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You can check your wifi signal quality easily in the wifi menu (press Alt and click) as was described in article referred to above.
 
So I'll have no internet connection on my Mac. How do I link the Mac and ATV via wifi without my router?
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I heard it was better to connect both base stations together via ethernet for better signal strength.
Yes. It is best to hard wire all your routers / access points. Then WiFi to the closet one to where you are using a mobile device. In most cases Ethernet for things that do not move and wifi for things that are mobile. Unless you have no choice.
 
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