I've got a single-board computer with an on-board wifi adapter. I've noticed that I can never get maximum link quality. The best I've ever seen is 60/70. Furthermore, the link usually fails if the quality drops below 30-35. I've tried higher gain antennas and different routers but with the same results.

Why is this the case?

You seem to have little experience with radio, which WiFi is between modem & router. Get a short direct line of sight with antennae optimised for max reception at the shortest possible distance for optimal reception.

60/70 is good. Signals dropping at 30-35/70 indicates a problem, perhaps shadowing where the signal is received via a number of paths with different phase, or a neighbor using the same channel.

I get maximum speed beside the router, which recedes as I go away. Try fiddling with the channel you use. See if everything does WiFi 802.11/n, which is the fastest protocol. If you run and see what channels your neighbors use and use another if possible.

Co-channel interference is indeed a suspect. Contrary to let's say FM radio, Wifi signals can co-exist operating on the same frequency. But there is interference and signals on the same frequency are separated by the transmitted codes. Your radio will however detect interference, signal quality will be lower and speed will be affected.

If there are many Wifi access point around, you will see congestion on channels 1, 6 and 11. This is what access points will choose automatically. There is total frequency division between those channels, it means they do no overlap. Which is good, but there will be heavy co-channel interference on channels 1, 6 an 11.

That also explains that when the wanted signal becomes weaker, it literally drowns in the other signals on the same frequency.

So you could try to move your router to one of the other channels. You will overlap with channels 1, 6 or 11. But still you have more clean frequency spectrum available than when sticking to the same channel.

Even better is to move to 5 GHz band if you can.

jlinkels

this helped me recently.
i suspected an inferior driver or even hardware, but when i changed my router's wifi settings from Auto to manual (i chose channel 13), all problems were gone.

I just checked the neighborhood and I'm the only one on channel 8. Every other router is on 1, 6, or 11.
Even so, I've seen similar behavior when I'm quite literally in the middle of a several square mile field. I never get a link quality of 70 regardless of proximity.
I have noticed the difference between a rubber-duck antenna that's new and one that appears new but may be broken internally. I also don't see any difference between a 5 dBi antenna and a 12 dBi antenna. Seems like the only thing that makes a difference is raw power.

Mine works excellent inside a all metal building. 250 MB dist-upgrade in about 2 minutes. Like a land line.

Side view

Router is 200 feet away which is also surrounded by 4 walls. One wall is lead lined. Used to be the X-ray room. I went off using a satellite service contract , switched to roku, and re-purposed the antenna they did not take off my roof of the house. Saves me about 100 bucks a month.

One cannot expect to set something like this in the room of a house. My house is a former 1950's hospital. The metal building is my shop in the hospital parking lot.

But a coffee can or pringles potoato chip can with a usb wireless like I used on my shop building , in a room is feasible. Mounted pointed toward a radio signal. You tube also has dipole wifi antenna videos also.