The first Directional broadcast Antenna was designed by Ray Wilmotte. This was for 620 WFLA (dayshare with WSUN) in Tampa back in 1932. It was the right solution for a specific problem. By 1947 there were 646 AM radio stations on air in America, and 39 of them were using directional antennas.
Interestingly enough the whole ordeal had to go to court. When WFLA powered up 620 WTMJ lost coverage. On appeal it was ruled that the earlier coverage area of 620 WTMJ Milwaukee should be restored. Thus the system was installed to avoid a power reduction. Coverage map Here. [WFLA moved to 940 AM in 1941 leaving 620 to WSUN which in turn changed called to WDAE.] The purpose of the WFLA antenna was two fold:
1. Protect the contour of Milwaukee station, WTMJ.
2. Reduce wasted power sent out over the Atlantic ocean. Then redirect it over land.
Its success began a series of conversions. In 1934, 700 WLW-AM, Cincinnati used a directional antenna to protect CFRB Toronto, Ontario, about 375 miles away. The FCC restricted their power output to 50 kW toward the North East.
Other early directional stations include, WOR, WADO and WEAF. All were experimenting with Bell Labs on this approach. None of them have any indications of experiments prior to 1933.
Ray Wilmotte was actually born in Paris and educated at Cambridge University in England. He moved to the U.S. to work to work on blind landing equipment for aircraft. During the second World War he worked on direction finding systems for airports and radar. Later at RCA on the development of communications satellites. In the 1970s he became a full time FCC consultant. There at the age of 78 he directed a UHF task force that developed a high-performance tuner that permitted greater use of the UHF spectrum. He died January 27th 2000 at the age of 98.