Shortwave radio diversity reception provides a way to combine several fluctuating signals and get a solid result. It provided the foundation for most radio news received in America for years.
During World War II, most countries around the world relied on Britain’s shortwave radio broadcasts for the latest news from Europe. In the days before transatlantic audio cables or satellites, distant news traveled fastest by radio. Networks in the America’s, Australia, New Zealand and elsewhere re-broadcast shortwave radio news domestically.
Getting reliable, good quality audio programs over shortwave is always a challenge because of fading. As signals bounce off the ionosphere, they split over multiple paths. Often they fade and flutter, sometimes significantly, as the nature of the layers change with time. Here are several examples of shortwave signals fading, so you know what it sounds like. Skywave radio signals are subject to complex patterns of travel and interference.
Eventually, domestic networks found a clever way to get better audio from these distant signals. Diversity reception works like this. Instead of one signal, you monitor several signals at once and blend them together. Harold Beverage and RCA pioneered work on shortwave radio diversity reception in 1920’s. Commercial solutions arrived by 1933. Typically, you would use three receivers with three different antennas, spaced 1,000 feet apart. When antennas are widely spaced, signals arrive with different fading. Just combine the signals and let the strongest signal dominate. As long as the fading is not correlated across all three antennas, improvement can be significant.
Diversity reception can be achieved in several ways. The most popular – spatial diversity – is described above. Other methods include frequency diversity – mixing together the same program received on several different channels. The picture above shows racks of RCA AR-88 receivers used for triple diversity reception during World War II. The automatic gain control circuits in this apparatus was specially designed for the sets to work together and track the best signal.
Engineers still use diversity reception for many types of radio signals across all frequencies. For example, wireless microphone set-ups in auditoriums always use multiple receivers with outputs mixed together. Here is a more detailed technical explanation.
Shortwave Radio Diversity Reception – Canada’s Britannia Park Listening Post
In Canada, CBC engineers built a listening post just west of Ottawa in 1933. Britannia Park served the dual purpose of shortwave monitoring and providing the ears for foreign broadcasts on the national network. Receivers were connected to giant rhombic antennas mounted on 90 foot poles. These diamond shape antennas were aimed at Europe and covered 5-25 MHz. Operators used two sets of triple diversity receivers.
During the war, Britannia Park relied mostly on frequency diversity. Audio was sent to CBO in Ottawa, and from there to CBC’s three networks: Eastern, Ontario and Western. In addition to news and leadership speeches, the BBC also relayed the voices of Canadian soldiers serving overseas.
Finally, to complement live BBC relays, this listening post also supported recording of foreign programs from many sources for archiving or later broadcast. Similar listening posts existed across the Commonwealth including Makara Receiving Station near Wellington, New Zealand.