Hacker News Reader

Comment by modeless | Hacker News Reader

What is the legality of this? I have a vague suspicion that operating a device like this would violate some FCC regulations.

View on HN

Good intuitions. FCC regulations restrict EIRP[0], which is essentially "what is the signal strength in the strongest direction". It's possible that increasing directionality of your antenna puts you over the EIRP limit (given how routers already try to get _really_ close to it to improve performance).

[0] https://en.wikipedia.org/wiki/Equivalent_isotropically_radia...

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.

OK, so if by the time the signal exits my property it has dropped below mandated limits, there is no harm done right?

If I aim my antenna such that I contain the illegal power region within my property, will there be an unintended consequence?

To be clear, I'm referring to a house not an apartment so I don't have any concerns about upstairs or downstairs neighbours.

I think FCC laws technically apply within your property as well. Hypothetically, what happens if your non-compliant device blocks a visitor on your property from calling 911?

I think this falls under the pragmatic category of "you're only breaking the law if you get caught."

Oh of course, and there's actually a surprising number of FCC rules you'll never get caught breaking. GMRS license violation is another.

No, that is incorrect.

The domain of the FCC extends to all property of the United States of America, be it public or private.

The reason this is important is that it prevents private corporations from interfering with public air space (as discussed previously on HN [1]), and prevents landlords from interfering with the rights of tenants to legally place outdoor antennas (such as satellite dishes).

[1] https://news.ycombinator.com/item?id=8406022

RF energy doesn't quite work like this. If your transmitter has a high EIRP (either due to antenna directionality or high TX power), then its signal is going to travel a long way, even after the signal level has dropped to that of a lower-EIRP transmitter. That is simply the inverse square law at work.

You could place sufficiently absorptive materials that this isn't so (e.g. enclose your house in a Faraday cage). But as hydrogen18 points out you're still limited by the FCC.

He's not operating under FCC part 15. He's using FCC part 97 instead, with the appropriate license.

His rules: 1500 watts PEP, unlimited EIRP, channels 1 to 6 only, no encryption

That's better than a normal microwave oven.

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)

You're not allowed to mix and match antennas on mass produced Part15 devices, regardless of the ERP. That never seems to stop anyone though.

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.

http://www.ecfr.gov/cgi-bin/text-idx?SID=c2dc3c7e7691816d126...

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...

I remember reading that unless you amplify the radio above 5W or something, there's no way you're above the regulated limit.

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 regulations (afaik) only apply to the power output of the device, modifying the antenna like this shouldn't change the power output.

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.

To be fair, the FCC regulates both EIRP (4 W / 36 dBm) and max transmit power (1 W / 30 dBm). E.g. you can't just have a 34 dBm transmitter hooked up to a 2 dBi dipole.

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.

Right? Why wouldn't the router companies include this somehow in their designs if it actually improved performance?

Because it's illegal, is why.

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.

Dipoles are directional too. There are nulls directly aligned with the antenna (above and below the router in this case).

No doubt this accounts for some of the "my WiFi doesn't work upstairs" stories.

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 is illegal, according to another comment:

https://news.ycombinator.com/item?id=13459834

As I said in that comment, it's perfectly legal (and useful) so long as you turn down your TX power a bit.

Antenna gain is a function of directionality and vice versa. Patch antennas are also good because they have often very low "backward gain" but at the same time they are not very directional forward, so they are good to be placed in a corner of a room or near a wall.

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.

Reply on news.ycombinator.com