Unable to connect to internet sharing after upgrade to El Capitan

I upgraded my mid-2007 iMac from Mavericks to El Capitan 4 days ago. I normally connect my iPhone 4S and Apple TV via the internet sharing feature.

After the upgrade the signal strength was significantly degraded. There are no other WiFi devices on the system, one bluetooth device (trackpad); the keyboard, mouse. and printers are wired.

My iPhone dropped the signal about 15 feet from the iMac (it used to hold the connection up to 40 feet through my condo). The ATV worked fine from the Mavericks OS internet sharing; after the upgrade it could "see" the network and log the new, longer, password, but it is not connected to the internet when I select an app for viewing.

I have turned the network on/off. Unchecked and rechecked the internet sharing feature, rebuilt the internet sharing, shutdown. Repeated the above, deleted the password in keychain (as suggested in a support thread i read).

On the iPhone side I have told the phone to "forget This Network" and attempted to rejoin multiple times. As I write this, my iPhone is sitting 4 inches in front of the iMac and will not connect to the WiFi (Internet Sharing) Network.

Any ideas why the devices are not connecting, even though they can see the network and seem to accept the password?

Any suggestions on how to boost the signal strength?