zmarcus@ found a kefka in pool:wificell that was showing SISO rates in `iw wlan0 station dump` on both AP (managed0) and DUT (wlan0) sides while running VHT80 perf tests. 
Signal levels showed -14 and -60, IIRC, indicating one connector was broken. 
 
This was detected in software and a physical check showed that one pigtail was indeed not connected. crbug.com/845755 tracks adding a router-side check for this (to make sure the wificell is setup correctly). 

I think this is a useful check to have on the DUT side as well - ronaldho@
sent https://listnr.corp.google.com/product/208/report/85459346174 today where I suspect 
one of his antenna on the Fizz is not connected properly. 
Here's the station dump from his feedback report: 

Note the rx at 1Mbps (basic rate) and wildly different signal on each antenna. 

w dev wlan0 station dump:
Station f4:f2:6d:##:##:## (on wlan0)
	inactive time:	97 ms
	rx bytes:	63851271
	rx packets:	37947
	tx bytes:	7915853
	tx packets:	29070
	tx retries:	2502
	tx failed:	20
	beacon loss:	0
	rx drop misc:	889
	signal:  	-12 [-12, -51] dBm
	signal avg:	-13 [-13, -43] dBm
	tx bitrate:	130.0 MBit/s MCS 14 short GI
	rx bitrate:	1.0 MBit/s
	authorized:	yes
	authenticated:	yes
	associated:	yes
	preamble:	long
	WMM/WME:	yes
	MFP:		no