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Intercept receiver (Super Skyrider)
The SX-28 was an AM/CW communication receiver,
developed and built by
Hallicrafters Inc. in Chicago (USA) in 1940, a few years before the US
got involved in WWII. The receiver was intended for use by the
Government, the Army and by Radio Amateurs (Hams). It is one of the most
popular receivers every built by Hallicrafters. The receiver is also known
as Super Skyrider.
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The SX-28 was known for its excellent audio performance, but also for
its stylish art-deco exterior. Although the receiver needs some
expertise when put to use, it is generally loved by engineers,
even today. During WWII, it became a popular intercept receiver
alongside others like the National HRO
and RCA's AR-88.
The image on the right shows an SX-28A that was probably produced
mid-1944. It was owned for several years by a Radio Amateur who
complete reworked and restored it, before it ended up in the
Crypto Museum collection.
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The SX-28 is a superheterodyne receiver, that covers all frequencies
between 550 kHz and 43 MHz, divided over six ranges.
The circuit is built around 15 valves (tubes)
and features a double pre-selection
font-end in the highest four frequency ranges.
A single pre-selection front-end is used on the lowest two ranges.
Other design features of the
SX-28 are the Amplified Automatic Volume Control (AVC), the Lamb
Noise Silencer, Calibrated Bandspread and Push-Pull Audio.
The SX-28 was developed in 1940 and was intended to be the 'ultimate
receiver' of the moment, using the input from over 600 report, filed
by Radio Amateurs and Government Engineers [1]. It performed better
than any previous Hallicrafters receiver and was on par with
its competitors.
The receiver was first announced in July 1940, but production didn't
start until August of that year. The design of the SX-28 was improved
several times during the war and around February 1944 it was replaced
by the SX-28A, a model that stayed in production for some time after
the war, until June 1946.
During the war, a special ruggedized version, known as the AN/GGR-2
was released to the Army, and an airborne version, known as the
R-45/ARR-7, to the Air Force.
In the UK, many SX-28 receivers were used in the so-called Y-Stations
for the interception of coded enemy radio signals, mainly from Germany and
Italy. Such messages were subsequently passed on to
Bletchley Park
for decoding [6].
When production of the SX-28 ended in 1946, an estimated 27,500
of these receivers had been manufactered [5].
Many of these have survived, especially in the US, and are still
operational today. The price of an SX-28A in 1944 was US$ 223.
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The block diagram below shows the basic operation of the SX-28.
The antenna input at the left passes one or two pre-amplifiers
(depending on the band) before it reaches the mixer.
The main oscillator produces a
signal that is 455 kHz higher than the desired frequency,
resulting in a 455 kHz Intermediate Frequency (IF) that is
amplified in two stages before it reaches the detector.
For the reception of CW (morse) signals, a Beat Frequency Oscillator
(BFO) is present. The signal from the BFO can be mixed with the
detected signal, before it is passed on to a two-stage audio amplifier.
The 2nd AF stage is a Push-Pull Amplifier, built around two 6V6 valves.
It produces enough signal to drive a speaker directly.
At the bottom right is the Power Supply Unit (PSU). It converts the
AC mains (110-125V) to suitable LT and HT voltages. Alternatively, the
radio can be fed by a DC source, by directly applying 6V DC and 270V DC
to a special connector at the rear.
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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.
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A good example of a Y-Station is the one at Beaumanor Hall [9],
a large estate in the small village of Woodhouse (Leicestershire, UK)
that was used for military intelligence during WWII.
At Beaumanor Park, a series of radio intercept huts, disguised
as stables and cricket pavilions, was set up, using a variety
of Intercept Receivers including the
SX-28 [6]. Most of these were 19" rackmount versions,
modified for 240V AC [1].
Other receivers that were used by the Y-Stations include the
RCA AR-88,
the National HRO,
the British R-107,
the R-109
and the DST-110 [5].
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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.
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- SX-28 (August 1940 - February 1944)
- SX-28A (February 1944 - June 1946)
- AN/GRR-2 (Ruggedized version)
- R-45/ARR-7 (Airborne version)
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- 550 kHz - 1.6 MHz
- 1.6 - 3.0 MHz
- 3.0 - 5.8 MHz
- 5.8 - 11 MHz
- 11 - 21 MHz
- 21 - 43 MHz
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- Mains voltage: 110-125V AC
- AC Power consumption: 138W at 117V/60Hz
- DC Power consumption: 108W at 6V/18A
- AF Power output: 8W undistorted
- Sensitivity: 2mV (band1-5), 4mV (band 6) at 0.05W AF output
- AF output impedance: 5000 or 500 Ω
- Intermediate Frequency (IF): 455 kHz
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| Index
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Valve
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Description
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| V1
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6AB7
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1st RF Amplifier
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| V2
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6SK7
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2nd RF Amplifier
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| V3
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6SA7
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Mixer
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| V4
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6SA7
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HF Oscillator
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| V5
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6L7
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1st IF Amplifier Noise Limiter (ANL)
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| V6
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6SK7
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2nd IF Amplifier
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| V7
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6B8
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Detector and S-meter Amplifier
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| V8
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6B8
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AVC Amplifier
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| V9
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6AB7
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Noise Amplifier (ANL)
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| V10
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6H6
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Noise Rectifier (ANL)
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| V11
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6J5
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Beat Oscillator (BFO)
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| V12
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6SC7
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1st Audio Amplifier
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| V13
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6V6
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Push-Pull Output Amplifier
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| V14
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6V6
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Push-Pull Output Amplifier
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| V15
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5Z3
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Rectifier
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The following receiver can be regarded as contemporary competitors of the SX-28:
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- Henry Rogers, The Hallicrafters Inc. SX-28, a pre-war masterpiece
Website: Radio Boulevard. Western Historic Radio Museum.
1997-2012. Retrieved January 2013.
- Philip I. Nelson, Hallicrafters Model SX-28 Communications Receiver (1941)
Detailed information about restoration of an SX-28.
1995-2012. Retrieved January 2013.
- Philip I. Nelson, Rebuilding the Hallicrafters SX-28 Gearbox
Detailed information about restoration of the SX-28 tuning mechanism.
1995-2012. Retrieved January 2013.
- Wikipedia, Hallicrafters
Retrieved January 2013.
- Wikipedia, Hallicrafters SX-28
Retrieved January 2013.
- Personal correspondence with Kevin Coleman
Volunteer at Bletchley Park (Station X) and Beaumanor (Y-Station).
December 2008 - January 2009.
- The Hallicrafters Co., SX-28 Operating and Service Instructions
Operating, Alignment and Servicing Instructions. 1941.
- War Department, Radio Receiver AN/GRR-2
Military version of the Hallicrafters Model SX-28-A.
22 November 1944.
- Wikipedia, Beaumanor Hall
Retrieved January 2013.
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© Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Friday, 01 February 2013 - 15:35 CET
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![Beaumanor Hall. [unknown source]](img/beaumanor.jpg "Beaumanor Hall. [unknown source]")