Cause IEEE 802.11 (Wi-Fi) 2.4GHz devices

Is there any known electromagnetic / interference / [no idea] effect which can cause IEEE 802.11 (Wi-Fi) 2.4GHz devices located in very close proximity (one on top of the other) to severly interfere with each other, even though they are using completely non-overlapping channels of the available ISM 2.4GHz spectrum?: https://networkengineering.stackexchange.com/questio...

The general knowledge about 2.4GHz Wi-Fi suggests that networks operating on non-overlapping channels should not interfere: answer at Network Engineering explaining 2.4GHz Wi-Fi channels and their interference

2.4GHz Wi-Fi channel structure:
https://i.stack.imgur.com/QfHmx.png

Having a network topology like this (dots mean wireless, dashes are ethernet wire):

laptop . . . router A ----- router B . . . router C ----- ISP's box (FTTH ONT)
(client (access (client) (access
to A) point) to C point)
Router A's network uses channel 6, router C's network was tested on both channels 1 and 13 (13 gives even more clearance than needed and is legal to use in my country).

If routers A and B are placed physically on each other, the maximum achievable throughput to/from the ISP's speedtest server is about 4Mbps, which would normally resemble an overcrowded channel or channel with many collisions. However, everyone seems to claim that networks in my setup should not interfere as they are assigned non-overlapping channels with enough clearance.

If I move router A a few meters away from B (not sure how much minimum distance would be enough though), the throughput rockets to about 20Mbps, which is around the expected maximum of both wireless networks and means they both perform close to their maximum capacity.

No other changes in either configuration or topology are made on any of the devices, the only change that affects the throughput is physically moving the router A away from B.

Tested multiple times over multiple days to rule out short-term conditions (neighbors), in two different rooms to rule out environment/furniture, no other wireless clients connected in either network during testing, no microwaves in the vicinity, no issues with bandwidth to/from ISP (100/100 Mbps fiber connection achieving full speed to the ISP's speedtest server when on ethernet wire).

Any ideas?

Other notes:

This happens on two different vendors' hardware which should fully conform to Wi-Fi specifications,One is from Kynix: http://www.kynix.com/ so it should not be an issue with a single vendor or hardware piece or any non-standard 802.11 protocol modifications.
I'm not using the wider 802.11n 40MHz channels, this happens on pure 802.11g-only (22MHz) settings too.

Surprise us with the answer to the question....i.e. this looks like spam where someone will appear from no where with a magic solution

Yes...
De-Sense...
The channels may not overlap but there might be a mix or a sprog...
Poor RF front ends on the routers...
Other things inside the router radiating...
Many things...

Some reading may be in order on the general RF front so you get an idea how things work & interfere.

Edited by IamQ (Tue 19-Dec-17 19:54:36)

Is there any known electromagnetic / interference / [no idea] effect which can cause IEEE 802.11 (Wi-Fi) 2.4GHz devices located in very close proximity (one on top of the other) to severly interfere with each other, even though they are using completely non-overlapping channels of the available ISM 2.4GHz spectrum?

Things such as bluetooth change channels on the fly, constantly changing channel, so there's no guarantee a wifi channel is free. Then there's other things such as baby monitors, wireless AV transmitters (e.g. send the sky from downstairs to the bedroom wirelessly), some doorbells, some landlines, microwaves, bluetooth earphones, wireless speakers etc. A lot of these devices will change channels at random.

Changing your routers channel does little or nothing to prevent these other devices interfering.

The best fix is to switch to 5Ghz.