Given that it's only somewhat directional, this would actually be useful for the common case that the AP sits in a corner of a one-story house. No sense sending all that RF energy outside, so direct it inside.

My current (rental) place has the router and AP in one corner of the apartment, so signal strength at the other end is terrible. Is there a better (cheap but effective) option than this?

This would be great if you lived on a boat, then you could have a wifi router on the shore/marina with signal directed at your boat.

It also makes the antenna more sensitive in the direction its pointing. The elements make the antenna more directional, sort of focusing the energy. This increases the transmit side by concentrating the energy into a smaller area, but the same effect also helps it collect more energy.

The trade-off is worse performance perpendicular to the extra elements.

Reciprocity says that path loss is equivalent both ways.

I'd guess it goes both ways, just like a speaker cone and the cone of an ear.

Antennas are coupled. Transmitting is receiving, and receiving is transmitting.

When you pull on a chain or a rope, it doesn't matter if the other side is "receiving" the pull, or "sending" it. The two sides are physically coupled.

Similarly, antennas are physically coupled to each other. Increase the gain on one side, the other side still benefits. You're physically connecting a "rope" of electrons over the air.

itll be applicable for any 2.4ghz, 50 ohm radio

You might start at the ARRL's Antennas page [0] and consider the ARRL Antenna Book [1]. Most of that is aimed at amateur radio in particular but the theory is the same.

There's tons of free and/or open-source software (of varying quality) for modeling antennas and such as well.

[0]: http://www.arrl.org/antennas

[1]: https://www.arrl.org/shop/ARRL-Antenna-Book-23rd-Softcover-E...

Take a microwave design class. This would be a second or third course after electrostatic, electrodynamics, then something like microwave theory. At my school we had to petition for the course as a special section. It was great, we built cavity resonators, and final project was an antenna design. All the way from theory, then using modern fea antenna software, and then building it and testing performance. If you have any interest take it. I was not going to but the professor said: "Phil, what's the lower end of the microwave spectrum?", I said "1 GHz", he then asked "How fast is your computer?", I replied "2.8 GHz", He said "I'll see you next semester then?". these days having an understanding of microwave theory will help you no matter your specialization.

1. Find a university course on RF, look up the course textbooks.

In my university we had several courses on "analog electronics" which covered topics like free space path loss and RF concepts (e.g. bandpass filters).

2. Use Google or another search engine (DDG) to find the book in PDF or EPUB version.

3. Enjoy reading the book without forking over hundreds of dollars to a greedy publisher.

balanis

Depends. Some routers have multiple antennas because there are multiple transceivers inside it, and some technologies (i.e. MU-MIMO in 802.11AC) will most likely react strangely having one of their antenna modified. In the end, it would probably make your performance even worse.

theres a two reasons your router probably has 3 antennas:

-beamforming

-split 2.4ghz/5ghz/5ghz-wave2 antennas

if your router is using those three antennas for beamforming, you might trigger some strange behavior if you yagi a single one of them. if run into issues and can turn beamforming off, you should be able to yagi a single one no problem.

if your router pumps each antenna into a separate radio, then you can reliably pump a single radio into that corner of your house w/ a yagi. however, you won't have seamless roaming between the 2.4ghz and 5ghz networks when you move into and out of that corner of your home.

i recommend getting a mesh network, they're the bees knees and are a huge step above the traditional access point + range extenders model

No, Yagis have a single driven element like a dipole, and you have longer elements behind the driven element (reflectors), and shorter elements in front (directors I think?).

Certainly, though the output power is quite a bit less (20dBm for WiFi vs 4 dBm for typical bluetooth).

http://www.tomsguide.com/us/how-to-bluesniper-pt1,review-408...

Yep. Specifically you'd need to adjust your TX power down to 27 dBm (from a max of 30 dBm) for an AP, or 29 dBm for a station. http://www.air802.com/fcc-rules-and-regulations.html gives a nice summary.

Basically APs on 2.4 GHz are only allowed 36 dBm (4 W) EIRP, no matter what antenna shape. This is usually not a problem though, since stations tend to have weaker transmitters anyway, but will still benefit from the AP's enhanced antenna gain.

Note that licensed ham radio operators are allowed to operate at (much) higher power on the WiFi channels that happen to overlap the 13cm ham band.

http://w5vwp.com/wifihams.shtml

(Of course then you are subject to all the Part 97 restrictions like no encryption, no commercial use, etc)

The FCC has stated that the RP-SMA connector (the connector used on nearly all wifi routers that have detachable antennas) shouldn't be allowed on part 15 devices, but also that it will delay enforcement of this indefinitely[1]. So even the FCC is fairly "meh" on enforcing this unless actual interference is reported.

1: http://hraunfoss.fcc.gov/edocs_public/attachmatch/DA-00-2225...

Citation? Plenty of commercial part 15 devices don't even come with antennas, requiring you to "mix and match", unless I misunderstood what you meant by that.

4 W (36 dBm), and the 8.5 dBi gain of this homemade Yagi puts you over that if you're operating at 1 W (30 dBm) max transmit power (as some home routers do).

FCC regulates EIRP, which is output+antenna gain. For this to be strictly legal you would need to adjust the power down until the EIRP is within the legal limit. Actually to be strictly legal I think it would need to be type excepted by the FCC.

That being said if you're using this in your home/office on a legal channel it would be really difficult to even tell from outside you were using it and almost certainly not worth the FCCs time to even try to watch for it. It's not that much extra gain over the stock antenna.

If it's like here in Australia, they do consider EIRP. So a directional antenna would increase the EIRP, but with just 100mw tx at the output, I doubt EIRP would be over the limit (4W for 2.4Ghz ISM in Australia).

FCC regulations apply to the device with the supplied antenna in place.

The antenna is not supposed to be replaceable, hence the reverse SMA connector on most modems.

Modifying the antenna in any way invalidates the type approval.

No, because no-one wants a home AP that only works if it's pointed at you.

You can buy directional APs if you want, but they're usually used for outdoor point-to-point links, or as sector antennas. e.g. I just installed one of these today: https://routerboard.com/RBSXTG-2HnD in a place that needed a signal far from where the AP was. Usually patch antennas (like this product) are preferred over Yagis for microwave links due to their smaller footprint, and capability of dual polarization in a single package.

Better is if your indoor AP can transceive in more of a "pancake" shape, like this: https://routerboard.com/RBOmniTikU-5HnD Note that's almost the same gain as this home-grown Yagi (7.5 dBi vs. 8.5 dBi) but with a more useful directionality.

It's not illegal per se, if you adjust transmission power accordingly. Router manufacturers generally don't use yagi because it's directional, requiring aiming, unlike omnidirectional dipole antenna. Modern wifi stations have much more sophisticated ways to increase range than passive antenna designs.

It's only illegal if you get net gain out of it[1]; if you turn down the power so that the maximum power transmitted in any direction is the same as before, then it's legal. You can get a small performance improvement by using directional antennas in this way as you reduce the received strength of off-axis signals which can reduce both multipath interference as well as interference from other stations.

It's not all that useful indoors though as 2.4 and 5GHz signals bounce off of walls really well so you lose much the directionality at the very first wall it hits.

1: Actually there is a total ERP requirement, but most routers with external antennas are already close to that requirement, so more than about 3dB gain will often put you over.