as parabolic dish and no light anywhere else. With zero interference (using the word quite freely) this beam "will be just as strong no matter how far away the receiver is". With roles reversed, you do not necessarily get directional transmitter. For the sake of argument consider omnidirectional light bulb (the same bulb without parabolic dish) as sender. Classical intro/ELI5 description of transmit power is fixed number of spatially evenly distributed omnidirectional "rays". The larger the distance, the lower number of "rays" you receive per unit of area (receive power scales with surface area of transmit profile, usually r^2). By adding a parabolic dish you effectively increase receive area to size of dish instead of sensor size. When sending, the area over which power is dissipated is constant instead of scaling with distance.

> You're claiming it won't matter how far away the sender is from the focus, which makes no sense.

You are correct in that it makes no sense, but the claim was "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions", key word improvement. While this is not entirely correct, this approximation (that antenna improvement works both ways equally) is pretty close to reality in practical far field applications.

> With roles reversed, you do not necessarily get directional transmitter.

I don't get why you put "necessarily". You do not, period.

> [...snip...] but the claim was "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions", key word improvement.

Yes, the claim is wrong:

1. With the sender at the focus and the receiver farther away, the mirror reduces the loss (due to distance, since everything else is assumed ideal as you already acknowledged) to ZERO. You literally cannot do better than reducing your loss to zero.

2. With the receiver at the focus and the sender farther away, the mirror DOES NOT even remotely reduce the loss due to the distance to zero. Loss is still ~1/r^2. Because the mirror only gets so much of the omnidirectional light. This should be very obvious.

I don't care how you slice and dice this, and I'm tired of arguing something this obvious, so I won't keep commenting. 1 is not equal to 1/r^2.

> I'm tired of arguing something this obvious

You're arguing a scenario that is impossible both practically and theoretically and then calling others pedants for pointing out the fundamental flaws in your thought experiment. You're literally arguing against the textbook and then asserting that your thought experiment is close enough to make your point. It might be time for you to stop assuming that everyone else is an idiot and entertain the notion that you might actually be wrong.

> Light will come out parallel

No it won't, it's diffraction-limited. The best you can theoretically do is an https://en.wikipedia.org/wiki/Airy_disk

You cannot construct, with any arrangement of mirrors or lasers, a beam that does not diverge.

I was assuming geometric optics... because it was simple enough to get the point across. Which of course you knew, but took the liberty to 'correct' anyway. It's quite impressive how you can always count on HN pedants to deliberately go out of their way to technically-correct you while making sure to completely miss the actual point you're trying to make.

You're the one that started in by trying to wrongly "correct" the statement "every improvement in sending is completely equivalent in receiving for a system that uses the same antennas for both directions of communication" (which is actually correct).

The fact that you can't have perfectly collimated beams and must make do with antenna pattern "lobes" is actually important here.

I'm not going to get into to the "gain wars" (yes, text bok, real-world you can have just one "good" (high gain) antenna, and it will work just fine for send and receive) - but your answer prompts the obvious question: What about lasers? Turns out, before optical lasers there were masers - microwave lasers, bit they were (are?) very limited power:

https://www.scientificamerican.com/article/first-practical-m...

You are assuming the mirror is parabolic.

Your thought experiment is not in line with what OP is claiming. You would need to vary the dish size and see how that affects tx/rx.

Have you studied physics and antenna theory?

I don't know why people are arguing so much with this when it's straight out of textbook antenna theory. Evidently it's very HN to work things out inaccurately from first principles.

Edit: http://www.cv.nrao.edu/course/astr534/AntennaTheory.html

"An antenna can be treated either as a receiving device, gathering the incoming radiation field and conducting electrical signals to the output terminals, or as a transmitting system, launching electromagnetic waves outward. These two cases are equivalent because of time reversibility: the solutions of Maxwell's equations are valid when time is reversed."

Thanks for the reply. I'm on the move, in my phone, and has regretted on posting the comment as it is hard to elaborate further given the situation.

> Evidently it's very HN to work things out inaccurately from first principles.

I find this quite common among people that consider themselves technically minded. One of the key indicators I have for truly 'smart' people is an inclination toward curiosity with new things, and perhaps a degree of humility.

It has been ten years, since I had antenna classes in the uni, so I might be wrong, but I do not completely agree.

Gain is the same in both ends, but my concern about receiving is that gain is applied to everything. This includes interference and noise. So at the receiving end if the SINR is already bad, the antenna gain is not going to help much. This is why you apply the "Low Noise Amplifier" right after the antenna. To make sure that an already bad SINR will not get even worse in the rest of the receiving stages.

I don't know much about radio, so your question had me stumped for a second, and the other replies didn't help me much. Here's my explanation:

Some celestial bodies like the moons of Jupiter, are too dim for us to see, because the amount of light that hits our pupils is too small for our retina to react to. But the light from the moons isn't pointed directly at our pupils, it goes in all directions. If the area of our pupils were twice as large as it is, we would be getting twice as much light from Jupiter's moons — and everything else (except a laser pointed at your eye, please don't point lasers at people's eyes). Telescopes work by being a HUGE pupil, collecting all the light from a wide area and pointing it all right at our pupils (that's not the only thing they do, but it's an important part).

An antenna is like a pupil, and a satellite dish, or this piece of aluminum foil, are like a telescope. Some of the signal from your laptop in the bedroom goes and hits the antenna with a weak signal. A lot more of the signal misses the antenna and hits the foil, and then gets reflected where some more hits the antenna (since the foil was curved, more of the signal hits the antenna on the rebound than on the first pass).

Are you from Svishtov? Cheers buddy. I did something similar to OMV free wi-fi from my balcony :)

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