At night, solar radiation ceases and the free electrons recombine with their host molecules. The D-layer completely disappears and offers no signal loss. The E/F layers merge into a single layer, but remain reflective to HF signals.
However, this combined layer has a lower electron density than daytime levels, lowering the MUF. Astronomers call these ionization layers plasma layers and the lowest frequency that escapes into space the plasma frequency, fp. QRPers look at it just opposite – what is the highest frequency that does not escape into space?
We call this the maximum usable frequency or MUF. In reality, the MUF and plasma frequency are exactly the same.
Built by AT&T in Houlton Maine, this array of 4 phased beverage antennas was used for the first transatlantic phone system which began operation on the 7th of January 1927… The first conversation took place betweenWalter S Gifford, the president of AT&T, in NY, and Sir Evelyn Murray, the Secretary of the General Post Office, which managed the British telephone system at the time.
History Channel… The First Transatlantic Conversation (1 minute commercial before it starts)
Wikipedia … Good source for Beverage antenna history and more….
EDN Network … Good article with links on the first transatlantic phone call
Harold H Beverage invented the Beverage antenna while working for General Electric Corporation on transoceanic receiving antennas and in April of 1920 got the patent for the Beverage Antenna…. Original Patent drawing
Small section on Harold H Beverage at Wikipedia