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National HRO
General Coverage Communications Receiver

The National HRO was a valve-based (tube) shortwave general coverage communications receiver, manufactured by the National Radio Company (National) in Malden (Massachusetts, USA) from 1935 onwards. The receiver was intended for military and amateur use and became very popular for intercept work during WWII. Different versions of the radio were in production until the 1960s.
 
There are many different versions of the HRO and each new production run saw a number of smaller or larger modifications being made to the design. The most well-known version is arguably the HRO-5 and the earlier wartime variant the HRO-M, both of which played an important role during WWII and both of which are still being used today by Radio Amateurs.

The image on the right shows a rather late HRO-5 model that has been preserved really well. It was produced towards the end of WWII and is still in pristine and fully operational condition.
  
National HRO-5

The receiver measures 48 x 22 x 33 cm and weights approx. 22 kg. It's case is usually finished in black wrinkle paint and has a lid at the top for easy access to the valves. In order to avoid hum, the receiver is powered by an external Power Supply Unit (PSU). Apart from the PSU, an external speaker is the only accessory needed to operate the receiver. It is unknown how many HROs were actually manufactured, but it must have been tens of thousands. The UK ordered about 10,000 of them during WWII. They were used in the so-called Y-Stations and for mobile direction finding.
 
National HRO-5 Front view of the National HRO-5 Using the frequency dial Close-up of the S-meter Looking inside the HRO-5 Coiol pack taken out of the HRO-5 Placing the coil pack

 
Controls
The HRO-5 covered the entire Short Wave Band and was suitable for the reception of AM, CW (morse) and SSB signals. All controls are at the front of the receiver. The two most obvious key features of the HRO, are the precision tuning dial, the tuning section and the pluggable coils.


Most of the controls are self-explanatory. The SELECTIVITY adjustment at the right is used when the single-signal crystal filter is used. Setting the SELECTIVITY to zero, turns the crystal filter off. The B+ switch at the bottom right can be used to turn the receiver off, when it is used in combination with a transmitter. It turns off the HT voltage (B+) but leafs the filaments of the valves on (LT voltage). The CW Oscillator at the bottom left is in fact a BFO that is used to make CW (morse) signals audible. It can be also be used to find weak AM carriers.
 
Interior
The interior of the HRO receiver can easily be accessed by lifting the hinghed top panel. This reveals the extremely neat and clean design of the receiver, where all valves and internal adjustment are easily accessible.

One of the most prominent design features is the 4-stage tuning section that is mounted at the front, directly behind the tuning dial. When operating the dial, the 4 sections are driven simultaneously via a worm drive at the center. The image on the right shows the tuning section when viewed from the rear of the receiver.
  
HRO tuning section

 
Opening the HRO-5 HRO-5 interior HRO tuning section Close-up of part of the tuning section Operating the tuning dial Close-up of the HRO-5 interior Coil pack slide contacts Close-up of the valves

 
Frequency Ranges
The HRO was usually supplied with a range of plug-in coils; one for each frequency band. When not in use, the coild packs were stored in a wooden storage case. For use on the amateur radio bands, coils A to D can be used in bandspread mode. The following coils were available:
 
Plug-in Range Bandspread 1
A 14 - 30 MHz 10m
B 7 - 14 MHz 20m
C 3.5 - 7 MHz 40m
D 1.7 - 4 MHz 80m
E 900 kHz - 2 MHz  
F 580 - 920 kHz  
G 180 - 430 kHz  
H 100 - 200 kHz  
J 50 - 100 kHz  
AA 27.5 - 30 MHz  
AB 25 - 35 MHz  
AC 21 - 21.5 MHz  
AD 50 - 54 MHz  
 
  1. Only coils A-D can be set to bandspread the amateur bands (10, 20, 40 and 8 metres).
HRO coil pack Front view of HRO coil pack Bottom view of coil pack Close-up of notice at bottom side of coil pack Single coil pack Wooden box with three coil packs Wooden box with three coil packs Collection of coil packs

 
Power Supply Unit
The HRO needs an external Power Supply Unit (PSU). Two different mains PSUs were available: the Type 697 and the Type 5886. Both were suitable for the 115V AC mains voltage, with the latter delivering a higher HT voltage. Optionally, the PSUs were also available for 230V AC.

The HRO could also be operated from, say, a car battery, using the Type 686 power pack with built-in vibrator. It converted 6V DC into suitable LT and HT voltages. A nice example of the Type 686 vibrator pack is shown in the image on the right. The HRO is connected to the 4-pin socket.
  
