The WiFi in my bedroom was terrible. The router was in the living room, maybe 30 feet and two walls away. In the living room I got 200 Mbps. In the bedroom I got 12. Streaming anything was a buffering nightmare. I looked into mesh systems, range extenders, powerline adapters — all at least 50 dollars. Then I found a research paper from Dartmouth about using aluminum foil to shape WiFi signals.

The Science Behind the Foil

WiFi signals are radio waves. They radiate outward from your router’s antennas in all directions. Some of those waves go toward your bedroom. Most go toward the neighbor’s apartment, the backyard, the ceiling — wherever. Aluminum foil reflects radio waves — the same principle behind satellite dishes.

By placing a curved piece of foil behind your router’s antennas, you can redirect some of those wasted signals toward the direction you actually need coverage. The Dartmouth study found that a properly shaped foil reflector could improve signal strength by up to 55 percent in the target direction while reducing it behind the reflector (which you probably do not care about).

How to Build It in 5 Minutes

1. Cut a piece of cardboard to about 12 inches wide by 10 inches tall. A cereal box works. The flap from an Amazon box works. Anything rigid enough to hold its shape.
2. Wrap the cardboard in aluminum foil, shiny side out. Tape the foil to the back of the cardboard so the front is smooth and reflective.
3. Bend the foil-covered cardboard into a gentle C-curve, like a half-pipe. The curve should focus the reflected signals roughly where you want them.
4. Place it about 3 to 6 inches behind your router’s antennas, with the concave (shiny) side facing the direction of your dead zone — the bedroom, the back office, wherever. If your router has external antennas, position the reflector behind them so the antennas are between the reflector and the target room.

Does It Actually Work

I tested it. Before: 12 Mbps in the bedroom. After: 47 Mbps. Not the full 200 I get in the living room, but fast enough that Netflix stopped buffering and I could actually load webpages without waiting. The improvement was immediate and measurable.

It is not a perfect solution. The reflector does narrow the overall coverage area, so rooms behind the router might lose some signal. And if your router has internal antennas (no visible sticks), the foil trick is less effective because you cannot easily position it relative to the antennas. But for routers with external antennas and a specific dead zone you want to fix, it works better than it has any right to.

A Few Tweaks

• Experiment with the angle. Radio waves reflect at the same angle they hit the surface, like light off a mirror. If pointing the foil straight at the bedroom does not help, try angling it upward or downward slightly.
• Do not let the foil touch the antennas. Direct contact can interfere with the signal instead of reflecting it.
• If you want to get fancy, the Dartmouth team provides a 3D-printable reflector design on their website. The cardboard version is good enough for most people.

I have had a piece of foil-covered cardboard behind my router for a year. It looks ridiculous. It also means I can watch Netflix in bed. No regrets.