THE STORY OF CONELRAD
by Dan Roach
This
month's article is concerned with Conelrad ("Control of Electromagnetic
Radiation"), a broadcast system that was put in place at the height of the
cold war to protect the
In
the aftermath of the Pearl Harbor attack, the oft-told tale of Japanese zeros
homing in on Oahu by using radio direction-finding (RDF) on Hawaiian broadcast
signals must have preyed on defense planners' minds. That's the only excuse I can conjure up for
what ensued with the ill-fated Conelrad project. In today's world, where a couple of hundred
bucks will buy you a handheld GPS receiver that can locate your position in
three dimensions almost anywhere on the planet to an accuracy of a few yards,
it's hard to believe that RDF could ever have been such a threat. Before
The
problem was that broadcast signals were essential to inform the public of
impending air attack. So some
enterprising types tried to figure out how to keep broadcasters on the air, but
make their signals untraceable. Conelrad
was born.
In
the event that enemy bombers were approaching, regular broadcasters would
direct the public to a local emergency frequency, then
most would sign off. There were
originally two, then a third emergency frequency was
added to the AM band. Older radios show
the triangular civil defense logo on their tuners at these locations. Several transmitter sites in a given area
would switch to the same frequency and would transmit simultaneously, carrying
the same emergency programming information.
Although there would be tremendous co-channel interference between the
various transmitters, their signals were judged to be "intelligible"
most of the time. The sound would be
unpleasant, but the essential message would get through. And RDF efforts would be stymied by the beats
between the various transmitters. The
free world could be saved for future generations!
Except
that it didn't work. Fine in theory, it
fell down in actual practice. Field
trials were attempted in the
Time to rethink the project. It was then decided that some of the
co-frequency stations would transmit intermittently, on for four minutes, off
for two, on for five, off for 2.5, etc.
This was tried, unsuccessfully: the station's on and off cycles became
predictable, and accurate time at each location to
coordinate the overall effort properly was a problem. So remote control circuits were installed,
with a central control point turning the transmitters on and off in a
pseudo-random manner. Transmitter plants
needed to be modified extensively, to operate on emergency frequencies, even at
reduced power. Transmitter technicians needed to be on-hand at the sites to do
the retuning and adjusting required during the tests or the emergencies.
Luckily,
the system was never actually used, because there was essentially zero chance
that it would ever have worked as hoped.
It was later dismantled in favor of the EBS system, which recently was
replaced in the
Today,
the only vestiges of the once-ubiquitous Conelrad program are those triangles
on the tuners of old radios, and the Conelrad switch on old RCA transmitters,
that switched in that third, odd oscillator (that no-one in this country ever did
have a crystal for!).