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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.
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There are many versions
of the HRO receiver and each new production run
saw a number of smaller or larger modifications being made to the design.
The most well-known
versions are arguably the late-war HRO-5 and the pre-war variant
the HRO-M, both of which played an important role during WWII and both
of which are still being used by Radio Amateurs today.
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.
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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.
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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+ toggle 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 leaves the filaments of the valves ON (LT voltage).
The CW Oscillator at the bottom left is in fact a Beat Frequency
Oscillator (BFO) that is used to make CW (morse) signals audible.
It is also used to find weak AM carriers.
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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, mechanical parts and internal
adjustments 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.
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The HRO was commonly supplied with a range of plug-in coils; one
for each frequency band. When not in use, the coil 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:
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Plug-in
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Range
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Bandspread 1
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A
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14 - 30 MHz
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10m
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B
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7 - 14 MHz
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20m
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C
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3.5 - 7 MHz
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40m
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D
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1.7 - 4 MHz
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80m
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E
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900 kHz - 2 MHz
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F
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580 - 920 kHz
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G
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180 - 430 kHz
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H
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100 - 200 kHz
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J
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50 - 100 kHz
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AA
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27.5 - 30 MHz
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AB
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25 - 35 MHz
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AC
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21 - 21.5 MHz
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AD
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50 - 54 MHz
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Only coils A-D can be set to bandspread the amateur bands (10, 20, 40 and 8 metres).
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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.
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The story of the HRO starts in 1932, when
General Electric Company (GE)
was awarded a contract by the US Government's Bureau of Air Commerce, to
supply shortwave transmitters and receivers. GE had built transmitters
before, but had no experience in designing receivers.
It was therefore decided to ask
James Millen at the National Radio Company to design one.
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 and 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, no RF pre-selection and featured a simpler mechanical
construction. It was supplied with a single coil pack for the 80 meter
amateur radio 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 Automatic Volume Control (AVC), also known as Automatic
Gain Control (AGC).
Furthermore, they wanted the receiver to contain an S-meter
and have a very high reliablity.
Finally, 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).
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
epicyclic dial, which allows the operator to tune
the frequency scale in 1/500th units (with the aid of a calibration chart).
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 the best engneering 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 (e.g. the so-called Y-stations),
for diplomatic communication, aboard ships and for clandestine use,
before and during WWII [1].
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During WWII, the British intelligence service,
GC&CS
(now: GCHQ),
ran a massive operation for 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, which were spread all over
the country, but were also present in other parts of the world, such
as North Africa and Australia. These Y-Stations were operated by
Ham Radio operators and specially trained house-wifes, organised in
the so-called Y-Service, using a variety of intercept receivers, of which the
HRO was arguably the most important one.
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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
Bletchley Park codebreaking center,
either by despatch rider or via teleprinter lines.
There, a team of over 12,000 people,
consisting of codebreakers, engineers and WRNS 1 ,
broke the German codes at large scale on a daily basis.
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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 already 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.
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WRNS = Women's Royal Naval Service, sometimes written as WRENS.
→ Wikipedia
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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 preparing the receiver for production,
National didn't want their radio's to become known as HORs (whores) and
changed its name to HRO.
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The HRO design was so successful that it was copied several times,
before, during and after WWII.
During the war it was (partly) copied by
Kingsley in Melbourne (Australia) and became known as AR-7.
In Germany, Siemens made the R2
(succeeded by the R4)
and Kórting produced the KST.
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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).
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Before WWII, the National Company had made the tuning section of their receiver
available to other manufacturers. In fact, there were four design
variants it, which allowed manufacturers to integrate it into virtually
any type of radio. One example is the pre-war
Siemens R-II (R2) receiver.
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At the outbreak of WWII, the National Company stopped supplying components
to the Germans, but some companies, notably Körting, managed to source them through 'neutral' countries like Portugal, whilst Siemens built
their own units.
The history of the R-IV
has always been vague.