6V DC power inverter

 
The 4-pin power plug of the HRO 6V DC power inverter Close-up of the Type 686 vibrator pack Valve-base reproduction Mains PSU for the HRO Reproduction mains PSU with mains cable Close-up of the LT/HT socket on the mains PSU

 
History
The story of the HRO starts in 1932, when General Electric Company (GEC) was awarded a contract by the government's Bureau of Air Commerce, to supply shortwave transmitters and receivers. GEC had built transmitters before, but had no experience in designing receivers, so they asked James Millen at the National Radio Company to design one.

General Electric logo

The resulting radio was a 1.5-20 MHz superheterodyne, that was designated Aeronautical Ground Station (AGS) Receiver. It consisted of a single RF pre-amplifier, two IF amplifiers and had an IF frequency of 500 kHz. The circuit contained 9 valves (tubes) and featured pluggable coil packs. The latter avoided engineering difficulties and signal losses, and improved reliable repeat tuning.

The AGS Receiver was soon joined by the SW-58C, a superregenerative receiver that covered the 200 - 400 kHz band used by the airlines at the time. The basic AGS was sold for US$ 165. A cut-down version, called the FB-7, was later released for the amateur market. It had only 7 valves, nor RF pre-selection and featured a simpler mechanical construction. It was supplied with a single coil pack for the 80 meter band and was available for just US$ 55.

The airlines were not entirely happy with the AGS Receiver and called for a design with better image rejection, better selectivity, and a good Automatic Volume Control (AVC, also known as AGC). They also wanted the receiver to contain an S-meter and have a very high reliablity. As a result they specified their own requirements, which were advocated by Herb Hoover who was then in charge of Radio Communications at Western Airlines (later part of TWA).

National Radio Company logo

The new radio was also designed by James Millen at the National Radio Company, but this time with two RF amplifiers and two IF amplifiers at 455 kHz with a 20Hz crystal filter. He kept the pluggable coil packs as part of the design and added the now famous large epicyclic dial, allowing the operator the scale in 1/500th units.

The design was finished in 1934 and National pushed hard to get the receiver out by the end of that year. When creating the tools for the first production run, the tool makers had to work overtime and used HOR (Hell Of a Rush) as a job number on their overtime slips. As National's marketing department didn't want their radios to become known as HORs (whores), the name was changed to HRO (Hell of a Rush Order). Despite their best efforts, technical problems delayed the release of the the radio until March 1935. The price at the introduction was US$ 233.

The HRO was improved and modified numerous times and remained in production until 1964. It is difficult to estimate exactly how many units have been produced, but it must have been tens of thousands. At the outbreak of WWII, the US Military told National: 'Start building HROs. We'll tell you when to stop' [7]. An estimated 1000 units were initially ordered by the UK, but a total of approx. 10,000 units saw use by the British intercept operation, for diplomatic communication, aboard ships and for clandestine use, before and during WWII [1].
 
Bletchley Park
During WWII, the British intelligence service, GC&CS (now: GCHQ), ran a massive operation of intercepting and decoding German radio messages in morse code, mainly encrypted using the well-known Enigma cipher machine. The messages were intercepted by the so-called Y-Stations, that were spread all over the country, but were also present in other parts of the world, such as North Africa and Australia. Those Y-Stations were operated by Ham Radio operators and specially trained house-wifes, the so-called Y-Service, using a variety of intercept receivers, of which the HRO was arguably the most important one.
 
At the start of the war, the HRO-M was the most common model, gradually being replaced by the later HRO-5 models. Other receivers that were used by the Y-Stations include the RCA AR-88, the Hallicrafters SX-28, the British R-107, the Army Set R-109 and the DST-110 [5].

Once the messages were intercepted, they were sent to the codebreaking center at Bletchley Park by despatch rider or via teleprinter lines (telex). There, a team of over 12,000 people, consisting of codebreakers, engineers and WRENS broke the German codes at a large scale on a daily basis.
  
A woman of the ATS using a HRO receiver in a Y-Station [source unknown]

Initially, about 1000 HROs were ordered by the UK. By the end of the war, that number had risen to approx. 10,000. They were used before and during WWII by the British for intercept operations, the diplomatic wireless service, aboard ships and ashore, and for clandestine activities [1]. As the HRO was very popular before the war with radio amateurs (HAMs) who could afford them, a number of them were confiscated for the war effort. Sometimes even complete with the operator.
 
The name H.R.O.
The name HRO is the abbreviation of Hell'va Rush Order (Hell of a Rush Order), although this is not the name that was originally given to this receiver. According to designer James Millen, who left National in 1939, the earliest design papers were stamped 'H.O.R.' instead [2]. HOR stands for 'Hell Of a Rush', but in the finalization phase of the receiver, National didn't want their radio's to be called HORs (whores) and the name was changed to HRO.
 
