According to Engadget, a Wi-Fi extender simply re-broadcasts whatever signal it receives, so putting it in the wrong spot can make a weak connection worse. The usual advice is to place it halfway between the router and the dead zone, which under ideal conditions works out to about 25 to 40 feet from the router for models with external antennas, and 20 to 35 feet for those with internal antennas.
Obstacles such as thick concrete walls, brick, floors, metal surfaces, microwaves, and baby monitors can all interfere with the signal, meaning the halfway rule may not always apply. Engadget recommends starting near the router and moving the extender gradually toward the dead zone while watching its signal indicator light, then keeping it about four feet off the ground with antennas pointed straight up.
When no placement works, the source suggests two alternatives: running an Ethernet cable to the extender and switching it to access point mode, which preserves bandwidth and avoids signal loss, or upgrading to a mesh Wi-Fi system. Mesh systems keep devices on a single network and route traffic to the strongest node, but they cost more than extenders. The source notes that not all extenders support wired mode, and Ethernet cables have a maximum effective length of 100 meters. The article does not compare multiple sources, so these recommendations reflect Engadget's guidance alone.