In his book Die deutschen Funknachrichtenanlagen bis 1945 [11],
Fritz Trenkle suggests that the wartime
Siemens R-IV (R4) is a copy of the
National HRO. Although some features are obviously 'borrowed', the surviving
sets clearly show that it is quite different from the HRO.
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Furthermore the R-IV has a different tuning
unit that was made in-house by Siemens.
According to personal notes from the estate of a late collector [12],
the design of the wartime R-IV was based on the pre-war
R-II (R-2).
It therefore seems more likely that the R-II
was a copy of the HRO, whereas the R-IV
was designed by Siemens themselves after the supply of components from
National had dried up. Once the war was over,
National components once again became available worldwide,
as is demonstrated by their 1948 catalogue [10].
➤ More about the Siemens R-II
➤ More about the Siemens R-IV
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- 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) 1
- 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) 1
- HRO-50 (1950)
- HRO-50T1 (1951)
- HRO-60 (1953)
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A suffix indicated the use of this model: T=tabletop, R=rackmount [4].
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Körting KST Germany, WWII - Germany, WWII
- Germany, WWII
Japanese HRO Japan, WWII Kingsley AR-7 Melbourne, Australia FWB AQST East German post-war copy (around 1950)
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- Power pack Type 697: 0.6A at 115V/60Hz
- Power pack Type 5886: 0.4A at 115V/60Hz
- Sensitivity: 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)
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The following receiver can be regarded as contemporary competitors of the HRO:
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- National, Instruction Manual for The New National HRO Communication Receiver
National Company Inc., Malden, Massachusets (USA), 1935.
- National, Instruction Manual for The National HRO
National Company Inc., Malden, Massachusets (USA), 1939. 1
- National, Instructions for the HRO Series 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.
- Technical Manual TM11-885, Radio Receiver R-140/FSM-1
War Department. August 1946, 88 pages. 1
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- Wikipedia, National HRO Receiver
Retrieved December 2012.
- Henry Rogers, HRO - Communication Receivers
Website: Radio Boulevard. Western Historic Radio Museum.
1997-2012. Retrieved December 2012.
- Unknown author, The HRO Story
Gloucester Amateur Radio and Electronics Society.
Website. Retrieved December 2012.
- Niel Wiegand (W0VLZ), HRO Models
Radio Bay Website. Retrieved December 2012.
- Kevin Coleman 1 , Information about Y-Station receivers
Personal correspondence, December 2008 - January 2009.
- Arthur Bauer, Körting KST Receiver
Foundation for German Communication and Related Technologies.
Website. Retrieved December 2012.
- National, Instruction Manual for The National HRO
National Company Inc., Malden, Massachusets (USA), 1939. 2
- John J. Nagle, A Brief History of the National Company Inc.
Website. Retrieved December 2012.
- Brian Page, N4TRB, The National HRO Receiver and the Nation Company
Website. Retrieved May 2013.
- National Company Inc., National Precision Condensers
Malden, Massachusetts (USA). Catalogue 1948. p. 31. 3
- Fritz Trenkle, Die deutschen Funknachrichtenanlagen bis 1945
Band 2 'Der Zweite Weltkrieg' (German). pp. 151-154.
- Richard Auerbach (DL1FK), Notes about Siemens R-II and R-IV
Personal notes, 17 April 1978. Retrieved June 2014. 2
- Garett Coakley, Photograph of HRO receivers in Y-Station
18 July 2009. Obtained via Wikimedia Commons.
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Volunteer Guide at Bletchlay Park (Station X) and Beaumanor (Y-Station).
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PDF file kindly supplied by Fernando Arroyo, EA4BB.
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Copy of advertisement kindly supplied by Günter Hütter.
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© Crypto Museum. Created: Saturday 01 December 2012. Last changed: Tuesday, 26 December 2023 - 13:22 CET.
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![A woman of the ATS using a HRO receiver in a Y-Station [source unknown]](img/YStation.jpg)
![Typical setup with several HRO receivers in a Y-Station. Photograph by Garett Coakley [13].](img/HRO_opr.jpg)
![Photograph courtesy Arthur Bauer. Center for German Communication [6].](img/kst.jpg)
