On Fri, 29 Aug 2025 at 01:51, Bjoern A. Zeeb <bzeeb-li...@lists.zabbadoz.net>
wrote:
> >> Can you turn TX power down on th AP?
> >
> > Yes, I can do that too.Its current setting is 20 dBm / 100 mW, the
> default
> > and the maximum too.
>
> Given mine is ocked down I only have high/middle/low and can hardly say
> to what that resembles...
>
Hopefully more than 3dB difference for each. :-)
>
>
> > Another thing that I discovered is that if I change the channel width
> from 80
> > MHz down to 40 MHz (without changing anything else) the ghost beacons
> > completely disappear too.
>
> That interessting. I cannot do that on that device; I could only force
> the sta.
>
> In my case the problem is that it was CH 100 (correct) and the ghost beacon
> was on ch 36: 5500 - 5180 = 320
>
> There's no support of 320 wide channels on 5Ghz due to no sufficiently
> contigous
> spectrum.
>
> I would assume we are more likely dealing with physics here.
>
Yeah. There's a couple things it could be. This'd be a lot easier to solve
if y'all had
a spectrum analyser, but alas. :-)
* if it were dupe frames on adjacent channels then it could be the AP
sending duplicate
OFDM frames on the other 20MHz channels on the larger 40/80/160MHz
channel.
Normally you'd only see this for control stuff like RTS/CTS/ACK, but who
knows.
* if the transmitter amplifier is being non linear due to being over-driven
then i'd expect
some harmonics, and it's too early in the morning to do the math to see
if this would
overlap as seen.
* if the baseband frequency is leaking into the output stage then THAT
could cause
mixing artifacts and that could cause all kinds of hilarity.
* Heck I've even see the baseband frequency leak out and get into
unshielded ICs nearby,
leading to the fun behaviour of a 5GHz device seeing 2GHz beacons,
because of the
/baseband/ RF.
What I found interesting is that we both discovered this within a few
> days and I hadn't seen this in the years before.
>
>
Yeah, and I've not seen it locally at all, but I also don't have the same
branded APs
as yours. My "mesh" tplink devices all run low power on 5GHz.
> Given iwlwifi seems to report scan results in 5Ghz high to low channels
> my STA was always associated during boot before I would even see the
> first ghost (but I also checked that I do see them if not assoc) I would
> likely not have noticed in the last 20 days since that setup stands
> where it stands. Also the ghosts for me always came in with the last
> channel scanned. That said, apart from the channel they were valid;
> content was the same, seq/timestamps got updated, ...
>
> What also got my curiousity was that the AX210 monitor node next to it
> would not see the ghosts nor did the phone with a brcm chipset. I wanted
> to put a mediatek/linux sta there as well and a Realtek but shuffling
> cards etc. is currently too intrusive in that case.
>
>
> I was indeed thinking of filtering out the weaker one somehow but need to
> sort the RSSI bits out first (D50928) and flush other work before I open
> the next stack. I like the idea of logging duplicate BSSIDs on
> different channels.
>
>
Yeah, those are good ideas. I DO recall some err, hella broken old APs
reusing
MAC addresses on different frequencies though. :-) (But let's not worry
about THAT.)
Let's sort through the rest of that RSSI clean-up and see about finishing
cleaning up
how we represent noise floor, signal level, RSSI (relative signal level),
etc.
In the meantime, I can see about acquiring an AP that matches what you two
are
using and try to reproduce it. I /do/ have VNAs and spectrum analysers here
that I
can crime into service to see if this is an RX or a TX problem..
-adrian
