Today some of these letter beacons are grouped in with those spooky shortwave number stations. But it's inaccurate. These have a well known origin in the early development of Air navigation technology of the 1930s. The frequency they occupy is not relevant, it does not imbue them with any mysterious purpose. They are for air navigation, and have been for almost a century now. More here. Abbeville is about eighty miles from Paris. It's population today is only about 24,000. So you can imagine it wasn't much to look at then either. But try to understand that the first transcontinental air mail flight was only 10 years earlier. That year, 1921 the U.S. Post Office used bonfires to light the runway. A lot changed in 10 years.
This beacon in Abbeville was designed to aid travel from Dover to Paris. It consisted of two antennas a short distance apart. One of these sent out a constantly repeated "F" in Morse code" The other sent out a constantly repeated"L" in Morse code on the same frequency. These are similar letters but that is to it's design. A letter F in Morse code is dot dot dash dot, and L is dot dash dot dot. On pilot was quoted as saying:
"It eliminates both instrumental and human error. It is almost like following an invisible road or railway, and removes most of the uncertainty of fog flying. Where before, in thick weather, we were continually asking the control tower for positions, we now need do so only once in a complete Journey. Although both outgoing and Incoming planes use the same road, there is no danger of collision, as they fly at different heights... We fly now in weather which would formerly have held us up. The sole disadvantage is the monotony of listening to the signal."If the pilot hears both on the same frequency, the signals intersect to form a continuous straight line between Dover and Paris. As long as an airplane is flying along this line the pilot hears in his earphones a continuous buzzing, caused by the co mingling of the two lines of code. If he deviates form his course he will hear more clearly an L or F. At the time they were already testing a "lamp" that would flash the signal so spare pilots from the buzzing sound in their headsets, and enable them to hear other information.