101–110 of 181 posts
What´s the 'household shelter' for?
some countries mandate civil defense shelters in private dwellings, switzerland and singapore both come to mind.
Large public buildings maybe still need shelters by law.
Earlier quoted context omitted.
No regulatory issues. The aluminum acts to increase the antenna gain (i.e. concentrate the signal in a particular direction). The total power does not increase instead you take some from one place and add it to another. There are tons of products for sale. You can get a replacement antenna that is a directional dish for example, instead of the typical omnidirectional dipole (stick).
> No regulatory issues. The aluminum acts to increase the antenna gain (i.e. concentrate the signal in a particular direction). AFAIK, many countries limit the EIRP, which increases with the antenna gain.
It's more complicated for 2.4 GHz point-to-point. At 1 watt transmitter power you are allowed a 6 dBi antenna. But for every 1 dBi you reduce transmitter power, you are allowed an additional 3 dBI of antenna gain to a maximum of 30 dBi antenna gain at 160 mW transmitter power.
5 GHz has even more complicated rules, I believe.
I was thinking of doing something similar a few days ago but came to the conclusion — perhaps wrongly — that the inevitable creases in a DIY aluminium foil sheet would render it ineffective as a dish, due to the scatter created. Anyone more knowledgeable able to chime-in on the effect of creases here? I couldn’t find any mention of it in the article...
speed of light / 5 GHz / 4 ~= 1.5 cm. If you can keep the size of the largest crease/feature/defect in your foil smaller than this, you should be okay.
So something like:
Source ----------------------------------- Dest
|
| lambda/4
| stub
By the time the reflection gets back to the transmission line, you have a phase offset of half a wavelength relative to original signal on the transmission line. If you add two signals that are half a wavelength apart, they combine destructively (cancel each other out). So the quarter wave stub acts like a very basic filter, and you actually won't see much of your signal at "Dest" when your transmission line has a quarter-wave stub like this.
Microwave engineers use all sorts of tricks involving transmission line segments that are a quarter wavelength long. So when you're trying to build an antenna (e.g. out of foil, or if you're adding metal support structures to a large antenna), it's helpful to use the heuristic: "if you need to add features that are not part of the original design, try to keep them smaller than a quarter wavelength."
blackguardx is right that restricting the maximum feature size to 1/10 of the wavelength is probably a better rule of thumb if you want good performance. If you can get it down to say, 1/2 of a quarter wavelength (1/8 wavelength), there's a reasonable chance it'll still work.
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…
This seems great for sending but how does it work for receiving? Or did you somehow do this on both ends?
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).
I got this guy and this allows me to read / play games in my bathtub https://www.aliexpress.com/item/Original-Portable-Light-Weig...
If you want to to do this "properly", see http://ham-radio.com/k6sti/wifiyagi.htm When messing with antennas, things like distance needs to be thoroughly calculated. Foil can help in many ways, but it can also make things worse—antenna calculations are tricky.
theoretically, are there any regulatory issues on a piece of aluminum sold (and marketed) for this purpose? if not, why aren't there cheap and effective such designs on alibaba - why have to DIY?
No regulatory issues. The aluminum acts to increase the antenna gain (i.e. concentrate the signal in a particular direction). The total power does not increase instead you take some from one place and add it to another. There are tons of products for sale. You can get a replacement antenna that is a directional dish for example, instead of the typical omnidirectional dipole (stick).
Earlier quoted context omitted.
> No regulatory issues. The aluminum acts to increase the antenna gain (i.e. concentrate the signal in a particular direction). AFAIK, many countries limit the EIRP, which increases with the antenna gain.
Correct. In the US, for example, for 2.4 GHz point-to-multipoint, EIRP is limited to 4 watts, and maximum power to the antenna is limited to 1 watt. So, if your router does 1 watt, you are limited to a maximum 6 dBi antenna gain. Lower the router power, and you can use higher gain antennas. It's more complicated for 2.4 GHz point-to-point. At 1 watt transmitter power you are allowed a 6 dBi antenna. But for every 1 d…
[1] http://www.qsl.net/kb9mwr/projects/wireless/allocations.html
If you want to to do this "properly", see http://ham-radio.com/k6sti/wifiyagi.htm When messing with antennas, things like distance needs to be thoroughly calculated. Foil can help in many ways, but it can also make things worse—antenna calculations are tricky.
In short, I'm looking for logic to repeat this for my router's dipole antenna and possibly with custom wires (not #14 or paperclips of unknown thickness mentioned there)