Radio prepares for war, part 2
Last month, in part one of Radio Prepares for War we looked at the National HRO receiver, which set the bar for the standards required by military radio communications. The HRO was a superb receiver for land or ship based use, but it was too large and heavy for use in aircraft. It also would have been difficult to redesign it for use on 28 volt aircraft power systems. The HRO also was relatively difficult to operate in a fast paced combat environment. It required changing coil packs to change from one group of frequencies to another. On a ship or land station, there was ample room for several HROs, each dedicated to a different frequency and function, and ample room to store the coil packs.
In the heat of an air battle, when seconds can make a difference, a radio operator needs to be able to change frequencies instantly. He or she needs to be able to see what frequency they are on at a glance without the need to interpret charts and graphs. I say “he or she” not just to be politically correct. Although women did not see active combat in WWII, hundreds of B-17s and other aircraft were flown from the U.S. to combat staging areas in Europe and the Far East by women’s air corps.
The B-17 was the work horse of high altitude heavy bombers when the United States entered World War II. The BC-348 was the main receiver on the B-17. It was paired with the BC-375 transmitter to provide two way communications. The BC-348 was an upgraded version of the BC-224, originally designed in 1936. The earlier version was designed to work on 12 to 14 volts that was used by smaller multi engine transports and bombers, and continued to see use as a vehicular radio in mobile command centers. The newer BC-348 took advantage of more advanced octal based tubes that not only offered better performance but were more compact and weighed less. The radio was also designed to operate on the B-17’s 28 volt power source.
The picture shows the advancement in the miniaturization of tubes between the early 1930s, and the end of World War II. At the beginning of the war, most equipment used octal based tubes (meaning that they had 8 connection pins). These tubes were about half the size of those from the early 1930s. Further miniaturization would be required though. Even smaller radios were needed to fit in the cockpits of fighter planes. The miniature tube shown above, met that requirement.
To make the radio easy to tune and more selective, it’s tuning range of 200 kHz to 18 MHz was broken down into 6 bands. It did not include the broadcast band which was unusual for that period of aircraft receivers, as broadcast stations were often used homing beacons for navigation when other methods failed. Instead, the B-17 had a separate system known as a “radio compass” for this purpose. I would be amiss if I didn’t mention that Japanese pilots effectively used Honolulu radio stations to guide them in for the Pearl Harbor attack.
To avoid confusion, the band selector switch also rotated a mask that covered the dial of all but the selected band. This idea was not new, a few consumer radios such as the Zenith "shutter dial" series and the GE model A-64 also used mechanical methods to display only the selected band prior to the war. Like the National HRO, the BC-348 included a crystal filter for improved selectivity, an automatic volume control circuit, and a beat frequency oscillator that was used to control the audio pitch of a Morse code signal. For operation in the darkened radio room of a night flying bomber, the radio included an illuminated dial with a dimmer to allow the operator to adjust brightness as desired.
The earliest manuals for the BC-224/348 indicate that it was an RCA design and used heterodyne topology (note 1). After the start of the war, exact, interchangeable replicas of the radio were also produced by Belmont Radio, Wells-Gardner and the Stromberg Carlson companies. Even the Russians produced their own version of the receiver, known as the CY-9, a near exact copy of radios believed to have been recovered from downed allied aircraft.
Although not as feature packed or as sensitive as the National HRO, the radio was an excellent performer. It was used by other aircraft including the B-24, the B-29, and the Canadian version of the Lancaster Bomber. The radio was still in use on some active older military aircraft into the 1970s. Over 100,000 of them were produced. (note 2) Like the HRO, they were highly prized by amateur radio operators. Although not available to the public prior to the war, and even its service manual considered classified, after the war great numbers of them were sold off as surplus for as little as $17 each. Still considered desirable by both amateur operators and military enthusiasts, you will probably pay a bit more if you want to buy a working or restorable one today. Typically, a good one will sell for about $225 on E-Bay. Apart from their hot filaments, tubes typically require higher voltages in the range of 200 to 350 volts to operate. To obtain this from the aircraft 28 volt power source, the radio included a device called a "dynamotor," which is a miniature motor/generator used to step up the 28 volts to 225 volts. On the bench, the device produces a high pitched whine, but with the covers on and installed in the radio, it can barely be heard.
After the war many electronic and hobby stores sold AC power supplies that took the place of the dynamotor and allowed the BC-348 to be operated from household power. My BC-348N has a conversion kit (most likely a Lafayette Radio Electronics version) but is otherwise all original. It is completely operative and still performs to original specifications. (note 3)
Pictured below is a BC-603 receiver that was used in the WWII Sherman tank. It also saw duty on Jeeps and in mobile command centers. It tunes from 20 to 30 MHz and has push buttons that can be mechanically preset to frequencies, like on older analog car radios. The unique thing about this radio is that it and its matching transmitter used Frequency Modulation (FM) for voice communication. FM, like early television also saw some broadcast activity prior to the war, but neither was fully implemented until after the war ended. I’ll spend more time on early FM in the future.
The system would have had a range of ten to thirty miles on the ground (depending on flatness of terrain) and was primarily used for local communication between vehicles or between vehicles and a nearby command center. Like the B-17 receiver, both the tank receiver and its transmitter used dynamotors to step up the vehicle’s power to the high voltage required by the tubes
If you were to look inside the BC-603, you would see the neat and compact layout of a military design from that time period. The radio also used metal jacketed octal base tubes which were more rugged than their glass equivalents. Like the BC-348, an aftermarket transformer was installed and allows the radio to operate from house current. With both radios, the power supplies simply unplug, allowing the radios to be returned to original condition if desired, powered by dynamotors. The radio is in working condition with no restoration required. I occasionally use it to receive 10 meter band FM ham radio transmissions.
In addition to the use of frequency modulation, other notable WWII advancements included increases in usable frequencies. At the start of the war, 30 to 50 MHz was considered the high end of usable frequencies. By the end of the war, radio was using frequencies up to 450 MHz, and radar was using frequencies of 1 GHz (1000 MHz) or higher. Early forms of digital communication such as teletype and facsimile (fax) transmissions were common. Next month I’ll take a look at Atwater Kent, the man, his radios, and the empire that he built.
Note2: Of the 100,000 produced, at least 17,000 flew on B-17s, 18,500 flew on the B-24, 3900 flew on the B-29 and about 400 flew on the Canadian version of the Lancaster Bomber. Others were used on reconnaissance aircraft.
Note 3: Western Historic Radio Museum's web article on restoring a BC- 348.