The National HRO Communications Receiver

Last updated 1/1/2002

general view of the National HRO

The 'HRO' radio communications receiver made by the American National company of Malden, Mass, USA, was a groundbreaking example of 'lateral thinking' in electronic design in the early 'thirties, and proved to be of enormous importance in British intelligence gathering in the Second World War. My use of one such receiver in the late 'fifties when I was in my early teens opened my eyes to the world of electronics and communication.

My HRO was a very early rack mounted version, well past its best days, and for some reason painted canary yellow. An 'amusing' feature was that the intermediate frequency transformers were wound on wooden formers, which meant that even if I lined it up correctly, the damp would take the edge off its performance if the weather changed! I still remember the excitement of listening to amateur transmissions from American missionaries in Africa AM on the10 metre band, thanks to high sunspot activity at the time I bought it .

As radio communication developed in the thirties designers were confronted with the conflicting demands of increasing congestion, higher frequencies, and increased demand for sensitivity. In high performance receivers the convention approach of ganged wavechange switches plus banks of trimmers led to large and clumsy 'front ends' with ensuing stray capacitances limiting performance. Add to this the need for one or often two stages of tuned RF amplification, and the solutions became difficult and expensive. In the case of the National HRO the designers 'went back to the drawing board', and returned to the system used by early home-made designs, with plug-in coils. The RF section was turned sideways so that the coils could be built into one unit, plugged in from the front. The four gang tuning capacitor evolved into a gear-driven design driven at a right-angle from the centre, using a worm drive and a micrometer dial with an effective length of twelve feet. This guaranteed accurate logging, although absolute frequency was a matter of interpreting a graph on the coil pack. The resulting receiver was simple and (by the standards of the day) compact, and performance was maintained up to 30MHz with ease.

The HRO first appeared around 1935, and the price of $300 made it a luxury item until its adoption for extensive wartime use. There is a short but very informative article in 'Radcom' Jan. 2002, P.69.

The sub-pages listed below are from a 1939 instruction manual. Pages 14 - 17, which I haven't included, feature a detailed discussion of the design principles. I have scanned the pages individually and transcribed them to Acrobat files. I assume this manual is well out of copyright, but it should be noted that I have posted it for information for fellow enthusiasts. Please do not use it for commercial purposes.

John Hoare, G3PJI


Front cover
Inside front cover
Page 3
Page 4 (circuit diagram)
Page 5
Page 6
Page 7
Page 8 (inside top view)
Page 9
Page 10
Page 11
Page 12 (chassis underside)
Page 13 (parts list)
Page 18 (electrical characteristics)
Inside back cover (price list)
Back cover

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