[kurthr]

This is because capacitive touchscreens are inherently a near-field phenomena. They are designed to detect extremely small capacitances (<fF) at relatively low frequencies (<500kHz) relatively low voltage (<5V) at high report rates (>120Hz) in the presence of very significant display noise (5-10V >1nF coupling). They require >120dB of OutOfBand rejection to operate. Cost per transceiver is ~$0.01.

They fundamentally measure the channel between arrays of neighboring electrodes. That makes them self shielding (like a Faraday shield) since they typically are designed to be relatively immune to uniform changes in coupling. From any distance comparable to the size of the touch screen the differential coupling to electrodes falls off exponentially. Near the screen they fall off 1/d then 1/d^n dominates farther out.

In order to avoid interfering with other parts of the device, they tend to be highly encoded narrowband signals and change (to known good) frequencies when they detect interference. That makes most narrow band techniques less effective. High voltage impulsive noise is most likely to have an effect, but triggering ESD detection/rejection might create a sweet spot in amplitude.