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WSHU Chief Engineer Paul Litwinovich explores aspects of vintage radio, from the radio sets themselves to the people and technology that made it all possible.

Radio Prepares for War, Part 1

P. Litwinovich collection

As the roaring twenties came to a close, radio technology would continue to evolve with significant improvements to consumer sets, particularly in the area of shortwave reception. Read my past articles for more details as to how this transpired. The price of radios would continue to fall as availability continued to increase. Herbert Hoover could have added "a radio in every home" to his famous "a chicken in every pot and a car in every garage" campaign slogan. This radio boom would continue right up until December of 1941, when the first bombs were dropped on Pearl Harbor. After that, new radios would be a scarcity as almost all production and materials were diverted to the war effort. 

One can make the philosophical argument that war brings very little benefit to mankind. Technology may be one exception.  The following is but a small part of that story.

Let's go back for a moment to 1935. With growing political unrest, the U.S. Navy decided that it should have amongst other things, a state of the art receiver for shipboard and land based use. The receiver would have to be able to tune all of the frequencies usable by ships at sea for long distance communication. At the time, this was considered to be from 50 kHz to about 30 MHz.  It would need to be selective, sensitive and rugged. Enter the National Company, Inc. Read details about the history of the National Company in my March 2014 article, "Regeneration under Glass."

By 1935, The National Company was well established as a producer of technically advanced commercial radios and components. (1) They employed, or were associated with, pioneering engineers such as Fred Drake and Glenn Browning. (2) The company had produced a line of consumer radios in the 1920s, including the one featured in my March 2014 article, but with the advent of the Great Depression, they dropped the consumer line in favor of the niche market of commercial, military, and high-end amateur radio receivers.  All of the consumer radios used TRF or regenerative topologies (Note 1).

Another prominent engineer, James Millen began work as National's Chief Engineer in 1924. Millen had already established a name for himself writing magazine articles about radio and other technical subjects as far back as 1916 as a way to pay for his education at the Stevens Institute of Technology in Hoboken, N.J.  (1a)

In 1932, General Electric was contracted by the Civil Aeronautics Authority (predecessor to the FAA) to design and build a system using short wave radio to communicate with aircraft as the need arose from the growing airlines industry. GE built the transmitters and asked the National Company to design and build the receivers. Millen chose the heterodyne circuit (Note 2) to do the job. The result was the National AGS (Aeronautical Ground Station) receiver. It was the first commercial grade, high-performance short wave receiver to be mass produced. Most were bought by the government, but a few were sold to amateur radio operators with deep pockets.

In 1934, Millen envisioned producing a high end receiver targeted at radio amateurs. His goal was to bring it to market by the holiday season in December of that year. The target was missed due to production problems and material shortages. By 1935, political unrest in Europe and the Pacific Rim made the need for advanced radio technology a high priority for the military. The U.S. Navy sought bids with less than three months notice for an advanced high performance receiver. The National Company's high-tech ham receiver would meet the requirements with some modifications. None the less, completing a prototype based on a radio not yet in production, in such short time, would be a challenge. The engineers nicknamed it the "HRO," which stood for "Hell of a Rush Order." The name stuck.  The National HRO receiver met the deadline, and outperformed the competition.

With still a few years before the outbreak of war, the radio was not considered a military secret, and the company produced a non-military version for the consumer market, primarily the amateur radio community. They were expensive. A stripped down version of the radio, without tubes, speaker, or power supply retailed for $175. Add those other parts and the price tag could exceed $250, which was a lot of money when the average Joe, if he had work at all, was earning $25 - $50 per week.  Gasoline was under $.15 per gallon at the time. The basic ham radio version, with a few coils and power supply, typically sold for $229 and the commercial version for about $320. Both the commercial and military versions were rack mountable, the military version having its components coated with a liberal application of fungus resistant varnish. 

By the start of the war, the National Company employed a little more than 200 people. After the Pearl Harbor attack, the defense department told the company to "start building HROs and we will tell you when to stop." Within months, the company grew to more than 2,500 employees.  The National HRO, in one form or another, remained in production well into the 1970s, finally ending its run with a solid state version.  (3) Most importantly, the HRO set the stage for what would turn out to be the largest and fastest growth of technology since the start of the industrial age. The pace of technological advancement that would occur in World War II would be unsurpassed until the invention of the personal computer in the 1970s.

Credit P. Litwinovich collection
HRO rack installation.

Pictured above is a National HRO, a nine tube TRF/heterodyne hybrid. It is displayed here in a military/ commercial configuration, mounted in a rack with its power supply, speaker, and coil storage box.  Compared to the consumer radios that I have featured in the past, it would not win any beauty contests. The HRO none the less earned its place in history. It was an extreme high performance receiver that pushed 1935 technology to the limit and beyond. It could tune from 50 kHz (far below the broadcast band) up to 30 MHz, which was considered the practical limit for short wave communication at the time.  This particular set, the HRO-M, the M meaning that it is the military version is from the first year's production and it used tubes with 2 volt filaments. Within a year or two, the model was upgraded to use tubes with 6.3 volt filaments, which would remain a standard in tube design until tubes went out of favor in the early 1960s.

