I have a radio that shows the relative strength of available AM stations. When I tune to a frequency that contains at least two signals, the dial moves either slowly or a bit more rapidly from right to left. In late fall and winter in the morning or late afternoon for example, WWKB-AM 1520 and WIZZ-AM 1520 (which oddly does NOT offer streaming audio; I wonder if WHIZ radio in Zanesville, OH does) will battle each other; also 1540 in Exeter, NH and Albany, NY; 1590 in Nashua, NH and Warwick, RI, etc. It was a long time ago, but the prize instance of this occurrence happened when the station then known as WITS-AM 1510 was installing its Waverley Oaks Road facility in Waltham, while still broadcasting from North Quincy with 50,000 watts days. I assume the testing may have been done at night, but one day I heard them running both transmitters at the same time during daylight hours! My signal strength meter was swinging from one direction to the other like Willard Mitt Romney changing positions. Anyway, lst night (Monday, 11/28) I tuned it to 1090 and WBAL was, as suspected dominant. But I did notice a slight oscillation, regular in nature, but no interfering audio, as if some OTHER station not far away but weaker was broadcasting.