Without saying anything, I went over to the
stairway switch and turned it on and off a few times. "On" caused the
interference. When we swapped the bulb with one in a floor lamp, the
floor lamp caused the same interference. It has been one of my enduring
regrets that I didn't ask to keep the bulb. It was probably thrown out.
================
That bulb was probably an old coiled 'Edison' type . They were /are known to
form a tuned circuit resonating in the old TV band 1 ( about 48 - 62 MHz)
acting as a TX when power is applied.

Frank GM0CSZ / KN6WH