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Siemens R-IV   R4
WWII intercept receiver

The R-IV, or R4, was a communications intercept receiver, developed by Siemens and used during WWII by the German secret service, the Abwehr. The R4 was based on the pre-war Siemens R-II (R2), that in turn was a copy of the National HRO receiver, similar to other copies, like the Körting KST. Although the design of the R4 is based on the HRO, it has a number of differences.
The receiver roughly has the same size as the National HRO and covers a frequency range from 3.15 to 33 MHz. It was suitable for the reception of AM and CW (morse). A major difference with the HRO is that the coil packs are not inserted at the front, but at the top left of the receiver.

The frequency dial resembles that of the HRO, but contrary to popular believe [6], the tuning section was not obtained from National during the war, like it was done for the Korting (KST) receiver. Although it is similar in design, the Siemens one differs from the original HRO one.
Siemens R-IV with coil pack installed and spare coil pack on top

In his book Die deutschen Funknachrichtenanlagen bis 1945 [6], Fritz Trenkle classifies the R-IV an HRO-copy, but this seems to be incorrect. There are far too many differences with the original HRO design to call the R-IV a copy. 1 Furthermore, according to personal notes that were recently discovered in the estate of a late collector [4], the design of the R-IV, introduced in 1940/1941, was based on the pre-war Siemens R-II (R2) receiver, that in turn was a copy of the National HRO.

It appears that Siemens bought the original HRO tuning units directly from National until 1939 [5]. After the outbreak of war, Siemens no longer had access to the original parts from National and was forced to redesign the radio, As far as we know, there are no surviving R-II receivers.

Based on the serial numbers of the surviving R-IV receivers, it seems likely that only a couple of hundreds were built. There appear to be two production runs: the so-called 4-series, of which the serial number starts with a '4', and the 5-series, that have a serial number starting with '5'. The receiver was available in two colours: dark green, as shown in the image above, and grey as shown below. Siemens R-IV receivers are extremely rare and are highly wanted by collectors.
  1. Trenkle is not the only source that mentions the Siemens R-IV as a copy of the National HRO, but it appears that most authors are quoting from his publication Die deutschen Funknachrichtenanlagen bis 1945 [6].
Siemens R-IV with coil pack installed and spare coil pack on top Siemens R-IV with removed coil pack Siemens R-IV with coil pack installed Coil pack Frequency tuning dial Operating the tuning dial Siemens R-IV with power supply R-IV power supply unit

Grey series
The Siemens R-IV shown in the images above is housed in a dark green enclosure and has a serial number starting with '4'. Another R-IV unit, shown below, is housed in a grey case and has a serial number that starts with a '5'. This suggests that all 5-series machines may have been grey.
The machine shown here has been 'amateurized' and has a few extra knobs at the right half of the front panel. Furthermore, the front panel has been partly restored. Eventually, this machine will be brought back to its original condition.

Apart from a few manfacturing differences, such as the square capacitors in the compartment at the right, this version is identical to the 4-series R-IV (i.e. the version in the green case).
5-series Siemens R-IV receiver in grey enclosure

Perspective view of the Siemens R IV (seeN from the front top) Coil Pack 1 (17-40 metres) seen from the top Top view of the Siemens R IV Interior seen from the rear bottom Interior seen from the bottom right Close-up of the steal valves (tubes) behind the coil pack Close-up of the components at the right side of the receiver

The image below shows the front panel of the Siemens R-IV receiver, that holds all controls and connections. The antenna connections are at the left center, with a socket for the earth (ground) at the bottom left. At the center of the front panel is the typical tuning dial, which resembles the micrometer tuning dial of the National HRO. The dial of the R-IV however, was manfactured by Siemens and has an additional fine tuning knob at the left, that drives the main dial. This is very useful on the 80 meter band, which is packed with narrowband signals. The fine tuning dial can be disengaged by pushing it to the left, after which the main dial can be moved freely again.

Siemens R-IV control panel

The remaining controls are at the right half of the front panel. At the top are two meter: one for measuring the signal strength and one for checking the internal voltages. The three knobs are for adjusting the bandwidth, the BFO (tone) and the audio volume. At the bottom are the switches for enabling the filter, the AGC and the BFO. The rightmost switch is the power switch (on/off). To the left of the switches are sockets for two pairs of headphones. At the bottom right is a 5-pin socket for connection to the mains power supply unit (PSU).
The R-IV is extremely service friendly. Its interior can be accessed from two sides: the top and the bottom. All valves can be accessed from the top of the unit, simply by lifting the upper panel (just like on the National HRO). The upper panel has been constructed in such a way that the coil pack can be left installed when the lid is opened.

