What is the best compatibility USB adapter for OpenWrt? I am searching and can't find good working adapter, all of them is not fully works with OpenWrt as Access Point, just works only as client. Have tried RTL8812AU — as client it's no problem, but it did not works as AP at all.
Now looking on RT3572L, but i am not sure is it works good as AP?
Please, recommend a good USB adapter for Access Point that work without problems on current master branch of OpenWrt (2.4 GHz 802.11n 300 mbit is suitable for me).
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There are no good USB wlan cards for 24/7 operations, even less in AP mode.
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Really no support for all USB adapters in AP mode at all? 
The firmware for USB devices is usually not tested and tweaked for the needs of AP mode, which often results in problems with AP mode specific functionality (e.g. supporting powersaving clients, DFS, etc.). The only USB wireless cards that were better in this regard (but still weren't stable in my own tests) were Atheros AR9170 and AR7010/AR9271, neither of them is still available on the market - and even those had quite a few hardware quirks to consider (4 or 8 connected clients at most, due to on-device RAM limits). Another issue is antenna quality (typically too small and too close to each other for effective MIMO support) and a tendency to overheat in long term usage, resulting in instability.
The various Realtek USB wlan cards don't support AP mode at all (their drivers require a proprietary hostapd backend, which isn't available in upstream hostapd and requires an ancient/ insecure hostapd 0.4.8(?) fork).
The traditional RaLink/ rt2800usb don't support powersaving clients (firmware limitation), so basically anything smartphone/ tablet, IoT or notebook like. I'm not up to speed about Mediatek's current mt76-usb chipsets, the caveats regarding antenna design or overheating probably apply though.
I've used AR9170 and AR9271 in AP mode for about a year, yes, it works, but there were multiple issues in daily operations (connections stalling, timing out, taking ages to auth, ...).
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just recently had 30+ days of uptime with AWUS036NHA in STA mode.
with simple tweak you can have 15 clients connect on AR9271, AR7010 IIRC has 4x onboard RAM so you should be able to have 50+ clients easily: https://github.com/qca/open-ath9k-htc-firmware/pull/149/files
I only have access to AR9271+AR9285, unfortunately not AR7010+AR9280, but the maximum number of connecting STAs wasn't even an issue for my use case (although certainly something to keep in mind) - long(er) term stability issues were (not OS crashes, but stalled connections, inability to (re-)auth over prolonged periods of time, etc. - it always recovered on its own, eventually, but the overall user experience was inacceptable).
Accepting repeater effect by reusing the same (AR9340) radio for (WDS-)client and AP mode however provides a much more reliable functionality in my long term experience (so effectively forgetting about USB cards in AP mode, but relying on AHB/ PCIe instead), it just works and none of the previously observed issues occur.
I had a very good experience with a trendnet ralink usb adapter tew-624ub and some module found in a toshiba smart tv, also ralink
I am using J3060 CPU based board (2 GB RAM) with only one minipcie, and i need USB wireless adapter only for 2-4 devices maximum to connect old 2.4 GHz wireless devices for "Smart Home", it better to support Power Saving. For modern devices already using 5 GHz QCA9994 adapter.
Now i am looking on AR9271 , but still not sure is it will works good...
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My own experience recommends against it (and carl9170/ ath9k_htc are already the best options), it works - but long term stability is the crux. It just isn't fun, if you just want to check something in the net, but need to wait somewhere between half- a minute or two to auth, or have your connections stalling.
if you're looking for home use you can safely go for ralink usb I used it like a year or so, just in a setup like yours and never had stalling on wifi, I restarted the system only when I updated openwrt, now I have an ubiquiti ap so I don't use the usb adapter anymore
What benefits would a USB adapter provide over a secondary router in AP mode... ( think second hand ATHEROS n onwards = $30-$50ish )?
-space
-power possibly, if low client numbers
-adaptability....
--->can be used where no ethernet port is available ... ( android OTG etc )
Why would a USB option suit you better?
I have describe "why" in upper message, just need 2.4 GHz band for old devices (that can't hold 5 GHz). I don't want a second router, because for me it have no sense, all high speed devices works thru QCA9994, and second router just will take a space / LAN port and drain electricity.
How do you think - RT3572L will work good?
Understood.
For you, it's important that solutions;
-support 2.4Ghz
-take little space
-use minimal electricity
Thanks!
I think it should work just fine, don't know what's the difference between RT3572L and RT3572
If this is best adapter for Windows - it doesn't mean that it will be best adapter for OpenWrt, in most cases it will doesn't work as AP at all.
Has anything changed more than 2 years later? i.e. can anyone recommend a known working usb wifi AP dongle that also supports 5Ghz.
My use case is to have a single OpenWrt VM and have several usb wifi AP's on strategic locations around (a rather large) house.
What I really want to prevent is having routers in dumb AP mode spread around. I have that now and I am running into maintenance overhead.
Not really (in a way even to the contrary, looking in the general direction of wifi6).
I'm really curious, how would adding USB WLAN cards improve this situation? You'd still have to plug them into something fast enough to deal with wireless, which needs maintenance just the same.
I need 6 AP's. So I'd rather keep a single OpenWrt instance maintained than 7. I assume it speaks for it self how much less maintenance that would be right?
Or am I missing some nuances I had not thought about yet?
Yes, you are. USB isn't wireless, and maximum cable length is 2m, 5m with expensive active cabling - not enough for multiple 'USB APs'. And USB pass-through into a VM is yet another world of pain (it doesn't work reliably, as the USB bus needs to be emulated and can't be passed through as you could do with PCIe, this messes up the driver's timing requirements). The other (non-)alternative would be extending the rf feedlines, but that works even worse than USB cables (signal loss).
So I really wonder how you plan to connect your 6 USB WLAN cards scattered over 6 rooms (aren't you missing dual-band requirements as well?, so 12 USB cards...) to your single OpenWrt instance (and a VM at that). And adding one SBC or x86 system to stick your USB WLAN cards into per room/ AP wouldn't gain you anything compared to a ready made AP with OpenWrt.
Dedicated dual-band APs running OpenWrt connecting via cat-5 or better would be trivial in comparison (100m maximum cable length, PoE if you want it).
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