Bear with me if I get a bit techie for just a moment. The HRO featured three tuned radio frequency stages in front of a heterodyne stage, followed by three intermediate frequency filters/amplifiers.  A typical consumer radio would have one of each of these stages. Included were three new innovations, a tunable crystal filter that allowed the user to adjust the selectivity, a tunable notch filter to allow the user to "tune out" an unwanted adjacent signal, and an automatic volume control (AVC) circuit to keep the loudspeaker volume constant even when the received signal faded in and out.  The radio also included a beat frequency oscillator (BFO) which allowed the operator to set the tone of Morse code signals.

Credit P. Litwinovich collection.
Inside the National HRO receiver.

Now back to human speak. At a time when shortwave reception of foreign broadcasts was just beginning to catch on in consumer radios, the HRO took the technology beyond even the hard core enthusiast's wildest dreams. There was nothing available on the consumer market that could come close to what this radio could do.

Although produced for the military and commercial market, they quickly became the ultimate status symbol in the amateur radio community.

One of the identifying features of the HRO is the tuning dial and the way that the radio was tuned. The dial features five windows with numbers that change from 0-500 as the dial is rotated. Only the top number is actually read and each graduation between the numbers has a value of 1. Therefore, if you were three lines past the number 260, the value would be 263. In the picture shown, the dial is just past 260, probably at about 260.4. But what did this number mean? It would be difficult to tune any analog radio from 50 kHz to 30 MHz all on one scale. The resolution would be like trying to single out a particular blade of grass in the middle of your lawn, from 50 feet away. There needed to be a way to zoom in. The HRO accomplished this by featuring sets of plug-in tuning coils, each covering a small portion of the band, such as 900 to 2100 kHz. The coil set is the small drawer just below the tuning dial. Each coil set has a graph on it for converting the generic 0-500 scale to the actual frequency.

Credit P. Litwinovich collection
A National HRO plug-in tuning coil.

A minimum of nine coil sets were required to receive all of the frequencies between 50 kHz and 30 MHz. Normally, the radio was ordered with just the coils that the user would require, such as those used for aircraft or ship communication by the military, or commercial frequencies such as a news agency would use for receiving news stories from their overseas bureaus. Four coil sets were designed to cover the popular amateur bands.

Look for the HRO when watching World War II documentaries, and movies where the producer pays attention to authentic details.  You will occasionally see an HRO installation. Ships and shore stations were not the only branch of the military to use the HRO.  The war department set up listening stations often equipped with twenty or more HRO receivers. They were staffed round the clock with operators scanning the airwaves for enemy communications.  The HRO was used at the top secret British Bletchley Park project, where the allies intercepted German communications and broke the Enigma code. (4)

Credit P. Litwinovich collection

I am frequently asked the question, "How does a 1935 vintage HRO stack up against a modern shortwave receiver?"  The answer is a matter of opinion. It will clearly outperform most modern consumer analog radios hands down.  Digital signal processing, on the other hand, and the ability to squeeze large portions radio spectrum on one receiver without the need to change coils or interpret charts give the modern receiver a great advantage.  Still, the HRO and its later models remain highly prized amongst amateur radio enthusiasts and collectors.

Credit P. Litwinovich collection
A built in compartment stored up to five tuning coils.

The HRO was developed with ship and land based stations in mind. Although they were excellent radios for that purpose, they would prove to be too large and heavy for aircraft, and from the start, maintaining air superiority would be the deciding factor between victory and defeat. Next month, in part two of Radio Prepares for War, I'll feature a classic radio that flew on the B-17 bomber, and, if room permits, squeeze in a radio used in the Sherman tank.

Collector's Tip: Finding a complete vintage HRO with all accessories and coils is extremely rare, and quite expensive. It is best to assemble the set by buying the radio itself in good condition, and then acquire the various coils sets and accessories when they present themselves at auctions and flea markets. Within the amateur radio community, the HRO remains the status symbol that it was back when it was new.

Note 1:  Read about the Tuned Radio Frequency (TRF) circuit and the Regenerative circuit and the roles they played in the advancement of radio in several of my past articles.

Note 2:  Read about the development of the heterodyne receiver circuit in my articles, "Radio for the Great Depression," parts 1 and 2


(1) "A brief History of the National Company," by John J. Nagle, K4KJ, first published in the Antique Wireless Association Review, Vol. 1.  Read more at Wikipedia.

(1a) James Millen Society web site. More on Millen and the National Company. Includes one of the few pictures of Millen available on the web.

(2) Browning Labs web site,  and "Evolution of the Browning Drake Receiver," by Arthur H. Lynch, published in Radio News, April 1927.

(3)History of National HRO product line.

(4) A web site featuring great pictures from the Bletchley Park museum of both an HRO listening station and the early computers used to break the Enigma encryption.

Paul was a design engineer and engineering manager in the broadcast industry for14 years before coming to WSHU in 1990. He holds an FCC commercial radio license, and an extra class Amateur radio license.