National HRO
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| Frequency | RX: 1.7-30 MHz standard (coils A-D); ~50 kHz-35 MHz with optional coil sets |
| Power | Receiver only |
| Modes | AM, CW |
| Weight | 18 kg (39.7 lb) |
| Connector | binding-posts |
| Year | 1935 |
| antenna | binding-posts |
Estimated (unverified): Weight ~18 kg (about 40 lb) is approximate and not confirmed on a primary spec sheet; the receiver and its separate power supply are weighed differently across sources. Antenna connection is via binding posts/terminals (era-typical).
A landmark high-performance amateur communications receiver, introduced in 1935. The PW (parallel-worm) micrometer dial - 500 divisions over 10 turns - and the plug-in coil sets gave repeatable, accurate tuning and excellent selectivity for the era. Four standard coils (A/B/C/D: 14-30, 7-14.4, 3.5-7.3 and 1.7-4 MHz), each factory-aligned to its receiver, with optional coils extending coverage to roughly 50 kHz-35 MHz. A complete HRO with all coils and the matching speaker is a museum piece.
It is a tube receiver with a separate, heavy power supply. Plug-in coil sets are fragile and complete sets are hard to find. The dial cord breaks and is tedious to restring. It receives AM and CW only, and the cabinet is large - about the size of a small suitcase.
The HRO set the standard for high-performance amateur receiving. National sold thousands between 1935 and the war, and an estimated 10,000 saw service in Allied intercept work, including the Y-service stations feeding Bletchley Park. Finding a complete example with all four standard coil sets, the matching speaker, and a working power supply is a real achievement, and restoration is well documented by the vintage-radio community.
The National HRO is one of the most historically important communications receivers ever made - a precision plug-in-coil set announced in October 1934, with first shipments in March 1935, that helped define the high-performance receiver and went on to serve extensively in World War II, most famously with British codebreakers. It uses interchangeable plug-in coil sets to cover the spectrum; the standard A, B, C and D coils span 14-30, 7-14.4, 3.5-7.3 and 1.7-4 MHz, with optional coils extending coverage to roughly 50 kHz-35 MHz, receiving AM and CW.
Its signature is the famous micrometer 'PW' dial - a parallel-worm precision tuning drive with a numbered micrometer readout (500 divisions over 10 turns) that, with a calibration chart, gave repeatable, accurate frequency setting nothing else of its era could offer. A nine-tube superheterodyne with two RF stages and an optional crystal filter, the HRO set a standard for sensitivity and selectivity that influenced receiver design for decades.
During the war the HRO became the preferred receiver for Allied intercept work: Britain bought around a thousand to start, and an estimated 10,000 were used across the Y-service stations, diplomatic posts and shipboard and clandestine roles, with versions in production into the 1960s. As a landmark of radio history it remains among the most revered of all vintage receivers. See the later HRO-60 and the Hammarlund Super-Pro.