Wartime copies
The HRO design was so successful that it was copied a number of times both during and after WWII. During the war it was (partly) copied by Kingsley in Melbourne (Austria) and became known as the AR-7. Also during the war, it was copied in Germany by Siemens (R4) and Körting (KST).
 
The image on the right shows the Körting KST, which is a nearly full copy of the HRO, albeit with German valves. The tuning section, one of the key properties of the HRO, was indirectly bought from National during the war, via Portugal. The photograph was kindly supplied by Arthur Bauer, who has more KST pictures on his website [6].

In 1950, HRO copies were made in East Germany by (Funkwerkstatten Bernburg (FWB). They were known as AQST or: Allwellenempfänger mit Quartzfilter und Storausstattung (All-band Receiver with Crystal Filter and Noise Canceller).
  
Photograph courtesy Arthur Bauer. Center for German Communication [6].

 
Models
  • HRO, later renamed to HRO-SR (Senior)
  • HRO-SPC (self-contained rackmount version with PSU, speaker, etc.)
  • HRO-B (battery operated version)
  • HRO-JR (Junior)
  • HRO-M (late 1941)
  • HRO-MX (early 1942)
  • HRO-5 (1944)
  • HRO-W (Signal Corps version of HRO-5)
  • HRO-5A (post-war version, early 1946)
  • HRO-5A1 (March 1946)
  • HRO-5TA1 (May 1946)
  • HRO-6 (early 1947)
  • HRO-7 (mid 1947)
  • HRO-50 (1950)
  • HRO-50T1 (1951)
  • HRO-60 (1953)
HRO copies
  • Körting KST - Germany (WWII)
  • Siemens R4 - Germany (WWII)
  • Japanese HRO - Japan (WWII)
  • Kingsley AR-7 - Melbourne, Australia
  • FWB AQST - East German post-war copy (around 1950)
Technical specifications
  • Power pack Type 697: 0.6A at 115V/60Hz
  • Power pack Type 5886: 0.4A at 115V/60Hz
  • Sesnsitivity: 1µV input at 2W AF output into 7000Ω
  • Bandwidth: 4, 7.5, 14 or 21.5 kHz
  • CW Noise Equivalent: 0.2µV
  • S/N Ratio (at 5µV): 16dB
  • Antenna input impedance: 500Ω (average)
  • Max. undistorted AF output: 1.5W
  • AVC: flat withing ±10dB (between 10 and 100,000 µV)
Competitors
The following receiver can be regarded as contemporary competitors of the HRO:
 
Manuals
  1. National, Instruction Manual for The New National HRO Communication Receiver
    National Company Inc., Malden, Massachusets (USA), 1935.

  2. National, Instruction Manual for The National HRO
    National Company Inc., Malden, Massachusets (USA), 1939. 1

  3. National, Instructions for the HRO Aeries of Radio Receiving Equipments
    Including HRO, HRO-5, HRO-5T, HRO-5R, HRO-M, HRO-MX, HRO-M-RR, HRO-M-TM, HRO-SR and HRO-JR. National Company Inc., Malden, Massachusets (USA), April 1961.

  4. Technical Manual TM11-885, Radio Receiver R-140/FSM-1
    War Department. August 1946, 88 pages. 1

  1. Off site, via the website of Brian Page, N4TRB [9].
References
  1. Wikipedia, National HRO Receiver
    Retrieved December 2012.

  2. Henry Rogers, HRO - Communication Receivers
    Website: Radio Boulevard. Western Historic Radio Museum.
    1997-2012. Retrieved December 2012.

  3. Unknown author, The HRO Story
    Gloucester Amateur Radio and Electronics Sciety.
    Website. Retrieved December 2012.

  4. Niel Wiegand (W0VLZ), HRO Models
    Radio Bay Website. Retrieved December 2012.

  5. Personal correspondence with Kevin Coleman
    Volunteer at Bletchley Park (Station X) and Beaumanor (Y-Station).
    December 2008 - January 2009.

  6. Arthur Bauer, Körting KST Receiver
    Foundation for German Communication and Related Technologies.
    Website. Retrieved December 2012.

  7. National, Instruction Manual for The National HRO
    National Company Inc., Malden, Massachusets (USA), 1939. 1

  8. John J. Nagle, A Brief History of the National Company Inc.
    Website. Retrieved December 2012.

  9. Brian Page, N4TRB, The National HRO Receiver and the Nation Company
    Website. Retrieved May 2013.

  1. PDF file kindly supplied by Fernando Arroyo, EA4BB.
Further information

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