I have done a lot of testing on this issue. I am using a several year old Motorola/Arris SBG6580 cable router with 802.11n. I don't believe the problem if with WiFi itself. Here are my findings:
* Get network drop off every 10-15 minute, for about 90 seconds.
* WiFi link never disconnects
* Did not have problem on Yosemite. Problem started with El Capitan upgrade, and persists even with 10.11.4.
* Problem does not happen on any other device in house. All iOS and Windows machines are fine.
* Turn Mac OS WiFi On/Off clears the problem (although I don't think it is a WiFi issue)
* Simply doing a Mac OS DHCP Renew immediately clears the problem.
- It is as if El Capitan and the router can't agree on keeping the DHCP address in place
* Tried the IPv6 and Energy Save recommendations. Had no effect on problem.
* I tried an external known-good Cisco enterprise 1142 AP with L2 bridging. Kept the SBG6580 as the router and DHCP server.
- Problem persisted. Tells me the problem is more related to the router+DHCP, not so much WiFi itself
* Tried a different router (old Linksys) and problem goes away. But I don't want to use that router, but rather the 802.11n SBG6580.
* Clear all the System Config settings in Mac OS X and rebooted.
- Problem goes away for a few days, then re-appears.
So Apple did indeed break something in El Capitan with some home routers, and many people are having the issue.
If they want to try to troubleshoot it, they can get a used SBG6580 router and faithfully reproduce the issue.
-Larry