National HRO
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The original HRO was a 9-tube shortwave general coverage communications receiver manufactured by the National Radio Company. James Millen (amateur radio call sign W1HRX) in Massachusetts was in charge of the mechanical design. Herbert Hoover, Jr. (W6ZH), son of the president, and Howard Morgan (of Western Electric) designed the electronics in Hoover's garage in Pasadena, California. [1] Dana Bacon (W1BZR) was also involved and wrote about the receiver as second author with James Millen. Some of National Radio's tool makers marked their overtime slips with HOR for "Hell Of a Rush." Management decided that a version of that abbreviation should be the name of the new receiver, choosing the slight alteration HRO to make it less objectionable. [2] [3] That was quickly countered by saying that HRO stood for "Helluva Rush Order" [4].
The HRO receiver was first announced in QST magazine in October 1934 and shipped in March 1935, incorporating many design features requested by the fledgling airline industry [5] that were also attractive to the amateur radio community. According to the 1935 instruction manual [6], the HRO price was US$233, the external power supply (to reduce hum) was US$26.50 less tubes, and a 7000 ohm speaker in a rack panel was US$30.00.
The two most distinctive features of this radio were its use of a micrometer-type dial, and plug-in tuning coils that slid into a full-width opening at the bottom of the front panel. The dial, designed by James Millen, allowed for 500 tuning steps over a range of ten full turns of the large tuning knob that tuned with velvet smoothness. Ten times the circumference of the dial is 12 feet (nearly 4 m), which allowed for great frequency resolution. The four standard coils, A, B, C, and D, covered 14-30, 7-14.4, 3.5-7.3, and 1.7-4 MHz, respectively. Two other coils, E and F, sold separately, covered 960-2050 kHz and 480-960 kHz, respectively. (See p. 7 of the instruction manual [7] and [8].) Before each radio left the factory, a technician custom calibrated a set of A, B, C, and D coils for that particular radio, a process that took nearly 4 hours [9]. Each of the four main coils also had bandspread modes set by moving screws that limited the frequency range to 28-29.7, 14-14.4, 7-7.3, 3.5-4 MHz, respectively, for amateur radio use.
The HRO found widespread use during World War II as the preferred receiver of various Allied monitoring services, including the code-breaking group at Bletchley Park (Station X) in England. The US military told National, "Start building HROs. We'll tell you when to stop." [10], [11]. Before, during, and after WWII, the HRO concept of using plug-in coils with micrometer tuning was copied in several countries, including Germany and Japan. The best known copies are probably two German models widely used during WWII by German military services, the KST made by Korting Radio and the R4 made by Siemens [12].
Here is a listing of the main HRO models with their years of production: HRO (also called HRO-Sr, 1935-1943), HRO-Jr (February 1936-1943, US$100 version of Sr with only one coil not individually aligned to the receiver and lacking crystal filter, phasing control, and signal strength meter), RAS (1939-?, HRO-Jr for Navy with general coverage coils and 175 kHz IF so the radio could receive the 500 kHz distress frequency), HRO-4 (used at Bletchley Park, production dates?), HRO-5 (1944-1945, octal tube version). There were also several sub-variations on these models [13]. An instruction manual that applies to all of these can be downloaded [14].
Following WWII came the HRO-7 (1947-1949, 12 tubes, including 2 miniature tubes), HRO-50 (1949-1950, built-in tuning dials and power supply, improved styling and performance), and HRO-60 (1952-1964, dual conversion for coils B (7-14.4 MHz) and A (14-30 MHz)). These were followed by two solid-state receivers that did not use plug-in coils: the HRO-500 (October 1964-1972, 5 kHz - 30 MHz [15]), and HRO-600 (1970-1972?, 16 kHz - 30 MHz). Breaking with tradition, the HRO-600 used a frequency counter instead of a micrometer tuning dial [16]. All the models of HRO receivers were outstanding and relatively expensive, although even better and more expensive general coverage receivers from such companies as Collins Radio became available in the 1950's and later. On the Web, one can find HRO receivers dating back to the original model that have been restored and are in regular service by vintage amateur radio enthusiasts and other hobbyists as of 2006.
