Hacker News Reader

Comment by bongodongobob | Hacker News Reader

I have a hard time believing this. Wifi can go through multiple walls. And if these were directional P2P links, they can easily go through and even around trees, I've deployed them in the past and they don't need perfect line of sight.

Granted the equipment could have been cheap, but this sounds questionable. He's asserting that the a few leaves at the top of a tree were blocking it when it wasn't raining? Idk.

View on HN

I'm the author. Among other things, it's a distance problem -- WiFi can go through walls when the router is right there. But distance attenuates the signal as distance squared, and in this case we're talking about hundred+ yards/meters instead of just a couple.

> in this case we're talking about hundred+ yards/meters instead of just a couple.

Sure but didn't you say you were using directional antennas?

No antenna is directional enough to overcome n^2 scaling. Especially not the mid-tier consumer-grade hardware I would have had access to at the time.

Rough rule of thumb, a consumer-grade directional antenna (at least at the time, maybe they've improved in the last 10yrs) will give your signal strength a one-digit multiplier (say ~8x), meaning ~7-10dB. But that n^2 means that improvement only takes you 2-3x farther, not 8x.

Here we're talking about ~100x the distance, which would need a 10000x = 40dB improvement in signal strength. AFAIK an antenna like that would cost more than the entire city block where I grew up

Won't 20dB at each end do the job? You can get 16-17dB antennas for $35 these days. And a wire parabolic dish promising 23dB for $200.

7-10dB matches the antennas I personally use, but those antennas are smaller than my phone. You can get a lot more out of a three foot spike or a two foot dish.

(I'm assuming 2.4GHz and hoping they're not lying about the gain.)

Right. This is a multiple timelines problem.

This story happened a bit over 10 years ago at this point. At the time of the story, the Wi-Fi bridge was already almost 10 years old -- we'd had the same equipment since I first remember getting internet at home.

That means the antennas at the start of the story were mid-2000s mid-tier consumer level equipment. I don't think a 16-17dB antenna would have been $35 at the time, in either mid-2000s dollars or in today's dollars. I also grew up in a country much poorer than the US, where $35 could feed a family for a week.

At the end of the post, we upgraded to 802.11n antennas (but still tech from 10+ years ago) which solved the problem by being newer, nicer, and having beamforming + MIMO capability which let them be more tolerant of obstacles (effectively, get more dB at the same emitted power level).

I'm not trying to critique the solution, I just think the decibel issue isn't so dire and other factors were more important. Which is supported by the way the problem was fixed.

As far as what was available at the time, I can't think of a super easy way to check that far back, but I will note that the 17dB stick antennas on Amazon have been available at the same price or cheaper for about 10 years.

Free space path loss over 5 meters with non-directional antennas is the same as over 500 meters with 20 dB of gain at each end.

a directional antenna increases the gain of the signal, but it doesn't fix the problem of decreasing with square of the difference and physical objects blocking signal. If the distance was already near the maximum, it wouldn't take much to block it.

Fair point. I suppose it's very much device dependant. The P2P stuff I've used was good for a few km.

Wifi normally uses adaptive transmit power and data rates. If the signal gets a bit weaker, your link slows down from say 300 Mbps to 260 Mbps. No biggie.

But sometimes for direct links you set the modulation, power and data rate fixed. The end result is that changing channel conditions can turn the link from 'working perfectly' to 'not working at all'

(Author here) For our Wi-Fi bridge, the devices on both ends were set to max power.

I don't recall them being able to change data rates very much (if at all) because the ones at the beginning of the story were 802.11g devices, and 802.11g didn't have channel bonding capability or similar tricks up its sleeve. Newer equipment definitely has more options like this.

I cannot know whether the story is true, but wet leaves definitely would interfere more with the link. The typical water content (and conductivity) of tree leaves is relatively low, and it could also be a factor apart from the aforementioned sagging due to the water's weight.

It is also true though that rain water has low mineralisation, and therefore low conductivity.

Keep in mind this was a 802.11g network.

Reply on news.ycombinator.com