Drainage chambers and sealed cisterns

There are seven drainage chambers and two sealed cisterns scattered around the Capital Wasteland. These hidden locations house ham radios that broadcast a repeated signal once the broadcast has been turned on. All of the signals are turned on by flipping the electrical switch on a transformer at the base of the radio tower that starts the encoded broadcast, except Kilo Bravo where the switch is inside the broadcast station.

The entry to the underground ham radio station will be nearby (check your local map or the links below if having trouble). You can also zero in on the broadcast. It actually comes from the sewer grate or manhole entrance, not the tower itself. The signals have a short broadcast distance of roughly 200 yards (183 meters), slowly degenerating into static. At their source, there is no static and the signal can be heard clearly. Turning off the ham radio stops the transmission. As you might expect, this happens when turning the radio's dial ends in a click. It can be turned back on, but you have to select it in your Pip-Boy again to hear it.

Locations
Many of the stations do not have a map marker, but can be spotted easily from far away by looking for the tower.
 * WKML broadcast station: Signal Kilo Bravo (KB) - In the northwest of the Fallout 3 map, a bit southeast of signal Alfa Lima and far south of Fort Constantine.
 * Relay tower KX-B8-11: Signal Oscar Zulu (OZ) - In the northeast of the map, a bit southeast of the tower and northeast of the Temple of the Union. Bob Andersteen broadcasts a plea for medical help for his son.
 * Broadcast tower KB5: Signal Alfa Lima (AL) - Northwest of the map, just west of the Deathclaw Sanctuary.
 * Broadcast tower LP8: Signal Echo Foxtrot (EF) - Near the north edge of the map, south of Oasis.
 * Radio mast Oscar Tango: Signal Oscar Tango (OT) - In the south of the Jury Street township.
 * Broadcast tower KT8: Signal Sierra Romeo (SR) - On the west edge of the map, due north of Little Lamplight and northeast of Vault 87.
 * Radio mast Yankee Bravo: Signal Yankee Bravo (YB) - South of Canterbury Commons and camp RHO. Feral ghouls are broadcasting their heavy breathing.
 * Broadcast tower SV: Signal Sierra Victor (SV) - Southeast of Jocko's Pop & Gas Stop.
 * Broadcast tower PN: Signal Papa November (PN) - West of the charnel house, southwest of Fort Bannister and northwest of Evergreen Mills.

The reasons for these signals are unknown. The variation in messages suggests that they did not all exist to a common purpose, however. Their usage of the NATO phonetic alphabet is common of both military operations and HAM radio operators, as well as anyone with training in radio communications protocol. They could have been groups sent into the wasteland for reconnaissance, search and rescue, raids, search and destroy, pre-War distress signals Chinese espionage on loop. However, there is no information to support any specific theory, aside from the content of the radio messages themselves and the equipment found along with the remains of the radio operators.

Morse code
These are minor locations, and all are home to radio stations broadcasting Morse code (except Oscar Zulu and Yankee Bravo).

These signals broadcast the Morse code "CQ CQ CQ DE /  K" (Morse code: -.-. --.- -.-. --.- -.-. --.- -.. . /   -.-) as explained by Mark Lampert, the sound designer at Bethesda Softworks.

Parsing the message, we get the following:

CQ -- calling any station ("seek you")

DE -- 'from'. The reason this Spanish/French form of the word is used is simply to shorten it as it's a common word to use. Compare sending DE -.. . to sending FROM ..-. .-. --- -- Consequently, operating in CW means heavy use of abbreviations, dropping unnecessary letters, and other handy ways of compressing your message length by use of what are called 'prosigns' and 'Q-codes'.

repeated three times -- that way if you miss it the first time or copy a character incorrectly, there's a chance for error correction

K -- could mean 'back to you', or 'open invitation to transmit' or simply 'over'. The meaning is more or less the same in this case.

Last, but not least, there's a pause as the station's operator momentarily listens for any replies. Hearing no replies, he/she resumes 'calling CQ'. Of course in this case our operator is long expired and we presume that the radio is simply transmitting a message from memory, operating for as long as it's still supplied with power.

Appearances
Drainage chambers and sealed cisterns appear only in Fallout 3.