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WWII
RX
NL
Oranje
  
Panter receiver
Clandestine midget receiver · Radio Oranje

Panter 1 was a clandestine single-valve receiver for the Long Wave (LW) radio band, built during World War II (WWII) by amateur radio operator MP (Mart) Rooth (callsign: PA0MPR) in Rotterdam (Netherlands). It was used to clandestinely listen to the British Broadcasting Corporation (BBC) on 1500 metres (200 kHz), and in particular to the programs of the Dutch government in exile that were aired daily by the BBC European Service under the name Radio Oranje (Radio Orange). 2

The radio is housed in a metal sigar box of the era, that measures 106 x 53 x 19 mm without the valve. It has only one fixed lead – for the 4V DC power supply – that can be rolled up inside the device. The bare receiver weights just 98 grams. When closed, it is indistinguishable from a regular cigar box and could easily be hidden.

The valve — a Philips B405 or similar — was usually hidden somewhere else in the house. To receive the broadcasts of the BBC at 1500 metres (200 kHz), the power cable must be unrolled and the valve should be installed in the 4-pin socket.
  
Receiver with valve installed

It is one of the simplest and smallest designs of a clandestine radio of WWII. It uses just one valve and – interestingly – it uses just one DC power voltage – 4V – that was supplied by an external battery or a flashlight (4.5V). A cable with Edison E10 thread at one end is present to 'steal' the power from a flashlight by screwing the E10-end directly into the flashlight's E10 lamp fitting.

The valve – a simple triode – is used way outside its specified range and produces only a weak signal into a pair of high impedance (4000Ω) headphones, that are connected to the banana sockets at the left edge. The two banana sockets at the right edge are for antenna and ground.

During WWII, the Germans had made it illegal to listen to foreign radio stations. As most people ignored this, the Germans issued a directive on 13 May 1943 that made it illegal to possess a receiver. People who didn't hand in their radio, risked high fines or even the death penalty.
  
Panter sigar box

The receiver shown here is part of a series of three different designs, all built by amateur radio operator Mart Rooth (PA0MPR) in Rotterdam (Netherlands) during WWII, and based on the same circuit diagram. The other two are housed in a small wooden cube and a matchbox respectively.

  1. As this homemade device does not have a name, we have nicknamed it Panter after the brand name of the cigar box it is housed in. An identical receiver from the same maker has also been found inside a (same-size) cigar box with the brand name Radio [2].
  2. Named after the Dutch monarchy's House of Orange-Nassau.  More

Panter sigar box
Open box with cable rolled out
Receiver with valve installed
Valve
Headpohones
Extra cable for taking power from light torch
Receiver with valve and torch light power cable
Complete setup
A
×
A
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Panter sigar box
A
2 / 8
Open box with cable rolled out
A
3 / 8
Receiver with valve installed
A
4 / 8
Valve
A
5 / 8
Headpohones
A
6 / 8
Extra cable for taking power from light torch
A
7 / 8
Receiver with valve and torch light power cable
A
8 / 8
Complete setup

Features
The image below shows how the Panter receiver was used. The receiver was first removed from the metal cigar box. The cable was then unrolled and the receiver was put back in the cigar box. Next, the valve was installed in the 4-pin socket and a pair of high-impedance headphones was connected. A long wire – often hidden under the rooftop of the house – was used as the antenna, whilst the ground terminal was connected to the earth — usually the metal pipe of the water tap.

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Power was provided by an external rechargeable 4V battery, or by the 4.5V battery of a flashlight. In the latter case, the existing lamp was removed from the flashlight and an adapter cable was fitted in its place. The adapter cable was then connected to the power terminals of the receiver.


Circuit diagram
Below is the circuit diagram of the receiver, as it was found inside a similar Radio Oranje receiver from the same maker and featured chapter 12 of Louis Meulstee's Wireless for the Warrier Volume 4 - Supplement [2]. This circuit has been verified against the actual device featured here. It is a regenerative receiver with a space-charge detector (V1) and a feedback coil (L2) – connected in series with the anode of V1 – that is located above the coil (L2) of the tuned circuit (L1/C2). The feedback coil (L1) can be positioned with a screw, so that the regeneration level is adjustable.

Single-valve long-wave receiver with single 4V power supply - Mart Rooth (PA0MPR) ~1943-1944

What makes this design special, is the fact that a single 4V DC power supply is used both for the filaments and the anode. Although this is much lower than the minimum specified anode voltage of 50V [5], it actually works. This is done by using the valve in space charge mode [3][6][7]. Low-cost designs like this, were very popular in the 1930s, as they did not require HT voltages [4].


