National HRO

[4] The HRO receiver was 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 heat in the receiver cabinet and hum)[7] was US$26.50 less tubes, and a 7000 ohm speaker in a rack panel was US$30.00.

[4] An estimated 1,000 standard HROs were initially purchased by Great Britain, and approximately 10,000 total saw use by the British in intercept operation, diplomatic communications, aboard ships and at shore stations as well as for clandestine use.

[8][9] The two most distinctive features of this radio were its use of a micrometer-type dial, and plug-in sets of tuning coils that slid into a full-width opening at the bottom of the front panel.

"[12][17] Before, during, and after World War II, the HRO concept of using plug-in coils with micrometer tuning was copied in several countries, including Germany and Japan.