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Portable direction finder

The RZ-301, codename POSPISIL1, was a valve-based portable direction finder developed by the Military Technical Institute (Czech: Vojenský technický ústav) in Prague (Czechoslovakia) in the late 1940s. The radio was inteded for tracking clandestine transmissions and could be operated from within a briefcase. In 1948, in the hands of the Czech Secret Police, it became a feared tool.
The direction finder consists of a valve-based receiver with a rectangular 'window' antenna mounted on top. The receiver measures 30 x 8 x 13 cm and is powered by internal batteries. It has a frequency coverage from 1.3 to 20.3 MHz, divided over 4 ranges. The desired frequency range is selected by inserting the appropriate plug-in unit at the bottom of the device.

For unobtrusive surveillance, the device was usually hidden inside a leather briefcase of the era. The operator would wear a pair of small stethoscope-style headphones and rotated the briefcase in order to get a proper bearing.
RM-301 with three additional plug-in units

The PRZ-48 prototype was approved on 10 March 1948 after which the device was renamed RZ-301. The first production run was released in 1951 [2]. It is currently unknown how many RZ-301 units were built but, as they were used by the Secret State Police, it seems likely that they were produced in small quantities, probably a few hundreds. At least four different series were built, as the one in our collection shows 'SERIE D' on the front panel. Very few RZ-301 receivers have survived, most of which are now in the hands of museums and private collectors.
  1. Pospíšil is a Czech word which means 'a person who is in a hurry', or 'impatient'. It is also a common surname in Czechoslovakia.
RM-301 with three additional plug-in units RM-301 with three additional plug-in units RM-301 with three additional plug-in units Front view of the receiver Rear view Side view Bottom/front view Remake of contemporary photograph
Headphones Headphones connected to the Sluch. socket The four plug-in units Plug-in number IV (4) Bottom view Bottom view with plug-in unit removed Carrying the receiver by its antenna Briefcase, RZ-301, plug-ins and headphones

All controls and connections are at the small front panel, which is shown below. The power switch is at the the bottom. In the leftmost position, the unit is ON. Just above the ON/OFF switch is the volume knob that is used to adjust the audio level of the high-impedant headphones, which are connected at the bottom left. At the center is the frequency tuning knob, that operates the hand-crafted frequency scale at the top. The scale can be illuminated by pressing the button at the left.

Controls and connections of the RZ-301

The other end of the receiver holds the rear panel, which is actually a hinged door that covers the battery compartment. At the rear are two large locking bolts. The upper one locks the rear panel in place. Loosening it, allows the panel to be lowered, giving access to the battery compartment.

Rear panel

Inside the battery compartment are two bays. The one at the bottom left is for the LT battery which consists of 3 1.5V D-cells that are connected in parallel. The 1.5V LT voltage is used for the filaments of the valves. The bay at the right is for the HT battery that supplies the Anode Voltage for the valves (probably 67.5V). It should be connected to the A+ and A- terminals. At the bottom of the unit is a slot for the frequency plug-in, which is further explained below.
Front view of the receiver Rear view Tuning scale Serial number tag Battery compartment Looking into the battery compartment Battery contact block Removing the plug-in unit

During WWII, the Czech got acquinted with radio direction finders, when the Germans Secret Police intercepted many secret underground transmissions. One of the most advanced devices used by the Germans, was the so-called Gurtelpeiler, a body-wearable direction finder that could be hidden under the clothing of the interceptor. As a result, many agents and spies were caught.

Once the war was over, the Military Technical Institute (Czech: Vojenský technický ústav) decided to develop their own portable direction finder, which would be codenamed 'Pospisil'. The device was intended for direction finding at close range and consisted of a small receiver with a rectangular 'frame' antenna, small enough to hide it in, say, a briefcase of the era. By using a small pair of stethoscope-type headphones the receiver could be operated unobtrusively.

On 31 March 1948, the PRZ-48 prototype1 was approved by the Technical Committee and the device was taken into production as the RZ-301. Three weeks earlier however, on 10 March 1948, a coup had taken place and the Third Czech Republic (1945-1948) became a communist country, controlled by the Soviet Union (USSR) [3]. As a result, the RZ-301 fell into the hands of the Státní bezpecnost (StB)2, the new plainclothes State Secret Police, and became a much feared tool [1].

Photograph of the early 1950s, showing a complete the RZ-301 setup, with briefcase, frequency calibration table and headphones. Copyright Military History Institute Prague [1].

The image above was taken around 1951 and shows a complete RZ-301 setup. At the center is the direction finder with three additional plug-in units to its right. At the left are the unobtrusive stethoscope-style headphones, with a frequency calibration chart behind them. Behind the calibration chart is the leather briefcase in which it was carried inconspicuously by the StB.
  1. The letters PRZ are probably the abbreviation of Prenosny Radiova Zamerovic (Portable Radio Finder) whilst '48' indicates the year of development. The model name was later shortened to 'RZ', which probably stands for Radiova Zamerovic (Radio Finder).
  2. The StB, or Státní bezpecnost, was the powerful and repressive plainclothes (political) Secret State Police that was bound to and controlled by the Communist Party of Czechoslovakia, between 1945 and 1990. Following the collapse of the Soviet Union, the organisation was dissolved on 1 February 1990 [4].

