Why, using a EMF tester on a smartphone in airplane mode, do I get a very low measure (\$< 1 mG\$), whereas using some applications which exploit the phone magnetometer, I get a very high value, something as \$35 \mu T\$? I get also a significant difference when I switch airplane mode off.
The Earth's magnetic field is about 30uG at the equator and 60 uG at the poles.
A small solar flare can cause 200uG.
For RF measurements you must also consider the power and Bandwidth of your magnetic fields to make reasoned comparisons.
1 T = 10,000 G
So 1 mG= 0.1 uT