OpenWRT users:
I have a Linksys WRT3200ACM and a WRT1900AC router with OpenWRT 21.02.1 loaded onto it. If I were to plug a Realtek type USB WiFi adapter into the USB3 port on one of the routers, would it be possible to make use of the additional radios then on the router?
Thanks!
Stuart
Yes, but you need to get the right one.
Most of them are Realtek based, and won't support AP mode.
this could be useful https://github.com/morrownr/USB-WiFi
Here is what macOS told me about the adapter so far, but I suppose I'll have to plug it into a Linux box to interrogate it more greatly. If I plug it into the WRT1900AC (running the most current OpenWRT), what packages do I need or commands ought be there to interrogate the device?
USB3.0 802.11ac 1200M Adapter:
Product ID: 0xb812
Vendor ID: 0x0bda (Realtek Semiconductor Corp.)
Version: 2.10
Serial Number: 123456
Speed: Up to 480 Mb/s
Manufacturer: Realtek
Location ID: 0x14200000 / 16
Current Available (mA): 500
Current Required (mA): 500
Extra Operating Current (mA): 0
Thanks again!
depends on the device, but dmesg and lsusb would reveal its true identity.
iw list will show you the device's capabilities, you want to see AP in that list.
In this case, it's a Realtek, so probably a dead end, from an AP point of view.
Well this site seems to indicate that it is an RTL88x2bu
I am not sure if they are referencing OpenWRT here but this looks encouraging:
Realtek Linux drivers are a clusterf.k, I wouldn't trust them for a second.
If you want to play around with them, and get a ulcer in the process, go ahead 
Rule of thumb is stay away. You've been warned.
Well I plugged it into the router and rebooted the router and I do not see any more physical radios with "iw list" than I did before it was plugged in and rebooted. It sounds like you are saying that there are other drivers that will work better than those that are in the Linux kernel? Which drivers are you referring to or are you saying Realtek hardware will just never work right on Linux?
You'll need the proper kmod and probably a firmware file to get the adapter to work.
Having it run as an AP is something different.
Realteks work, but not for your use case.
Just out of curiosity, if I wanted to use an USB adapter to add an additional radio, what kind of adapter would you recommend that would work (just to experiment with it)?
For now, I do not need it, but when USB 802.11ax USB adapters come out I'd like to perhaps do it then.
The driver you mentioned, that is a PR from after the release of 21.02.1 or it's in 21.02.1?
Thanks again.
Those in the 1st link, from MediaTek, but they will never work out of the box, you'll always need a kmod and (usually) a firmware. The router firmware is slimmed down to a bare minimum, required for supporting the hardware they run on, they'll never come with additional hw support.
Whatever you get, don't get Realtek.
Can't find the driver, not even in the snapshots, not sure it actually made it into openwrt.
when you say "...and (ususally) a firmware", you mean building a custom firmware for my say my WRT1900AC or that with the kmod for the mediatek device I wish to use inclusive therein or I could I load it after the fact post boot up?
no, it's just another package you need to install, along with the kmod.
the firmware is for the USB radio itself, not the router.
If I were to pick an USB WiFi from the list on the first link, how would I know which kmod and firmware to use? I presume it would be easily installable like any other opkg install from the command line?
Anything else I'd have to do to get it to work besides installing the packages, rebooting, and then going into Luci and configuring things to leverage it?
Stuart
They're MT7612 (perhaps not all of them, didn't read that far) and should be supported by kmod-mt76 - https://openwrt.org/packages/pkgdata/kmod-mt76, which is installable using opkg.
The gist about USB wireless cards would be, "A strange game. The only winning move is not to play."
It's usually cheaper and a lot more reliable and performant to get a simple concurrent dual-band AP (wireless router with OpenWrt) to cover the wireless side, rather than wasting your time with USB wireless cards which will never work well enough. Depending on your needs both replacing the wrt3200acm or continuing to use it wired-only with a cheap mt7621 based AP attached would be the options.
Well, I do agree that replacing a router (if I need greater speed) is the best way ahead and surely the easiest and most convenient (convenience does count a lot honestly). Granted, I am for now the only person using my network, though I use it for work as well as my computer lab too. I bought the 802.11ax WiFi card because it was cheap and to have it to play with should I get an 802.11ax router at some point. Both of my routers honestly give me enough bandwidth for my purposes that I am not concerned about 802.11ax at this point as a "must have" and I am not a gamer either. However, I will say that in the future I will be more discriminating and most assuredly get a MediaTek based device be it a USB stick or a router.
The points you make about the proximity of antennas are interesting but I am not sure the spacing is much less than those already on my Linksys routers (each has 4 antennas). I do not think the antennas are at least one wavelength apart either but maybe at 5GHz they are. I'd have to check.
Stuart
Routers doesn't have to be MT, there's Qualcomm too. Realtek router hw isn't as bad as their consumer usb product line.
See if you can get your hands on https://www.amazon.com/dp/B00MPDPFMI , they were sold for $10 in the past, new.
Realtek based, not supported by openwrt, but get the job done, and are extremely stable.
EDIT: seems you can find them on eBay.