Frequency plug-ins
The RZ-301 coveres the entire shortwave band from 1.3 to 20.3 MHz, divided over four overlapping ranges. For each frequency range, a separate plug-in unit is available, that is inserted into a rectangular slot at the bottom of the receiver. It is locked in place by two locks that are operated by the bracket that is used for pulling the plug-in out of the slot.
The following frequency plug-ins are available:
  1. 1.3 - 2.8 MHz
  2. 2.5 - 5.4 MHz
  3. 5.3 - 10 MHz
  4. 9.5 - 20.3 MHz
Each plug-in unit consists of a rectangular aluminium enclosure with three equally sized compartments inside. Each compartment contains an adjustable filter section and has its contacts at the bottom, nicely lined up, so that they mate with the pins of the plug-in slot.
The four plug-in units

The three filter sections are seperated by aluminium panels that are part of the enclosure. In order to prevent short-circuits inside the plug-in, the panels are covered at both sides with celluloid sheets. Please note that celluloid becomes brittle as it ages and might need to be replaced. If this is the case, small folded plastic sheets could be a good alternative.
Bottom/front view The four plug-in units The four plug-in units Plug-in number IV (4) Plug-in number IV (4) Rear view of plug-in number IV (4) Bottom view with plug-in unit removed Looking into the plug-in slot
Locked Unlocked Unlocking the plug-in unit Removing the plug-in unit Plug-in unit removed from the receiver Disassembled frequency plug-in Frequency plug-in interior Celluloid insulation

The interior of the RZ-301 can easily be accessed by loosening the lower bolt at the rear panel, after which the entire receiver can be extracted from the case by pulling the front panel. Please ensure that the plug-in unit (bottom) is removed before you attempt to extract the receiver.
The receiver has a 7-pin socket at the rear, which mates with a set of pins that are mounted inside the case. These pins carry the LT and HT voltages but also supply the antenna signal.

The unit is extremely well-built for its age and is very service-friendly. It can roughly be divided into the upper half, which contains the passive components, and the lower half which contains the valves. The valves are all mounted upside-down and are kept in place by protective caps. The upper section that contains the passive components is shielded by a metal panel.
RZ-301 interior

The image above shows the receiver with the metal panel removed and the passive components exposed. The metal enclosure at the far end is the tuning capacitor. The trimmer at the top, just behind the front panel, can be accessed through a closeable hole in the top of the case.

A remarkable feat is the absence of any Russian influence in the design. It is fully built with Tungsram valves and only first class (German) passive components are used. Apparently Czechoslovakia was not yet under the Russian sphere of influence when the RZ301 was designed.
Extracting the receiver from the case The receiver aside the case Looking into the case after the receiver has been removed Bottom section of the interior. At the right is the front panel. Bottom view of the interior Bottom view of the interior. At the top left is the plug-in slot. Side view (upside down) Left view of the interior
Top view of the interior Bottom view of the interior RZ-301 interior Passive electronic components (normally shielded) Removing the protective cover from a valve Exposed valve Rear view of the bare receiver Connector block at the rear of the bare receiver
Closed trimmer access point Open trimmer access point Enclosed tuning capacitor Component detail Replaced power switch Close-up of the passive electronic components. Note the scale light at the right. Close-up of the scale light Close-up of the rear of the battery connector block (left)

Bottom view of the interior. At the top left is the plug-in slot.

The image above shows the RZ-301 as seen from the bottom of the device, with the front panel on the right. In total, there 7 valves (tubes) in this receiver, all of which are normally protected by an aluminium cap. In our case, the aluminium cap is missing from the valve at the top right.

A major problem with our RZ-301 was that nearly all text had disappered from the valves. Luckily, collector Günter Hütter was so kind to open the RZ-301 in his collection and check the position of the valves, which is given above [5]. Datasheets of the valves can be found below.
Help wanted
We are currently looking for additional information about this direction finder and its operational use. In particular, we are looking for a technical description and the circuit diagrams. If you ave any of these available, please contact us.

  1. Datasheets via The national Valve Museum [6].
  1. Vojenský historický ústav, Prenozny zamerovac RZ 301 'Prospisil'
    Military History Institute, Prague (Czech Republic). Portable Direction Finder RZ 301.
    Website. Retrieved July 2014.

  2. Karel Julis, OK1UHU, RZ-301 POSPISIL
    Website 2011. Retrieved July 2014.

  3. Wikipedia, History of Czechoslovakia
    Retrieved July 2014.

  4. Wikipedia, StB
    Retrieved July 2014.

  5. Günter Hütter, Positions of the valves of the RZ-301
    Retrieved 10 July 2014.

  6. The National Valve Museum, Valve Datasheets
    Allen Wyatt. Website. Retrieved July 2014.

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