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Numerous iconic British artefacts emerged from
the Second World War but nothing says US Military louder and clearer than the
Willys GPW and the BC-611 (aka SCR-536). You’ve probably worked out the Willys bit. The clue
is in the ‘GPW’ designation, which stands for General Purpose Willys. The story
goes – and it is one of several -- that US troops turned GP into ‘Jeep’, a
tradition that allegedly lives on with the HMMWV (High Mobility Multi-purpose
Wheeled Vehicle) or ‘Humvee’. 
The story begins in 1940, when Motorola
started development work on a lightweight two-radio. It was originally intended for
civilian use but it came to the attention of the US Army Signal Corps (hence
the alternative SCR or Signal Corps Radio designation). It was an immediate
success, replacing heavy and bulky radio backpacks. More than a quarter of a
million BC-11s were manufactured throughout the war, making it the world’s
first mass-produced, self-contained, battery-powered, hand-portable
transceiver. 
The BC-611 was designed to be used by
ordinary GIs so it had to be tough, really tough, very easy to use and fix, when it went
wrong. It ticks all of those boxes, and a few more besides, but the one thing
it couldn’t do – contrary to many Hollywood movies -- is communicate over
distances of more than a few hundred metres. That is about as far as you can
shout, unless it is used in flat, open terrain, in which case the range can go
up to around 1.5km. That wasn’t necessarily a problem for advancing ground
troops. Shouting orders whilst under fire or when trying to stay hidden wasn’t
a good idea and soldiers in those situations usually only needed to be able to
talk to their comrades and commanders who were hopefully close by. That was
precisely the sort of situations they were used in, on the Normandy beaches,
the battlefields of Europe and North Africa and numerous other war zones where
BC-611s made vital contributions to military communications and doubtless saved a great
many lives. 
Its durability is mainly down to the heavy-duty
alloy casing. It’s waterproof; with rubber seals protecting all possible entry
points and that includes the telescopic antenna, which normally lives under the
stubby removable cover on the top of the case. This is actually one of the
BC-611’s cleverest design features. The antenna cover is tethered to the case
by a short chain. To stop it rattling against the case, and possibly give away
your position, there’s a threaded mounting stud, which keeps it out of the way.
Fully extending the antenna switches the unit on and puts it into the receive
mode. Retracting the antenna is supposed to switch the unit off, but to stop it
being accidentally left switched on and draining the batteries, replacing the
antenna cap ensures that the aerial collapses fully and returns the power
switch to the off position. There is only one other control and that’s the
rubber covered press-to-talk (PTT) switch on the right side of the case. As you
can see the earphone and microphone are built into the case and arranged like
telephone handset. There’s also an adjustable canvas webbing wrist/carry strap
on the back of the case. One other nerdy tidbit; the BC-611 was designed and
balanced for left-handed use on the premise that most soldiers were
right-handed and needed that hand free to operate their weapons.
Inside all of the electronics are mounted on a
single, easily removable one-piece chassis module. In fact it can be taken out
in just a few seconds by slackening off a single screw on the top of the case.
The chassis then slides out, through the open battery compartment on the base. (Normally
it is protected by a waterproof hinged cover). This ingenious quick release
feature serves two purposes. Firstly it makes it easy to change radio channels
– more on that in a moment – and second, it greatly simplifies maintenance and
repair in the field. 
Since this is a 1940’s vintage technology it
uses valves (tubes). There are five of them, all miniature types that are held
securely in place by spring-loaded clips. As you doubtless know, unlike
transistors – which didn’t appear in a useable form until the early 1950s --
valves operate on high voltages; which brings us to the batteries. The BC-611
uses two custom battery packs. The first, a BA-37 delivers a low voltage (low
tension or LT) 1.5 volts supply for the valve’s heater filaments. The second is
a BA-38, which is the high voltage (high tension or HT) pack providing just
over 100 volts. Under normal conditions – whatever they might be – a fresh set
of batteries was expected to last for around 15 hours. By the way, the original
battery packs are no longer available but it is a relatively simple matter to
put together home made battery packs using two D cells wired in parallel, for the 1.5 volt LT
supply, and a stack of eleven 9 volt PP3 batteries for the HT pack. More recently
packs have been developed using inexpensive step-up or ‘buck’ converter modules
to generate high voltages from a couple of 3 volt lithium rechargeable
batteries. 
The BC-611 operates on the Short Wave band, on
a set of 12 channels or frequencies between 3.5 and 6MHz. It is single channel
only operation and channels are set by plugging in a quartz crystal and a
matching coil module into sockets on the chassis. There’s a small holder just
below the PTT switch where the user is supposed to insert a small card showing
the frequency being used. The RF output, and the reason for the limited range,
is just 360mW. To put that into some sort of modern context, Citizens’ Band
walkie-talkies, popular in the 80s and 90s, which also use the Short Wave band,
operate at power levels of up to 4 watts, giving a range of several miles in
good conditions.