Portable direction finder
The RZ-301, codename POSPISIL 1 , was a valve-based portable
developed by the Military Technical Institute
Vojenský technický ústav in Prague
(Czechoslovakia) in the late 1940s.
The radio was intended for locating clandestine transmissions and could be operated
from within a briefcase. In 1948, in the hands of the
Czech Secret Police,
it became a much feared tool.
It was also used by the military intelligence service
Zpravodajská Správa Generálního Štábu (ZS GŠ).
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
The desired frequency range is selected by inserting the appropriate
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
and rotated the briefcase in order to get a proper bearing.
Known as PRZ-48, the prototype was approved on 10 March 1948
after which the device was renamed RZ-301.
The first production run was released
in 1951 . 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.
Pospíšil is a Czech word which means 'a person who is in a hurry',
or 'impatient'. It is also a common surname in Czechoslovakia.
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 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
Inside the battery compartment are two bays. The one at the bottom
left is for the LT battery which consists of three 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.
During WWII, the Czech got acquinted with radio direction finders, when the
German Secret Police GESTAPO intercepted many secret underground transmissions.
One of the most advanced devices used by the Germans, was the so-called
a body-wearable direction finder that could be hidden under the operator's
clothing. 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 'window' 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 prototype 1 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) . As a result, the RZ-301 fell into the hands
of the Státní Bezpecnost (StB) 2,
the new plainclothes Secret State Police, and became a much feared tool .
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 the set
was carried inconspicuously by the StB.
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).
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.
The organisation was dissolved on 1 February 1990, just before the collapse
of the Soviet Union .
The RZ-301 covers 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
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 three filter sections are separated 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
Please note that celluloid becomes brittle as it ages
and might have to be replaced. If this is the case,
small folded plastic sheets could be used as a good alternative.
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 and is
shielded by a metal panel.
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 around 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.
The image below shows the RZ-301 as seen from the bottom of the device,
with the front panel on the right. In total, there are 7 valves (tubes) in
this receiver, all of which are usually 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 . Datasheets of the valves can be
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 can provide any of these available,
please contact us.
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Datasheets via The National Valve Museum .
- Vojenský historický ústav, Prenozny zamerovac RZ 301 'Prospisil'
Military History Institute, Prague (Czech Republic). Portable Direction Finder RZ 301.
Website. Retrieved July 2014.
- Karel Julis, OK1UHU, RZ-301 POSPISIL
Website 2011. Retrieved July 2014.
- Wikipedia, History of Czechoslovakia
Retrieved July 2014.
- Wikipedia, StB
Retrieved July 2014.
- Günter Hütter, Positions of the valves of the RZ-301
Personal correspondence, 10 July 2014.
- The National Valve Museum, Valve Datasheets
Allen Wyatt. Website. Retrieved July 2014.
- Miroslav Truksa, RZ301 Pospisil
16 January 2015.
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© Crypto Museum. Created: Monday 03 August 2009. Last changed: Monday, 12 April 2021 - 20:53 CET.