Interior
The interior of the receiver can be accessed simply by holding opening the metal cigar box and holding it upside down. With some help, the interior should come out. The image below shows a bottom view of it. All passive parts are mounted to the bottom side of the brown pertinax panel. At the four corners are 3 mm screws that act as spacers. They ensure that pertiax panel lines up with the metal cigar box, and prevents contact between the metal parts and the bottom panel of the cigar box. For extra isolation, a piece of thick black paper is present at the bottom of the box.

Bottom view of the clandestine 'Panter' long-wave receiver

At the heart of the receiver are two movable parts: a homemade adjustable capacitor (C2) and movable coil (L2), each of which are adjustable from the top surface. C1 is used to adjust the frequency, whilst L2 is just to adjust the regeneration level. The valve (V1) is installed at the top.

Interior
Interior
Interior
Interior with cable rolled up
Bottom view
Broken coil
B
×
B
1 / 6
Interior
B
2 / 6
Interior
B
3 / 6
Interior
B
4 / 6
Interior with cable rolled up
B
5 / 6
Bottom view
B
6 / 6
Broken coil

Restoration
When we obtained the receiver in March 2021 [1], it was in well-preserved condition, with the original wartime parts still present. The only problem was that the movable coil (L2) had come off the rotatable shaft and was floating around inside the case. This was probably caused by frequent handling of the device in the 76+ years that had passed since the war. As this would eventually cause the thin wires of the coil to break, it was necessary to refit the coil to the rotatable shaft.

In the original situation the coil was held in place by hot wax or paraffine, but this does not offer a long term solution, especially when the device is kept under varying environmental conditions (hot, cold, etc.). For this reason we cleaned the shaft and the body of the coil, and applied a small drop of a modern two-component adhesive. The result is shown in the images above. The radio has since been tested with a signal generator and is now fully operational again.

Fixed
  • Movable coil (L2) refitted to the axle
Connections
B405 was a 4-pin directly-heated triode valve (tube) with a 4V filament, introduced by Philips in 1927 for use in pre-amplifier and/or power amplifier stages. It has an anode voltage range of 50-150V, but could also be used in space-charge mode, with an anode voltage of just 4V [6][7]. The regenerative single-valve receiver featured above, uses the valve in this mode. Below is the pinout of the B405 as seen from the bottom. Usable alternatives are the A415, A425 and B415.

 B405 datasheet




Specifications
  • Era
    WWII
  • Years
    1943-1944 (est.)
  • Purpose
    Reception of BBC broadcasts at 200 kHz (1500 m)
  • User(s)
    Civil (clandestine)
  • Band
    Long Wave (LW)
  • Frequency
    160-200 kHz (1500 - 1875 m)
  • Design
    MP (Mart) Rooth (PA0MPR)
  • Principle
    TRF with reaction
  • Valve
    B405 or similar
  • Power
    4V from battery or flashlight (4.5V)
  • Output
    High impedance headphones (2000-4000Ω)
  • Dimensions
    106 x 53 x 19 mm
  • Weight
    98 grams (bare receiver, without valve)
Parts
  • Metal cigar box for 10 mignor cigars (branded 'Panter')
  • Receiver base unit (fitted inside cigar box)
  • Valve B5405 or similar
  • Headphones (4000&Ω) with 2-pin plug
  • Power cable with E10 fitting (optional)
  • Antenna and ground leads
References
  1. Cor Moerman, Miniature Radio Orange receiver in Panter cigar box - THANKS !
    March 2021.

  2. Louis Meulstee, Clandestine Midget Receivers #5
    Wireless for the Warrier, Volume 4, Supplement chapter 12.
    Version 1.00 may 2015.

  3. Jeff Duntemann, Low-Voltage Tubes — Low-Voltage Operation with ordinary tubes 1
    Retrieved September 2021.

  4. One-Tube Set Works on Six Flashlight Cells 1
    Popular Mechanics, September 1936. pp. 420-421.

  5. Philips Miniwatt B 405 (datasheet)
    Philips, date unknown.

  6. Wikipedia, Vacuum tube — Space charge of a vacuum tube
    Retrieved September 2021.

  7. Wikipedia, Space charge
    Retrieved September 2021.

  8. Peter Kievits (PE1RUF), Personal correspondence
    September 2021.
  1. Brought to our attention by Peter Kievits [8].

Further information
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Crypto Museum. Created: Monday 04 January 2021. Last changed: Thursday, 25 November 2021 - 17:49 CET.
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