Skin Effect

Metals can shield Electromagnetic (EM) waves. The distance an EM wave can penetrate into a metal sheet is known as the skin depth. At a frequency of 1 GHz, the skin depth of an aluminum sheet is about 2.6 microns, which is similar to the thickness of household aluminum foil used for kitchen purposes. Cellphones can both transmit and receive microwaves. Wrapping a cellphone with aluminum foil can help determine the reduction in signal transmission as a function of the number of foil layers.

There are different ways/apps to conduct this experiment. Based on our testing, method (iii) appears to be the most reliable at this time:

(i) Use a smartphone as a microwave detector to measure nearby cellphone tower signals:

For iPhones, turn off Wi-Fi before starting and dial 3001#12345#. Press the call button and the field test mode menu will appear. Click on "Serving Cell Measurements" and find the power value in dBm under "rsrp0".

For Android, go to Settings > More Options or More Settings > About Phone > Mobile Networks > Signal Strength. You will see your dB reading under Signal Strength or Network Type or SIM Status.

You will need to figure out a way to record the reading when the cellphone is wrapped in aluminum foil. The procedure to turn on field test mode for iPhones may differ for different iOS versions.

(ii) Use a smartphone as a microwave receiver to measure nearby Wi-Fi routers:

Download the "AirPort Utility" app.
Open the app and scan for nearby Wi-Fi sources.
Choose the one with the strongest signal, click on it, and it records several measurements in the past half minute.

(iii) Use one smartphone/smartwatch as a microwave transmitter and another smartphone as a microwave receiver:

Download the "BLE Scanner" app onto the smartphone used as the microwave receiver.
Turn on Bluetooth on both devices.
Open the app, and it will scan nearby Bluetooth devices.
Wrap the smartphone/smartwatch with aluminum foil as the microwave transmitter and read off signals from the BLE Scanner on the other smartphone.

The benefit of this setup is that the transmitter is wrapped with aluminum foil, making it easy to read the values.

Note that for all three configurations, you will need to measure the signal multiple times and take the average.