Electromagnetic interference may be a difficult problem in debugging. If you need to prove what causes your WiFi to slow down or your digital TV signal to decline, the ability to measure electromagnetic field (EMF) will be a great help. Professional equipment is usually very expensive, but making an EMF detector yourself is not even that difficult: just look at the handy handheld electromagnetic field detector of expert Arduino [Mirko pavleski].
The basic idea is very simple: connect the antenna directly to the analog input of Arduino and visualize the measured signal. Because the input of ADC is high impedance, it is very sensitive to any stray current received by the antenna. In fact, it is so sensitive that a ground resistance of several megaohms is required to prevent the sensor from triggering under any random noise. [Mirko] the resistance is adjustable through several knobs and switches, so the detector can be used in quiet and noisy environments.
In electromagnetic engineering, making the whole equipment work reliably is an interesting work: in the first few iterations, the detector will trigger its own LED and buzzer and trap itself in an endless loop. [Mirko] solved this problem by encapsulating Arduino in a closed grounded metal box, exposing only the required wires. The design of the antenna is mainly based on repeated tests; At present, the setting using 7 cm x 3 cm aluminum plate seems to work well.
Although this is not a calibrated professional instrument, it should be easy to find the interference source, or even simply find the hidden power cable. You can think of it as a more advanced version of [Mirko]’s dustbin EMF detector; If you have a second Arduino, you can use it to generate interference.