Radio silence

Per the manual, the protocol is: The Wilderness protocol (see page 101, August 1995 QST) calls for hams in the wilderness to announce their presence on, and to monitor, the national calling frequencies for five minutes beginning at the top of the hour, every three hours from 7 AM to 7 PM while in the back country.

Radio silence can be used in nautical and aeronautical communications to allow faint distress calls to be heard (see Mayday).

In extreme scenarios Electronic Silence ('Emissions Control' or EMCON) may also be put into place as a defence against interception.

Electronic emissions can be used to plot a line of bearing to an intercepted signal, and if more than one receiver detects it, triangulation can estimate its location.

Radio direction finding (RDF) was critically important during the Battle of Britain and reached a high state of maturity in early 1943 with the aid of United States institutions aiding British Research and Development under the pressures of the continuing Battle of the Atlantic during World War II when locating U-boats.

One key breakthrough was marrying MIT/Raytheon developed CRT technology with pairs of RDF antennas giving a differentially derived instant bearing useful in tactical situations, enabling escorts to run down the bearing to an intercept.

The U-boat command of Wolfpacks required a minimum once daily communications check-in, allowing new Hunter-Killer groups to localize U-boats tactically from April on, leading to dramatic swings in the fortunes of war in the battles between March, when the U-boats sank over 300 allied ships and "Black May" when the allies sank at least 44 U-boats—each without orders to exercise EMCON/radio silence.