The National HRO vintage radio receiver was developed s a consequence of a contract placed with US General Electric Company (GE) to provide the US Government's Bureau of Air Commerce with short wave receivers.
One stumbling block was that GE had no experience in designing and building radio receivers hence cue entry stage left for the National Radio Company to design the radio for them in 1934 leading to first production in 1935.
The design was referenced in house by the acronym to secure secrecy as HOR ( for Hell of a Rush). This acronym was soon deemed unsuitable as HOR could be mnemonically related to WHORE so a swift transposition of letters resulted in the amended acronym of HRO - Hell of a Rush Order and the rest is history!!
The HRO was widely used during WW II, not only in the USA, but also in the UK where over 10 000 units are estimated to have been shipped, many being used by the Y Stations that intercepted Nazi radio communications that were then sent on to Bletchley Park.
The HRO continued in use post war with a number of improvements and iterations until production came to a close some 29 years after start of production in 1965,
We have compiled a table showing the range of HRO variants below: -
|YEAR OF INTRODUCTION|
|HRO||Original HRO version renamed the HRO Senior (Snr) when the HRO Junior was introduced||1935|
|HRO-SPC||Rack mounted HRO|
|HRO-B||Battery powered HRO|
|HRO-JR||HRO Junior, a low cost device without crystal filter, S meter, and band-spread option capability being used only with coil sets JA, JB, JC and JD. One coil set - customer specified, was supplied on purchase.||1936|
|HRO-M||Most commonL seen HRO version used for British Y Stations and knowns as Receiver R106||1941|
|HRO-5||Upgraded version of the HRO-M using octal valves.||1944|
|HRO-W||US Signal Corps specified version|
|HRO-5A||Post War version of HRO-5|
|HRO-60||HRO Final runout version||1952 - 65|
The National HRO radio receiver used pluggable coil packs to provide the switching between different ranges which reduced the receiver's and increased RF sensitivity.
Coil packs enabled operation from frequencies as low as 100 kHz right up to 54 MHz as shown in the table below: -
|COIL-PACK REF.||FREQUENCY RANGE (MHz)||BAND-SPREAD (M)|
|A||14 - 30||10|
|B||7 - 14||20|
|C||3.5 - 7.3||40|
|D||1.7 - 4||80|
|E||0.9 MHz - 2.05|
|F||500 - 1000 kHz|
|G||175 - 400 kHz|
|H||100 - 200 kHz|
|J||50 - 100 kHz|
|AA||27.5 - 30|
|AB||25 - 35|
|AC||21 - 21.5|
|AD||50 - 54|
|JA||14 - 30|
|JB||7 - 14.4|
|JC||3.5 - 7.3|
|JD||1.7 - 4|