The top section of the receiver is divided into three compartments: the right compartment with 5 valves and some capacitors, a compartment at the rear with 4 valves and one at the front that is the slot for one of the three the coil packs.
Close-up of the valves and capacitors in the right compartment

Opening the top cover Siemens R-IV with coil pack installed and top lid open Close-up of the valves and capacitors in the right compartment Close-up of the valves and capacitors in the right compartment Close-up of the valves and capacitors in the rear compartment Valve layout chart inside the top lid Coil pack interior

Coil packs
Each R-IV receiver was supplied with three plug-in coil packs. Rather than at the front of the receiver, as with the National HRO, the coil packs of the R-IV were inserted into a slot on top of the device. A second coil pack was usually placed at the right side of the top surface. Each of the three coils packs had a different frequency range, giving the receiver a total frequency coverage of 3.15 - 33 MHz (9 - 95 m). The table below shows the frequency range of each coil pack.
Plug-in Range Frequency
0 9 - 18 m 16.5 - 33 MHz
1 17 - 40 m 7.5 - 17.5 MHz
2 40 - 95 m 3.15 - 7.5 MHz

Each coil pack consists of a die-case aluminium alloy case with eight compartments: four small ones and four larger ones. The larger compartments each contain a single coil wound on a ceramic body. The smaller compartments contain the adjustable capacitor for each section.
Power Supply Unit
The R-IV came with its own proprietary Power Supply Unit (PSU) that was usually placed to the right of the receiver. It supplied the various HT and LT voltages and was connected to the rectangular 5-pin socket at the bottom right of the front panel, with a tick rubber cable.
The image on the right shows the PSU of the Siemens R-IV after the top cover of its case has been removed. The design is rather straight­forward and consists of a transformer, a valve-based rectifier and several capacitors.

The unit is marked Netzanschlußgerät für Empfänger R IV (mains connection device for receiver R4). At the front of the PSU are two cables: on for connection to the mains (Netz) and one for connection to the reciver (Empf.). Also at the front panel are the fuse, a voltage selector and the on/off power switch.
PSU interior

R-IV power supply unit Small lid on top of the PSU The small lid gives access to the valve PSU front view PSU interior Transformer and valve Right side view of the interior Left side view of the interior

Spare parts
Each R-IV receiver came with a small metal box, roughly the size of a coil pack, in which the spare valves were kept. The box was padded with wood chips in order to prevent the valves from getting damaged in transit.

As the R-IV contains four different valves (3 x EF11, 3 x EF12, 2 x EF13 and 1 x ECH11), the box contained just four black metal valves (German: Stahlröhren).
Inside the spare parts box

The Siemens R4 in use
As the Siemens R IV was used by secret services during WWII, pictures of the radio in use are just as rare as the device itself. Below an overview of the pictures we have found so far. If you are aware of any other images that show this receiver, please contact us.
The Siemens R4 was used during WWII by the German intelligence service, the Abwehr. The image below shows a soldier at work in the secret radio station of the foreign department of the Abwehr (OKW-Amt Ausland/Abwehr). The device as the left is the Power Supply Unit (PSU). Above the radio, mounted on the wall, is the antenna selection unit. At the right is an identical setup.

Copyright German Federal Archives (Bundesarchiv) [2]

The image below shows another Abwehr radio operator at work. This photograph clearly shows the frequency dial being operated. A major difference with the HRO receiver is that the coil units were plugged-in from the top at the front left. A spare coil pack it placed on top of the receiver (at the right). Apparently this was common practice as it is also visible in the image above.

Copyright unknown [3]

Help wanted
We are currently looking for the following:
  • Circuit diagram of the R-IV.
  • Information about the Siemens R-II receiver.
  • Images of the R-II receiver.
  1. Günter Hütter, Siemens R IV featured on this page
    Personal correspondence. May 2014.

  2. Bundesarchiv, Bild 146-2005-0157 / CC-BY-SA
    German Federal Archives. Creative Commons License. Obtained via Wikipedia, May 2014.

  3. Arthur O. Bauer, Some aspects of the German military 'Abwehr' wireless service...
    Paper about the Abwehr wireless service during WWII. 15 September 2003.

  4. Richard Auerbach (DL1FK), Notes about Siemens R-II and R-IV
    Personal notes, 17 April 1978. Retrieved June 2014. 1

  5. National Company Inc., National Precision Condensers
    Malden, Massachusetts (USA). Catalogue 1948. p. 31. 1

  6. Fritz Trenkle, Die deutschen Funknachrichtenanlagen bis 1945
    Band 2 'Der Zweite Weltkrieg' (German). pp. 151-154.

  1. Document kindly supplied by Günter Hütter [1].
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